biology, geology, and chemistry), and investigate the effects of afforestation, forest management, and environmental changes on forest soil biogeochemistry.

SUE BENHAM

I have always loved nature and spent my childhood climbing trees and watching the
natural world in the woods around my home. Now I am a scientist at Forest Research
and I get paid to do the same thing! I spend my time working to understand how our
forests grow and what effect the changing climate is having on their condition. For
this I study all aspects of a tree's environment, from the soil around its roots to the
air around its leaves and the animals that rely on it.

<figure><img src="image_125.png" /></figure>

KEVIN R. BUTT

Kevin is an ecologist at the University of Central Lancashire. He has studied earthworms for more than 30 years and is interested in how these ecosystem engineers are able to assist humankind through their everyday activities, such as burrowing and eating organic matter. He has studied earthworms throughout Britain and undertaken research across Europe and in the USA. One of his current projects is examining the action of earthworms on Charles Darwin's estate, recreating experiments of the great scientist, whilst another is investigating giant rainworms in the mountains of Germany.

<figure><img src="image_126.png" /></figure>
<figure><img src="image_127.png" /></figure>

# THE WAY SOIL ORGANISMS LOOK CAN HELP US UNDERSTAND THEIR IMPORTANCE

Pierre Ganault<fnref n="1" />*, Léa Beaumelle<fnref n="2" /> and Apolline Auclerc<fnref n="3" />

## YOUNG REVIEWER:

<figure><img src="image_128.png" /></figure>

GIULIA

AGE: 13

There is a multitude of life forms on our planet. This is especially true under our feet, in the soil. Earthworms, spiders, and millipedes are only a few examples of the vast number of soil organisms. Once you look what lives in soils, you realize the tremendous diversity of shapes and colors. But what if we take the time to describe all their characteristics: color, size, shape, number of legs, type of wings, lifespan, and climate preferences? All these characteristics, called traits, help us to understand what types of organisms can be found in a particular ecosystem, what they feed on, and how far they can travel. Scientists use this information to understand the different roles of organisms in soils, and to restore degraded soils. Analyzing traits can reveal the importance of soil organisms and the fundamental roles they play for human societies.

<footnote n="1">¹CEFE, CNRS, EPHE, IRD, Université de Montpellier, Université de Paul-Valéry Montpellier, Montpellier, France</footnote>

<footnote n="2">²INRAE, UMR SAVE, Bordeaux, France</footnote>

<footnote n="3">³Laboratoire Sols et Environnement, Université de Lorraine, INRAE, LSE, Nancy, France</footnote>
## Figure 1

Methods of sampling and studying soil invertebrates. Small organisms are extracted from a small soil core by drying the soil and collecting the individuals that fall out of the sample. Large, fast-moving organisms living in leaf litter are collected when they fall into pitfall traps. Less-mobile organisms are extracted from a soil block of soil with a shovel and then sorted by hand. Earthworms living deep in the soil are extracted by pouring a mustard solution into their burrows. Less-mobile organisms living in the leaf litter can be isolated using a Berlese apparatus, which dries the litter and traps the organisms in a jar (Drawing credits: www.lesbullesdemo.fr. Picture credits: Apolline Auclerc, EcoBioDiv lab).

## INVERTEBRATE

A small animals with no internal skeleton, such as insects, worms, or molluscs.

## SOIL BIODIVERSITY

The variety of life forms in soils. It can be measured by the number of species, traits, or genes of these organisms.

## SOIL ECOLOGIST

A scientist investigating soil organisms, their interactions with their environment, and their role in soil functioning.

<figure><img src="image_129.png" /><figcaption>Figure 1</figcaption></figure>

# THE SOIL: A WONDERFUL BUT POORLY KNOWN WORLD

Under our feet, millions of organisms live in the soil [1]. These organisms span from the microscopic (called microorganisms) to **invertebrate** organisms (such as earthworms) more than 1-m long. The variety of organisms in the soil is called **soil biodiversity**. Biodiversity means the variations in all the life forms on the planet.

**Soil ecologists** are scientists who study the diversity of soil organisms. They usually sample the soil organisms living in various places, like tropical rainforests or agricultural fields. They use shovels, traps, or corers to remove soil samples, depending on whether the organisms of interest live within the soil or on its surface (Figure 1). Then, the scientists catch the organisms they can see in their samples by hand or with tweezers. To catch the smallest soil invertebrates, soil ecologists often use a technique called the **Berlese method**. In the laboratory, they place the soil sample in a funnel, with a heating lamp above the sample and a jar below it. The light and heat make the tiny organisms go down through the funnel into the jar. After a few days, scientist can study the organisms in the jar.

When all the soil organisms are collected, the long and meticulous work starts. Soil scientists count and closely observe each individual organism, to identify which **species** it belongs to. To do so, they use various types of microscopes along with identification keys and books. The total number of species found in one particular ecosystem represents that ecosystem's biodiversity. Soil scientists have a lot of work to do, because soils are among the most diverse and highly
## Figure 2

Differences in morphological traits between nine species of soil invertebrates belonging to three groups: earthworms, ground beetles, and springtails (Drawing credits: www.lesbullesdemo.fr).

<figure><img src="image_130.png" /><figcaption>Figure 2</figcaption></figure>

## BERLESE METHOD

A procedure to extract small-size organism from leaf-litter and soil sample by drying it and collecting organisms that migrate through the sample and fall in a jar.

## SPECIES

Individual organisms that belong to the same species can produce fertile offspring. It is the most commonly used unit to describe life on Earth. All human beings belong to the same species but there are many, many species of soil organisms.

## TRAIT

Any characteristic that can be measured on an individual to describe its shape, movement capacity, diet, behavior, or reproduction strategy.

human-impacted ecosystems on Earth. In addition, many soils in the world have not yet been studied, so many species of soil organisms have still not been discovered.

## SOIL ORGANISMS ARE EXTREMELY DIVERSE

Soil biodiversity is so large that it is almost impossible to describe the **traits** of all soil organisms at once. We will give you an idea of soil diversity by describing the appearance and behaviors of three well-studied types of soil organisms: earthworms, springtails, and ground beetles (Figure 2).

### Body Size

An important structural difference between earthworms, springtails, and ground beetles, and between species within these groups, is their body size. Size is an example of a morphological trait. The smallest earthworm measures several cm long, while the largest, found in tropical forests, can be 2 m long. In Europe, ground beetles measure between 2 mm and 8 cm from the top of the head to the last segment of the abdomen. Springtails are much smaller, with an average body size of only 2 mm, but their size varies depending on where they live. Some springtail species living in dead leaves are bigger than other species that live deeper in the soil.
UV LIGHT

Part of the sun's rays
that are invisible to the
naked eye and can
cause sunburn.

Movement

To find a habitat with enough food, other organisms with which to reproduce, and a low number of predators, soil organisms have developed numerous techniques for moving, both at the surface and within the soil. Earthworms do not have legs, but some species have strong muscles and small hairs that they use to burrow between soil particles. With their six legs, ground beetles can run on the soil surface to catch their prey. Many species of ground beetles have wings, allowing them to rapidly escape from a predator or other disturbance, or to move to a place where they can find more prey or mates. Springtails also move on their six legs, but thanks to a special appendage that acts like a spring, some springtails can jump several centimeters into the air to escape predators!

Color

Soil organisms can be colorful. Some earthworms that live in the few first centimeters of soil, in the dead leaves, or in compost or manure are reddish-brown, which enables them to camouflage from their predators against orange-brown dead leaves, but which also protects them from **UV light** [2]. Other earthworms live deeper in the soil and often have pale colors, such as pale pink, gray, or green. In the dark soil, pigmentation is not necessary because UV light does not penetrate. Still other earthworms live mostly in the soil, but they put their heads out of the soil to feed on dead leaves; consequently, only their heads are pigmented. Springtails show almost the same color patterns as earthworms: pigmented species live on top of the soil and unpigmented ones live within the soil [3]. Finally, ground beetles can have many wonderful color patterns, especially those of the *Carabus* genus. The vivid colors might discourage their bird predators or might help them to camouflage in their environments.

Mouth Types

Another stinking difference between our three groups is their mouth type. Ground beetles have strong mandibles (jaws) that can have different shapes and sizes depending on what they eat the most. For example, some species have very long mandibles, projected forward, to reach inside the shells of snails. Springtails have small mouths that allow them to eat fungi growing on leaves and small pieces of the leaves themselves, creating beautiful, skeletonized dead leaves. Earthworms do not have mandibles, but their muscular stomachs are strong enough to crush the soil and the leaves they eat.

THE TRAITS OF SOIL ORGANISMS ARE CLUES TO THEIR
IMPORTANT ROLES

Careful observations of the traits of soil organisms can tell soil
ecologists a lot about what the organisms eat, where they live, and
how they interact with their environments (Figure 3). The actions of soil
<caption>**Figure 3**</caption>

Soil organisms in action. (1) Earthworm living and feeding on dead leaves. (2) Earthworm reaching the soil surface through its large, deep burrow. (3) Earthworms living in the soil and digging many burrows. (4) Different springtail species transforming dead leaves into fecal pellets. (5) Springtail escaping from a predator by jumping with its spring-like appendage. (6) Ground beetle feeding on a snail. (7) Ground beetle ready to fly (Drawing credits: www.lesbullesdemo.fr).

<figure><img src="image_131.png" /><figcaption>Figure 3</figcaption></figure>

organisms are extremely important for maintaining healthy soils. These organisms can change the physical organization of soil by creating burrows, can add nutrients to the soil through the breakdown of dead leaves, and can help to control the populations of other soil organisms, [4]. Let us look at the important roles played by our three example organisms.

Earthworms play a crucial role in maintaining healthy soils by their intense burrowing activity. The earthworm species that live in the soil move through it, eating the food they find there and mixing the soil particles with pieces of dead leaves. As they move, they create a lot of burrows through which air and water can circulate more easily [5], helping other soil organisms to drink and breathe, and helping plant roots to grow. Some large earthworms create long, wide, vertical burrows (very much like chimneys). Others make thinner burrows, but still contribute strongly to mixing the soil. Earthworms therefore are quite important to reduce flooding and soil erosion, and to improve soil health.

Springtails also play crucial roles in soils, especially by recycling nutrients from dead leaves, which helps with plant growth. In some cases, springtails can reach densities of 10–100,000 individuals per m²! They can eat very large amounts of leaf litter and microorganisms (like fungi and bacteria). After eating, they produce many small fecal pellets composed of very tiny pieces of dead leaves mixed with some water. Fecal pellets are the perfect meal for microorganisms, which will continue to transform the dead leaves into nutrients that
the plants can use. This nutrient recycling performed by springtails
and microorganisms is extremely important for ecosystems and
plant growth.

Ground beetles have diverse diets, but can be predators feeding on a
wide range of prey, from small aphids to larger snails. Ground beetle
species are specialized for the prey that they eat; for instance, the
species *Cychrus caraboides* only feeds on snails. Some ground beetles
catch tiny springtails, thanks to their well-developed eyes (Figure 3).
Ground beetles are important for regulating the populations of other
animals. For example, in crops, they feed on the pests that would
otherwise damage the crop plants. Farmers can therefore use ground
beetles instead of chemicals to fight against pests. This is called
biological control, because it uses natural predator-prey interactions
between organisms to control pests. It is important to maintain a high
diversity of ground beetles in an ecosystem because not all species
feed on the same prey. Ground beetles vary in body size and mostly
eat prey that is smaller than they are. Therefore, a high diversity of
ground beetle species allows a better regulation of pests [6].

Body size, mouth type and size, hunting strategies, and type of
prey are important traits that soil ecologists commonly consider to
better understand the relationships between soil invertebrates and
their environments.

CONCLUSION

Soil organisms are incredibly diverse in shape and behavior. Soil
ecologists explore the wonderful world of the soil and have the
chance to discover new species and new traits. By looking at the
characteristics of the species they find, soil scientists can better
understand the interactions between organisms and ecosystems.
Taken together, the numerous roles played by the wide variety of
soil organisms are complementary and fundamental to maintaining
healthy soils. It is thus very important for us to maintain and conserve
soil biodiversity, which is facing the increasing impacts of human
activities, such as intensive agriculture and climate change. Raising
public awareness of the importance of soil organisms and improving
our knowledge of soil biodiversity will be key to decreasing our impacts
on the amazing ecosystems under our feet.

ACKNOWLEDGMENTS

The authors thank the TEBIS consortium (http://www.reseau-tebis.fr/) and the different NGO, such as *Les petits debrouillards* (https://www.lespetitsdebrouillards.org) and *CARABES* (https://assocarabes.com) with which the authors are working with to increase citizen awareness and encourage protection of soils and
their biodiversity. The authors also thank Morgane Arietta Ganault for the quality of the detailed drawings, the mentor and young reviewers for their suggestions that improved the manuscript quality, and Susan Debad for her help with English syntax that improved the manuscript clarity.

## REFERENCES

1. Orgiazzi, A., Bardgett, R. D., Barrios, E., Behan-Pelletier, V., Briones, M. J. I., Chotte, J. L., et al. 2016. *Global Soil Diversity Atlas*. Luxembourg: European Union. Available online at: http://esdac.jrc.ec.europa.eu/public_path/JRC_global_soilbio_atlas_online.pdf (accessed April 28, 2020).

2. Bottinelli, N., Hedde, M., Jouquet, P., and Capowiez, Y. 2020. An explicit definition of earthworm ecological categories—Marcel Bouché’s triangle revisited. *Geoderma* 372:114361. doi: 10.1016/j.geoderma.2020.114361

3. Potapov, A. A., Semenina, E. E., Korotkevich, A. Yu., Kuznetsova, N. A., and Tiunov, A. V. 2016. Connecting taxonomy and ecology: trophic niches of collembolans as related to taxonomic identity and life forms. *Soil Biol. Biochem.* 101:20–31. doi: 10.1016/j.soilbio.2016.07.002

4. Pey, B., Nahmani, J., Auclerc, A., Capowiez, Y., Cluzeau, D., Cortet, J., et al. 2014. Current use of and future needs for soil invertebrate functional traits in community ecology. *Basic Appl. Ecol.* 15:194–206. doi: 10.1016/j.baae.2014.03.007

5. Capowiez, Y., Bottinelli, N., Sammartino, S., Michel, E., and Jouquet, P. 2015. Morphological and functional characterisation of the burrow systems of six earthworm species (Lumbricidae). *Biol. Fertil. Soils* 51:869–77. doi: 10.1007/s00374-015-1036-x

6. Rusch, A., Birkhofer, K., Bommarco, R., Smith, H. G., and Ekbom, B. 2015. Predator body sizes and habitat preferences predict predation rates in an agroecosystem. *Basic Appl. Ecology* 16:250–9. doi: 10.1016/j.baae.2015.02.003

**SUBMITTED**: 15 May 2020; **ACCEPTED**: 12 January 2021;

**PUBLISHED ONLINE**: 11 February 2021.

**EDITED BY**: Rémy Beugnon, German Centre for Integrative Biodiversity Research (iDiv), Germany

**CITATION**: Ganault P, Beaumelle L and Auclerc A (2021) The Way Soil Organisms Look Can Help Us Understand Their Importance. Front. Young Minds 9:562430. doi: 10.3389/frym.2021.562430

**CONFLICT OF INTEREST**: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

**COPYRIGHT** © 2021 Ganault, Beaumelle and Auclerc. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original
publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

## YOUNG REVIEWER

### GIULIA, AGE: 13

I am Giulia. I am 13 years old. I like going to school and my favorite subject is English. In my free time, I love playing with my dog, playing tennis, and going to ride horse. In the summer, I like playing in my little swimming pool with my friends and going around my city, all together, by bike. Instead, in the winter, I like very much skiing with my parents and our neighbors.

<figure><img src="image_132.png" /></figure>

## AUTHORS

### PIERRE GANAULT

At each walk in nature, I cannot help myself from flipping logs and rocks over or searching into the dead leaves to see what wonderful animal I will find hiding there. This curiosity led me to study soil biodiversity and do a Ph.D. on the response of tree species mixture for soil invertebrates and the role of these animals for soil processes. I also work with associations to bridge the gap between scientist and citizen so we can work all together to study, better understand and protect the creatures living in the soil. *pierre.ganault@gmail.com

<figure><img src="image_133.png" /></figure>

### LÉA BEAUMELLE

I am a post-doc at the French National Institute for Agriculture of Bordeaux. My research aims to better understand the impacts of human activities on soil biodiversity and functioning. During my Ph.D. in Versailles, I was studying the response of earthworms to heavy metal pollution. I have expanded my research during my post-docs in France and Germany, by investigating the effects of multiple pollutants, the response of entire soil communities, and the consequences of biodiversity changes for ecosystem processes.

<figure><img src="image_134.png" /></figure>

### APOLLINE AUCLERC

I am an Assistant Professor in soil ecology, biology at University of Lorraine in France, Nancy. My research is focused on understanding how the urban and industrial soil ecosystems can host a surprising high level of biodiversity by assessing how invertebrates, such as earthworms, insects, spiders, millipedes... are adapted to the special features of these human-impacted soils. I also develop tools to help citizens to raise their awareness on the soil quality and its unknown biodiversity.

<figure><img src="image_135.png" /></figure>
<figure><img src="image_136.png" /></figure>

# THE FASCINATING WORLD OF BELOWGROUND COMMUNICATION

Cristiana Ariotti<fnref n="1" />, Elena Giuliano<fnref n="1" />, Paolina Garbeva<fnref n="2" /> and Gianpiero Vigani<fnref n="1" />,<fnref n="2" />*

YOUNG REVIEWER:

<figure><img src="image_137.png" /></figure>

SHASHI
PREETHAM

AGE: 13

If you are a microbe in soil, how do you say something to your neighbors? Well, speaking English, French, or Italian would not do you any good underground. Instead, you would have to use molecules as words! Soil microbes like bacteria and fungi communicate with each other and with other organisms, such as animals or plants, by producing different kinds of molecules. Many organisms use these molecules as chemical words. These molecules can be volatile molecules, which means that they are like gases, just like the air. They can travel easily through small pockets of air in the soil and can also travel really far. It is like long-distance communication. Other molecules are soluble so they can dissolve in water, allowing the communication between organisms close to each other. The organisms that receive this communication can respond in different ways, such as by growing faster or producing other molecules in response. In this article, we will explore the exciting and mysterious world of belowground chemical communication and its role in the interactions occurring between microbes and plants.

<footnote n="1">¹Plant Physiology Unit, Department of Life Sciences and Systems Biology, University of Turin, Turin, Italy</footnote>

<footnote n="2">²Department of Microbial Ecology, Netherlands Institute of Ecology (NIOO-KNAW), Wageningen, Netherlands</footnote>
### ECOSYSTEM

The set of different organisms (plants, animals, and microbes) that interact with the non-living matter in a specific area.

### RHIZOSPHERE

The portion of the soil surrounding the roots where plants and microbes talk to each other by using molecules.

### CHEMICAL COMMUNICATION

The communication that occurs between two or more different organisms (plants, animals, and microbes) by using molecules.

### SOLUBLE

A substance that dissolve in water, like salt and sugar.

### VOLATILE

A substance that tents easily to become gas and spread in the air, like the smell of a flower.

## LIFE IN THE SOIL

The soil is one of the most fascinating and complex **ecosystems** on Earth. It is not only the outer surface of our planet where plants grow, but it is also a spectacular and hidden world where many kinds of organisms live. The soil is composed of irregular pieces of rocks, small air-pores, and organic matter (dead plants and animals). This environment is a wonderful place for microbes, insects, and plants to live. Can you believe that soil is so full of life? Depending on the soil characteristics (size of the rocks, types of food, amount of water, etc.), different organisms can live in it. These different organisms build a specific community, which is a unique set of different organisms living together.

Do you know what soil microbes are? They are very tiny organisms that live attached to soil particles or other living things. There are fungi that have a shape similar to very thin roots, so that they can touch and exchange information with their neighbors. There are also bacteria, which are organisms that consist of a single cell that you can usually see only by using a microscope. If these organisms live in the **rhizosphere** (the portion of soil surrounding the plant roots), they are called rhizosphere microbes. They can also live on the root surface or inside the roots [1]. In the rhizosphere, you can find beneficial (good) microbes, which help plants to survive and grow, or harmful (bad) microbes, which attack plants and make them sick.

## HOW DO SOIL MICROBES AND PLANTS COMMUNICATE?

What is particularly exciting is the ability of soil microbes to communicate amongst themselves as well as with other organisms, including plants and animals. The communication between microbes and plants has been studied by many scientists. This communication is based on the use of molecules as words, and it is called **chemical communication**. You can imagine a molecule as a group of many little balls (that we call atoms), connected to each other. These atoms are important chemical pieces, like carbon (C), hydrogen (H), oxygen (O), and nitrogen (N), that join together (like a puzzle) to form molecules like water (H₂O) or carbon dioxide (CO₂). The specific combination of atoms creates molecules that have different properties.

Soil microbes can produce many types of molecules, which can be divided into two main categories: **soluble** molecules and **volatile** molecules. Soluble molecules dissolve in water, like a sugar cube in tea, and they can be transported by water in the soil. They are used to talk with plants that grow close to the microbes. Volatile molecules, also
## Figure 1

Plant-microbe interactions in the rhizosphere. The picture shows three types of interactions: (A) interactions between plants and microbes living on the root surface or near the roots, with the help of soluble molecules; (B) interactions between plants and microbes living far away from the roots, with the help of volatile molecules; and (C) interactions with microbes living inside the roots, directly in contact with root cells. These interactions take place in the soil, which is composed of rocks, air pores, and organic matter.

## VOCs

The volatile organic compounds that are produced by different organisms, like plants and microbes, to communicate with each other by distance.

<figure><img src="image_138.png" /></figure>

Figure 1

called **volatile organic compounds (VOCs)**, are used to communicate over long distances (Figure 1). These volatile compounds are gases, which travel easily through the air-pores of the soil. Plant roots can smell these gases, like your nose can smell a flower or freshly baked bread [1]. Chemical communication does not happen only in one direction (from the microbes to the plants), but in two directions—plants also produce molecules that can be understood by microbes.

## WHAT ARE THE EFFECTS OF COMMUNICATION MOLECULES?

Here are some examples of what happens between microbes and plants when they start to talk to each other.

### Plants and Microbes Help Each Other to Get Food

Many soil microbes can help plants grow, because the microbes make necessary nutrients more available for the plants. For example, a bacterium called *Rhizobium* can transform the nitrogen in the air (N₂) into a different molecule: ammonia (NH₃). This transformation is called nitrogen fixation and it happens in the soil. Nitrogen fixation is really important for plants because they need nitrogen to grow and they are only able to get NH₃ from the soil. By transforming N₂ into NH₃, *Rhizobium* helps plants to grow and makes them stronger!

What is even more interesting is that *Rhizobium* can talk with a specific group of plants, including beans and peas. But how does this dialogue work? First, the plant releases molecules called isoflavones into the rhizosphere, which can attract *Rhizobium*. The bacteria "hear" the
**Figure 2**

The effects of plant-*Rhizobium* interactions on root structure. (A) Normal root structure and the beginning of chemical communication. First, the plant produces isoflavones (1), then *Rhizobium* responds by producing nod-factors (2). (B) *Rhizobium* goes toward the root and sticks to a root hair, which then changes its shape to hug the bacteria so that the bacteria can enter into the root. (C) Formation of the root nodule, which is the space where the bacteria multiply and sugar and nitrogen are traded.

<figure><img src="image_139.png" /><figcaption>Figure 2</figcaption></figure>

**BIOTIC FACTORS**

The living components of an environment, like plants, animals, and microbes.

**ABIOTIC FACTORS**

The non-living components of an environment, like rocks, sunlight, and water.

plant "talking" and go toward it. On their journey toward the plant, the *Rhizobium* start to produce other molecules, known as nod-factors, which tell the plant to create a space in the roots, called root nodule, where the *Rhizobium* can live. In exchange for a place to live and food (sugar) from the plant, the *Rhizobium* gives the plant a lot of NH₃ (Figure 2). As a result, plants that allow *Rhizobium* to live with them grow much more. For both the plant and the *Rhizobium*, living together is an advantage, because both organisms have more food to eat than when they live alone, and thus they can grow better [2].

**Microbes Can Help Protect Plants From Pathogens and Pests**

**Biotic factors** are the living parts of an environment, such as plants, animals, and microbes (Figure 3). Biotic stress is stress felt by an organism, like a plant, that is damaged by biotic factors like bad microbes, called pathogens, or by bad insects, called pests. Good microbes can help plants to fight against pathogens and pests in two ways. First, the microbes can push the pathogen/pest away or kill it. For example, some good microbes produce VOCs that can stop a pathogen's growth or a pest's attack on the plant. The second way good microbes can help plants to fight pathogens is by telling the plant to prepare itself to fight by increasing its defenses. Probably also your mum, during the winter, tells you to eat oranges because they contain an important molecule, the vitamin C, which is able to increase your immune defenses protecting you from illnesses. The mechanism is the same for microbes and plants! For example, the molecules produced by the bacterium *Pseudomonas fluorescens* allow the plants to better resist the pathogen attack [3].

**Microbes Can Help Plants Survive in Difficult Areas**

**Abiotic factors** are the non-living parts of an environment, which include sunlight, temperature, and water (Figure 3). Abiotic stress is the negative effect of such factors on a living organism. Examples of abiotic stresses that weaken plants are low water availability and high levels of salt. Good microbes can help plants to live in difficult areas with bad living conditions. For example, the bacterium *Pseudomonas*
<caption>**Figure 3**</caption>

Important biotic factors and abiotic factors that interact in the environment. They have an impact on plants growth and they can influence the chemical communication between plants and microbes in the soil. Mushroom icon is from Flaticon.com.

<figure><img src="image_140.png" /><figcaption>Figure 3</figcaption></figure>

*chlororaphis O6* allows some plants to survive when there is not enough water [4]. Other microbes, like the bacterium *Bacillus subtilis*, help plants to survive in soil where there is a high concentration of salt, by reducing the amount of salt that enters the plant's roots [5].

## WHY IS IT IMPORTANT TO UNDERSTAND CHEMICAL COMMUNICATION?

Chemical communication between microbes and other organisms started evolving millions of years ago. Four hundred and fifty million years ago, plants moved from the sea and started to live on the land. Scientists believe that soil fungi first helped plants to make this move to land. The fungi helped the plants to use important nutrients so that the plants could survive [6]. So, for millions of years, chemical communication among soil organisms has been important for plant well-being and growth. In this article, we explained how plant-microbe interactions can give a plant more food or help plants to fight against pathogens or to live in difficult areas. Unfortunately, these interactions are in danger! The high use of antibiotics, pesticides, and fertilizers in agriculture can change the soil and in doing so can also change the soil community, causing some soil organisms to die and others to grow. Changes in the microbial community can have terrible effects on plants; for example, the microbes that grow might be pathogens!

The human population is still growing, and since people need food to survive, new methods of increasing plant growth and food production must be found [2]. Microbes are increasingly used as natural helpers to improve plant growth and food production. The study of microbe-plant communication is necessary to understand which microbes can be used to help plants grow. We must pay close attention to choose the correct microbes! For example, you could
think that the use of the fungus *Fusarium culmorum* would be a good idea, because it helps certain plants to grow in difficult areas that are rich in salt. However, *Fusarium culmorum* is a bad organism for maize—it is a pathogen! For these reasons, researchers want to understand as much as possible about chemical communication, to increase our knowledge about soil ecosystems, to understand how organisms interact in these ecosystems, and to help us use plant-microbe interactions to increase plant growth for food while protecting soil ecosystems.

## REFERENCES

1. van Dam, N. M., Weinhold, A., and Garbeva, P. 2016. Calling in the dark: the role of volatiles for communication in the rhizosphere. *ISME J.* 12:1252–62. doi: 10.1007/978-3-319-33498-1_8

2. Tomer, S., Suyal, D. C., and Goel, R. 2016. "Biofertilizers: a timely approach for sustainable agriculture," in *Plant-Microbe Interaction: An Approach to Sustainable Agriculture*, eds D. Choudhary, A. Varma, and N. Tuteja (Singapore: Springer). p. 375–95. doi: 10.1007/978-981-10-2854-0_17

3. Van Wees, S. C. M., Van der Ent, S., and Pieterse, C. M. J. 2008. Plant immune responses triggered by beneficial microbes. *Curr. Opin. Plant Biol.* 11:443–8. doi: 10.1016/j.pbi.2008.05.005

4. Garbeva, P., and Weisskopf, L. 2020. Airborne medicine: bacterial volatiles and their influence on plant health. *New Phytol.* 226:32–43. doi: 10.1111/nph.16282

5. Ortíz-Castro, R., Contreras-Cornejo, H. A., Macías-Rodríguez, L., and López-Bucio, J. 2009. The role of microbial signals in plant growth and development. *Plant Signal. Behav.* 4:701–12. doi: 10.4161/psb.4.8.9047

6. Field, K. J., Pressel, S., Duckett, J. G., Rimington, W. R., and Bidartondo, M. I. 2015. Symbiotic options for the conquest of land. *Trends Ecol. Evol.* 30:477–86. doi: 10.1016/j.tree.2015.05.007

**SUBMITTED**: 31 March 2020; **ACCEPTED**: 01 September 2020;

**PUBLISHED ONLINE**: 20 October 2020.

**EDITED BY**: Rémy Beugnon, German Centre for Integrative Biodiversity Research (iDiv), Germany

**CITATION**: Ariotti C, Giuliano E, Garbeva P and Vigani G (2020) The Fascinating World of Belowground Communication. Front. Young Minds 8:547590. doi: 10.3389/frym.2020.547590

**CONFLICT OF INTEREST**: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

**COPYRIGHT** © 2020 Ariotti, Giuliano, Garbeva and Vigani. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original
publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

## YOUNG REVIEWER

### SHASHIPREETHAM, AGE: 13

Hello, my name is Shashi, I am 13 years old and I go to Penglais School. I enjoy playing football and basketball. My favorite subjects are Maths and computers. I am currently studying year 8. I am a four times Guinness World Records holder in a game called Rocket League and my name is in 2018 Guinness World Record Gamers Edition.

<figure><img src="image_141.png" /></figure>

## AUTHORS

### CRISTIANA ARIOTTI

I recently graduated from the University of Turin with a degree in Environmental Biology. I am now a Ph.D. student at the University of Turin where I study the communication between plants and soil microbes growing in iron-deficient conditions. In my free time, I love climbing mountains (I live near the Alps!) and singing in choirs.

<figure><img src="image_142.png" /></figure>

### ELENA GIULIANO

I recently graduated in Environmental Biology from the University of Turin. I would like to apply for a Ph.D. in plant science. I am interested in plant-microbe interactions and plant protection against biotic and abiotic stress. I like sharing knowledge and ideas with people from different cultures and I love reading and taking photos in my spare time.

<figure><img src="image_143.png" /></figure>

### PAOLINA GARBEVA

I am group leader in the Department of Microbial ecology at NIOO in Wageningen. The focus of my current research is to understand the fundamental mechanisms of microbial chemical interactions and communication.

<figure><img src="image_144.png" /></figure>

### GIANPIERO VIGANI

I am researcher at University of Turin (Italy). The focus of my research is to understand how plants take up nutrients and water from the soil and how plant-microbe interactions belowground occur. *gianpiero.vigani@unito.it

<figure><img src="image_145.png" /></figure>
<figure><img src="image_146.png" /></figure>

# HOW DO PLANTS DEFEND THEMSELVES FROM ROOT-EATING CREATURES?

Axel J. Touw<fnref n="1" />,<fnref n="2" />* and Nicole M. van Dam<fnref n="1" />,<fnref n="2" />

## YOUNG REVIEWERS:

<figure><img src="image_147.png" /></figure>

AVANI

AGE: 10

<figure><img src="image_148.png" /></figure>

CATHERINE

AGE: 15

<figure><img src="image_149.png" /></figure>

HARRISON

AGE: 11

The belowground world is full of creatures that depend on plants as a food source. Belowground plant eaters, or herbivores, feed on roots and can cause considerable damage to plants. Roots are very important because they help plants take up water and nutrients from the soil. These are important resources that plants need for growth. To protect their roots, plants produce chemical defenses. The production of these defenses is costly because nutrients and energy used to make defenses cannot be used for growth or the production of flowers and seeds. Plants, therefore, must be efficient with their defenses. Scientists are very interested in understanding how plants defend themselves efficiently, because this can help us to develop more environmentally friendly ways of growing fruits and vegetables. In this article, we explain how plants defend themselves efficiently, and how plant defenses affect herbivores in the soil.

<footnote n="1">¹Molecular Interaction Ecology, German Center for Integrative Biodiversity Research (iDiv) Halle-Jena-Leipzig, Leipzig, Germany</footnote>

<footnote n="2">²Institute of Biodiversity, Friedrich Schiller University Jena, Jena, Germany</footnote>
### Figure 1

Examples of aboveground and belowground herbivores. (A) Cabbage aphids, (B) the caterpillar of the beet armyworm, (C) a plant-eating nematode, and (D) the larvae of the cabbage root fly. (Photo credits: Axel Touw).

<figure><img src="image_150.png" /><figcaption>Figure 1</figcaption></figure>

### HERBIVORE

An animal that
eats plants.

### NEMATODES

Tiny, wormlike animals that mostly occur in the soil, but also in seas or lakes, in the intestines of animals, and even in the guts of insects.

### PLANT DEFENSES

Features of a plant that affect the behavior, growth, or survival of herbivores.

## PLANTS: SURVIVING IN A DANGEROUS WORLD

Plants are an important food source for many creatures, including humans. Plant eaters, or **herbivores**, can be large mammals like cows, sheep, or horses, but most of them are actually much tinier, such as caterpillars or aphids (Figure 1). Because these tiny herbivores usually occur in large numbers, they can cause a lot of damage. Insects, for example, are the most diverse group of animals on Earth. There are around 1 million known species of insects, of which about half are herbivores. In comparison, there are only around 5,500 mammal species living on Earth. In addition to the large and small herbivores aboveground, there are many plant feeders living belowground as well. The soil is filled with many kinds of herbivores that feed on plant roots, including insect larvae, tiny worms called **nematodes**, and spider-like creatures called mites (Figure 1).

There are usually high numbers of herbivores present in the soil, just like there are aboveground. For example, there are 30,000 known nematodes species, of which around 10% are plant eaters. One single female nematode produces up to 200 eggs. This means that a single plant can be attacked by thousands of nematodes at the same time. Chewing insects are another danger. They may chew through the water transport system of plant roots, which can cause the leaves to droop and the plant to die from lack of water.

## HOW DO PLANTS DEFEND THEMSELVES?

With so many creatures trying to attack them, you can imagine that plants have a hard time surviving. Since plants cannot flee from their attackers, they had to evolve ways of defending themselves. Plants have developed several defenses against herbivores [1]. Some **plant defenses** are easy to see, like the thorns of a rose, the hairs on the leaf of a stinging nettle, or the thick skin of beetroots. Other defenses,
## GLUCOSINOLATES

Defense substances responsible for the sharp bitter taste of mustard and wasabi. Although most humans enjoy their taste, they are toxic to most insects, nematodes and bacteria.

such as chemical defenses, are less visible. Each plant produces thousands of different chemicals, all involved in essential processes. Some chemicals, like sugars, provide energy to the plant. Other groups of chemicals help to defend plants against attackers. These chemical defenses can make the plant taste bad, which prevents herbivores from eating plant tissues. In some cases, the chemicals can even be toxic. Chemical defenses can affect humans too. There are many plants that would make you feel very sick if you ate them, for example the berries of black nightshade. Some plants, such as poison ivy or hogweed, can give you a rash and even cause burns when you touch them. Most chemical defenses are not that bad, though. In fact, chances are high that you have been exposed to plant chemical defenses yourself.

We have grown to like the taste of some chemicals that plants produce. Have you ever put mustard on your hotdog or sausage, or enjoyed a nice Indian curry with mustard seeds? The sharp-bitter taste of mustard is caused by defense chemicals called **glucosinolates**. In the wild, glucosinolates help plants to defend themselves against insects, fungi, and bacteria. The caffeine in coffee, which helps people to wake up in the morning, is not made by coffee trees to please humans. In reality, coffee trees produce caffeine to protect their seeds—the coffee beans—from insect attacks. Caffeine not only gives coffee beans their bitter taste, but it can also paralyze or kill insects trying to feed on them.

These examples illustrate that chemical defenses are an effective way for plants to protect themselves against herbivores in their environments. Nevertheless, most plants are not completely defended by these chemicals. If you take a good look at the plants around you, you will notice that most plants show some damage, such as holes in their leaves. This is because the production of chemical defenses comes at a cost. Plants do not only have to worry about defending themselves, but they must also put energy into growth, producing flowers, and making seeds. So, the energy plants can spend on producing defenses is limited. Plants must make use of this limited amount of energy in an efficient way.

## HOW DO PLANTS DEFEND THEMSELVES EFFICIENTLY?

Fossils of herbivore-damaged leaves show that plants and herbivores have been living together on Earth for more than 400 million years. During this time, plants have developed several ways to produce defenses in a cost-efficient manner. One way is to produce defenses only when necessary, for example, when insects start eating them [2]. By only producing defenses when under attack, plants save energy when no dangers are present. The disadvantage of this strategy is that defense production will only begin after the herbivore starts eating. Because defense production takes time, the plant can suffer significant damage before the herbivore leaves or dies.
## TAPROOT

Main root from which lateral roots arise. Collects water and nutrients from the rest of the root system and distributes them aboveground. In carrots and beetroots, it stores starch and nutrients.

Another strategy is to always have some defenses at hand but in limited amounts. In this case, the plant moves most defenses to the plant parts that are most important for survival and are vulnerable to attack by herbivores [3]. This would be like defending a castle by putting the soldiers on the outer wall, where the first attack would occur and where the castle is most vulnerable. Clearly, the treasure in the castle would be well guarded too, as this is the most valuable. Aboveground, such valuable plant parts include young leaves, flowers, and seeds, which play essential roles in energy production or in producing the next generation.

Belowground, various parts of the root system also have different values. The root systems of plants like tomato or cabbage consist of three parts: the **taproot**, lateral roots, and fine roots (Figure 2). Lateral and fine roots help the plant take up valuable nutrients and water from the soil. The taproot is the main root that collects all the water and nutrients absorbed by the lateral and fine roots and distributes them to the aboveground parts. Simultaneously, sugars and other substances produced in the leaves move through the taproot in the other direction. The important role of the taproot in transport of nutrients and water makes it an essential part of the root system. When herbivores damage the taproot, the essential transport routes are broken, and the plant will die. In plants like beets, the taproot stores energy in the form of sugar. This is like the treasure in the castle. The taproot is therefore considered the most valuable root part and is defended the most, followed by the lateral and fine roots [4] (Figure 2).

## HOW DO PLANT DEFENSES AFFECT SOIL HERBIVORES?

Herbivores decide which root part to eat based on both its nutritional value and on how well it is defended [5]. Most herbivores would prefer to feed on the taproot since it is the most nutritious part of the root system. However, as mentioned earlier, the taproot is also the best-defended part. Not all herbivores can overcome these chemical defenses. Some herbivores, like the larvae of the cabbage root fly, can deactivate chemical defenses and feed on the taproot [4]. Other herbivores, like the larvae of the European June beetle, cannot deal with the high defense levels in the taproot and instead eat the lateral and fine roots (Figure 2). The distribution of chemical defenses across the root system and the ability of herbivores to overcome these defenses can therefore have a strong influence on where herbivores can be found in the soil.

## HOW CAN SCIENTISTS USE THIS KNOWLEDGE?

The knowledge gained by studying the defense systems of plants helps us to understand how plants interact with herbivores and other
## Figure 2

The distribution of chemical defenses over a root system, and how this affects belowground herbivores. Red indicates the highest defense level in the root system and yellow indicates the lowest level. Chemical defenses are generally highest in the taproot (red), followed by the lateral roots (orange) and the fine roots (yellow). Some insect herbivores, like the cabbage root fly, can deactivate a plant's chemical defenses and can eat the taproot where defenses are highest. Other herbivores, like the European June beetle, cannot deactivate plant defenses and therefore they eat the fine roots, where chemical defense levels are lower. (Image credit: Jennifer Gabriel).

<figure><img src="image_151.png" /><figcaption>Figure 2</figcaption></figure>

## PLANT BREEDING

The science of creating new plant varieties with desirable characteristics like taste, smell, color, or resistance to herbivores or certain environmental conditions like drought.

animals in their environments. In addition, knowing how plants defend themselves can help us to develop more environmentally friendly ways of growing crops. **Plant breeding** can create new crop varieties, such as plants with nicer colors, more interesting tastes, or bigger fruits. Similarly, plant breeders can create crops that are better defended against attackers. To do so, plant breeders must understand how plants produce defenses, and which attackers those defenses are effective against. Scientists who study plant defenses using lab experiments and field studies collect this kind of information. By creating crops with better defenses, we can help farmers to reduce the amounts of chemical pesticides they use. This is good news for both human health and the health of our environment.
ACKNOWLEDGMENTS

We thank Jennifer Gabriel (iDiv, Leipzig) for her great help with the
figures. We are grateful for the support from the editors of the Soil
Biodiversity collection during the writing process and for the kind
remarks of the young reviewers and their mentors whose advice
was essential for the completion of this paper. Finally, we would like
to thank the German Research Foundation (DFG) for funding our
research on cabbage root flies at iDiv (DFG-FZT 118, 202548816), in
collaboration with ChemBioSys (SFB 1127, 239748522).

REFERENCES

1. Ehrlich, P. R., and Raven, P. H. 1964. Butterflies and plants-a study in coevolution. *Evolution*. 18:586-608.

2. Karban, R. 2020. The ecology and evolution of induced responses to herbivory and how plants perceive risk. *Ecol. Entomol*. 45:1-9. doi: 10.1111/een.12771

3. Meldau, S., Erb, M., and Baldwin, I. T. 2012. Defence on demand: mechanisms behind optimal defence patterns. *Ann. Bot*. 110:1503-14. doi: 10.1093/aob/mcs212

4. Tsunoda, T., Grosser, K., and van Dam, N. M. 2018. Locally and systemically induced glucosinolates follow optimal defence allocation theory upon root herbivory. *Funct. Ecol.* 32:2127-37. doi: 10.1111/1365-2435.13147

5. Tsunoda, T., and van Dam, N. M. 2017. Root chemical traits and their roles in belowground biotic interactions. *Pedobiologia* (Jena). 65:58-67. doi: 10.1016/j.pedobi.2017.05.007

SUBMITTED: 29 January 2021; ACCEPTED: 28 March 2022;

PUBLISHED ONLINE: 25 April 2022.

EDITOR: Helen Phillips, Saint Mary's University, Canada

SCIENCE MENTORS: Ryan Thomas Weir and Ruchira Sharma

**CITATION:** Touw AJ and van Dam NM (2022) How Do Plants Defend Themselves From Root-Eating Creatures? *Front. Young Minds* 10:660701. doi: 10.3389/frym.2022.660701

**CONFLICT OF INTEREST:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

**COPYRIGHT** © 2022 Touw and van Dam. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
## YOUNG REVIEWERS

### AVANI, AGE: 10

Hello, I am Avani. I enjoy running and swimming. I am also a dancer. I love going on walks with my dog, or collecting stones. I love math, science and I love sports. I enjoy playing video games, and calling friends. I love nature and cold windy weather.

<figure><img src="image_152.png" /></figure>

### CATHERINE, AGE: 15

I love music and singing, I play the violin and guitar and I also enjoy writing! I am part of a highland dance troup and volunteer with children at local kids clubs and guides. I enjoy attending youth events at my church and doing fitness. I hope that by reviewing these articles I could learn about new and interesting stuff!

<figure><img src="image_153.png" /></figure>

### HARRISON, AGE: 11

I love playing sports such as hockey and go running and chasing my dog! I also love discovering new things, but not new food! Because I currently go to primary school I am excited to start my new secondary school and try lots of new subjects. My favorite subject at the minute is maths.

<figure><img src="image_154.png" /></figure>

## AUTHORS

### AXEL J. TOUW

Axel has been fascinated by nature since he was young. At that time, he was mostly interested in dinosaurs, cats, dogs, lizards, and frogs, but particularly in birds. Actually, the first thing he ever drew was an owl (with some imagination). While studying biology, he became interested in how plants communicate, especially with insects. Today, Axel studies how plants defend themselves against microbes, nematodes, and insects. In his free time, he likes to be outside, play football, read, and cook. He also tries to use the knowledge gained during his research in his garden, with varying degrees of success. *axel.touw@idiv.de

<figure><img src="image_155.png" /></figure>

### NICOLE M. VAN DAM

Nicole was born and grew up in the Netherlands with her parents and two younger sisters. As a kid, she liked to experiment with insects. For example, she tested whether ants can swim by putting them in puddles (in case you are wondering, they do quite well). She studied biology in Wageningen, the Netherlands. There she became interested in how plants can defend themselves and how farmers can use this knowledge to reduce pesticide use. After doing a lot more experiments with insects and plants in various places in the world, she became a professor. In her free time, she likes to do yoga and to watch movies with her two sons (19 and 21). Together with her husband, she likes to grow organic fruits and vegetables in her garden. There she also finds inspiration for new research projects.

<figure><img src="image_156.png" /></figure>
<figure><img src="image_157.png" /></figure>

# BACTERIA IN SOIL KEEP YOUR HAMBURGER "HEALTHY"

Stephanie D. Jurburg*

German Centre for Integrative Biodiversity Research (iDiv), Leipzig, Germany

## YOUNG REVIEWERS:

<figure><img src="image_158.png" /><figcaption>MADDIE</figcaption></figure>

AGE: 15

<figure><img src="image_159.png" /><figcaption>MATÍAS</figcaption></figure>

AGE: 14

<figure><img src="image_160.png" /><figcaption>TACY</figcaption></figure>

AGE: 13

In 1993, an outbreak of the bacterium *Escherichia coli* made over 700 people ill across the United States. A special kind of *E. coli*, called strain O157:H7, inhabits the guts of cattle and spreads to water and compost through cow manure. The strain O157:H7 can survive for many months in the water or compost, until it reaches humans through meat or vegetables, causing disease. However, *E. coli* survives a much shorter time in the soil because it must compete with the many kinds of bacteria already present there. To soil bacteria, strain O157:H7 is an invader, and invaders depend on the "leftovers" of the native organisms to survive. In more diverse communities, fewer resources are left behind, and it is harder for organisms to invade. This is why *E. coli* O157:H7 is least successful in soil, the most diverse environment on Earth, and it is one of many reasons why soil bacteria are important for our health.
## ESCHERICHIA COLI ATTACKS!

In 1993, an outbreak of the bacterium *Escherichia coli* caused over 700 people to become seriously ill across the United States. The culprit was uncooked beef patties. Thirteen years later, another *E. coli* outbreak caused massive recalls of prepackaged spinach across the country. This time, the source was a cattle ranch next to a spinach farm. Since then, *E. coli* outbreaks have started from the consumption of cheese, onions, soy, and most recently, romaine lettuce. These outbreaks were always caused by the same bacterium: *E. coli* strain O157:H7. Who is this bacterium and why is it still causing disease outbreaks? Keep reading to learn more!

### STRAIN

A subtype of a bacterial species that has slight differences in its genes from other strains in the same species.

## THE HISTORY AND TRAITS OF E. COLI

Most of the bacteria we know about today were discovered in the last 10 years. However, *E. coli* is the exception. This bacterium was discovered in the colon of healthy humans in 1885, by pediatrician Theodor Escherich, after whom the bacterium is named. *Coli* refers to their habitat, the colon. Because it grows so well in the laboratory, microbiologists have continued to study *E. coli* to understand how bacteria grow and respond to their surroundings.

Many of the characteristics that make *E. coli* so attractive as a study organism also make this bacterium stand out. First, *E. coli* grows very well if it is given the right food sources—and there are many food sources that are "right" for this bacterium. With enough food, *E. coli* can grow very quickly: from a single cell to a million cells within 7 hours! Second, bacteria can change their **genes**, and *E. coli* is especially good at this (Figure 1). Genes are the cell's instruction manual, and unlike larger organisms like animals, bacteria can trade genes with each other, receive genes from a virus, or pick up genes from the environment. When a bacterium changes its genes, its behaviors and abilities change too, and its descendants, which inherit the same new genes, become members of a specific strain—much like you are a member of your family and share many characteristics with them. Instead of family names, strains are usually identified by a code name of letters and numbers, like O157:H7.

### GENE

A DNA segment containing instructions for building a protein.

## THE SUPER STRAIN O157:H7

Thousands of *E. coli* cells from different strains live in the healthy human gut and protect us from other pathogenic, or disease-causing, bacteria like *Salmonella*. But that is not what strain O157:H7 does. First discovered in 1983, *E. coli* strain O157:H7 now infects ~73,000 people a year in the United States alone [1]. What makes this strain unique is the set of genes it has acquired: one set of genes gives strain O157:H7 the ability to produce Shiga toxin, which is the poisonous substance
**Figure 1**

Bacteria, unlike most
other organisms, can
change their genes. (A)
*Escherichia coli* strain
O157:H7 has genes
that were left behind by
a virus that infected
it. (B) O157:H7 has also
taken up genes from
the environment.
Bacteria can also get
genes from other
bacteria.

<figure><img src="image_161.png" /><figcaption>Figure 1</figcaption></figure>

PATHOGEN

An organism that can
cause disease.

that makes infected people sick. Producing poison is not quite enough
to become a threat to our health. Strain O157:H7 also behaves in a way
that makes it a **pathogen**: it actively tries to spread. Not all *E. coli* strains
do this. This spreading behavior is also a result of genes picked up from
the environment (Figure 1).

Since 1993, there have been almost yearly outbreaks of disease caused
by O157:H7. Why do they still happen? The simple answer is that there
are too many ways that *E. coli* can reach your food. If the beef used
to make your hamburger came from an infected cow, your burger is
probably contaminated, but this is not a problem, because hamburgers
are never eaten raw. When you cook your hamburger, the *E. coli* in it
is killed by the heat, and the meat is safe to eat. But vegetables like
lettuce are often eaten raw, and in that case, the bacteria may be alive
when you eat them.

*E. coli* normally lives in the guts of cattle for weeks to months, where it does not cause disease (Figure 2). The feces of these animals contain many *E. coli* cells: a single gram of feces from an infected cow can contain over 50 million *E. coli* cells, and it is extremely difficult to get
<caption>Figure 2</caption>

Many pathways to a hamburger. Beef from infected cows ends up in in raw hamburger patties, and *Escherichia coli* is shed from the cow gut onto the soil as manure, where the native bacteria compete with it. If *E. coli* persists and can grow in the soil, then it may contaminate crops like lettuce, spinach, and onions.

<figure><img src="image_162.png" /><figcaption>Figure 2</figcaption></figure>

rid of them once they leave the cow as manure. Manure can host these bacteria for over 21 months, giving the bacteria plenty of opportunity to reach the soil. If the manure reaches the water, then the bacteria may survive there for more than 8 months, and during this time, they may also get into the soil, when the water is used to irrigate crops. Once in the soil, *E. coli* may contact crop plants, result in contaminated fruits and vegetables.

## SOIL BACTERIA, A SOLUTION

Given how long these bacteria can live and how easily they spread, why is *E. coli* relatively rare? The answer: soil bacteria. To contaminate our vegetables, *E. coli* must survive in the soil, but its ability to do so is limited. In soil, *E. coli* can only survive for 3 months. Scientists think that the **diversity** of soil bacteria, or the number of different kinds of bacteria naturally living there, make all the difference [2]. No environment is more diverse than soil. A single handful of soil can contain 10,000 different bacteria [3], and very often, many of them are non-infectious strains of *E. coli*. In several experiments, scientists have shown that the more diversity a soil has, the harder it is for dangerous bacteria to successfully invade it. The reason for this seems to be the availability of resources that the invading
bacteria need to survive [4]. Resource consumption for bacteria is similar to food preferences in animals, and different strains of bacteria consume different resources. When an environment like the soil has a high diversity of natural bacteria, the bacterial community consumes a wide range of resources, leaving nothing behind. When *E. coli* lands on this environment, it is not able to feed or grow, and so it dies. Less diverse groups of bacteria cannot consume all the resources, leaving "leftovers," which *E. coli* can use to grow and spread.

## HOW ANTIBIOTICS HELP E. COLI

So, now you know that the diversity of bacteria in soil is important because it ensures that there are few resources left for disease-causing bacteria to use. Unfortunately, soil bacteria are constantly threatened by antibiotics. Humans use antibiotics to combat diseases in animals and in themselves, but soils are exposed to antibiotics in ways we did not initially expect. Consider how soil can become contaminated with the pathogenic *E. coli* strain: cattle feces. To prevent disease outbreaks, cows often receive large amounts of antibiotics. These are not fully used up by their bodies and can be released into the environment through the cow's urine and feces. But that is just the beginning. Sewage contains lots of antibiotics released in the same way by humans and by the water from fish farms, which is also high in antibiotics. These waters mix into rivers, which are then used to irrigate soils for agriculture. Once the antibiotics reach the soil, they kill most of the good soil bacteria, leaving behind resources that pathogenic bacteria can use to grow and multiply, potentially spreading disease. A growing group of countries around the world have created laws to limit the use of antibiotics on animals like cattle, as a way to reduce the spread of antibiotics in the environment and to improve the health of their citizens but also of wildlife.

## BACTERIA: THE TINY ROAD AHEAD

Humans have known that bacteria cause diseases since the 1800s, but we are still learning the ways in which bacteria can prevent diseases. In fact, we have only started to see the full extent of bacterial diversity over the past 20 years. We used to think that a clean environment was a sterile one, without any bacteria. As we gather more and more information about the bacterial world, our definition of what is "clean" is changing, from an emphasis on the lack of bacteria to a focus on having the "right" bacteria who can prevent the pathogenic bacteria from successfully invading. We now know that bacteria are everywhere, and it is not possible to live without them. Our research is shifting from trying to keep our world free of bacteria, to learning
how to select the right ones: those that help our environments and our bodies to stay healthy. There are still many unanswered questions. For example, who are the "right" bacteria, and what makes them so special?

## REFERENCES

1. Lim, J. Y., Yoon, J. W., and Hovde, C. J. 2013. A brief overview of *Escherichia coli* O157:H7 and its plasmid O157. *J. Microbiol. Biotechnol.* 20:5-14. doi: 10.4014/jmb.0908.08007

2. Thakur, M. P., Putten, W. H., Cobben, M. M. P., Kleunen, M., and Geisen, S. 2019. Microbial invasions in terrestrial ecosystems. *Nat. Rev. Microbiol.* 17:621-31. doi: 10.1038/s41579-019-0236-z

3. Le Roux, X., Recous, S., and Attard, E. 2011. "Soil microbial diversity in grasslands," in *Grassland Productivity and Ecosystem Services*, eds G. Lemaire, J. Hodgson, and A. Chabbi (CAB International). p. 158-65.

4. Eisenhauer, N., Schulz, W., Scheu, S., and Jousset, A. 2013. Niche dimensionality links biodiversity and invasibility of microbial communities. *Funct. Ecol.* 27:282-8. doi: 10.1111/j.1365-2435.2012.02060.x

**SUBMITTED**: 26 March 2020; **ACCEPTED**: 26 October 2020;

**PUBLISHED ONLINE**: 13 November 2020.

**EDITED BY**: Malte Jochum, German Centre for Integrative Biodiversity Research (iDiv), Germany

**CITATION**: Jurburg SD (2020) Bacteria in Soil Keep Your Hamburger "Healthy". Front. Young Minds 8:545905. doi: 10.3389/frym.2020.545905

**CONFLICT OF INTEREST**: The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

**COPYRIGHT** © 2020 Jurburg. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

## YOUNG REVIEWERS

### MADDIE, AGE: 15

My name is Maddie. I am 15 years old, and live near San Francisco.

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## AUTHOR

**STEPHANIE D. JURBURG**

Stephanie Jurburg is an enthusiast of all things tiny. For nearly a decade, she has studied how the bacteria living in and around us shape our world, making us healthy or unhealthy and keeping our soils fertile. She is particularly interested in how communities of bacteria change over time, and how they recover when they are disturbed. Currently, she is a researcher at the German Centre for Integrative Biodiversity Research (iDiv), where she studies how the bacterial communities in different environments are alike. *s.d.jurburg@gmail.com
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# DUNG BEETLES HELP KEEP ECOSYSTEMS HEALTHY

Paul Manning<fnref n="1" />*, Xin Rui Ong<fnref n="2" /> and Eleanor M. Slade<fnref n="2" />

## YOUNG REVIEWERS:

<figure><img src="image_168.png" /></figure>

FDR-HB_
PERU IGEM
TEAM

AGES: 14–17

Dung beetles are a group of insects that primarily use the dung (poop) of mammals for feeding and nesting. These beetles are important for the breakdown and recycling of dung into the soil, enabling the nutrients in the dung to cycle through the ecosystem. Dung beetles provide many benefits for the health and functioning of both natural and human-modified ecosystems, such as dispersing seeds, reducing livestock parasites, and promoting plant growth. In this article, we will explore the basic life history of dung beetles. We then dig a little deeper into the importance of dung beetles within tropical forests and agricultural ecosystems.

## DUNG BEETLE BASICS

Many readers will be familiar with dung beetles from nature documentaries. Once you have seen a beetle skillfully roll away a ball of dung (animal poop), the sight is hard to forget. Readers might be surprised to learn that dung beetles are found across most of the world (Figures 1A–D), on all continents except Antarctica! This group

<footnote n="1">¹Faculty of Agriculture, Dalhousie University, Truro, NS, Canada</footnote>

<footnote n="2">²Asian School of the Environment, Nanyang Technological University, Singapore, Singapore</footnote>
**Figure 1**

Examples of dung beetles in temperate and tropical environments. (A) The dweller *Aphodius rufipes* is a nocturnally active species found in high abundance in agricultural ecosystems. It is an important food source for bats. (B) The tunneller *Onthophagus coenobita* occurs in agricultural ecosystems, and often carries tiny mites that use dung beetles to transport them between dung pats. (C) Male *Proagoderus watanabei*, a tunneling species found in Sabah, Malaysia, feeding on dung. (D) *Paragymnopleurus maurus*, a rolling species found in Sabah, Malaysia, pushes a dung ball to safety using its back legs.

<figure><img src="image_169.png" /><figcaption>Figure 1</figcaption></figure>

of beetles is named for its habit of using the dung of mammals for food and nesting. Although, some species of dung beetles opt for decaying flesh, fungi, fruits, and even dead millipedes and ants! Like all beetles, dung beetles have two sets of wings: a flexible interior pair used for flight, and a tough exterior pair that serves as armor. All dung beetles have antennae that widen at the ends into a club, and the males of some species have impressive horns that they use when fighting for females (Figure 1C). There are more than 7,000 different species of dung beetles, and new species are discovered each year.

Dung beetles can be classified into three main groups based on their feeding and nesting habits (Figure 2). The first group are the dweller dung beetles. Dwellers arrive at a pile of dung and quickly take up residence. Inside the dung, they mate and lay eggs. Once hatched, the larvae (the immature form of the beetles) spend the entirety of their development feeding within the pile of dung that serves as a home and a food source (Figure 2A). The second group are the tunnellers. Female tunneller beetles arrive at the dung and begin digging a tunnel in the soil. They drag small pieces of dung into the tunnel, forming the pieces into lumps called brood balls. The males then compete for a female and her tunnel, which they defend until the female is mated and lays her eggs in the dung (Figure 2B). Then there are the rollers. A male arrives at the dung and sculpts it into a ball using his hind legs. If the dung ball is to the female's liking, he rolls it away and begins burying the dung in the soil. Once buried, the female lays an egg inside the ball and the larva feeds on the dung ball for the entirety of its development—safe and sound in the soil (Figure 2C). Whichever method a dung beetle uses for feeding and nesting, its activities add and mix **organic matter** into the soil. This is extremely important for other soil animals and microbes and provides a boost of nutrients to plant roots.

**ORGANIC MATTER**

Compounds that have come from the remains of dead organisms, such as plants, fungi, and animals.
**Figure 2**

Three different nesting strategies used by dung beetles. (A) Dweller beetles lay eggs directly into the dung where the larvae spend the entirety of their development. (B) Tunneller beetles dig tunnels in the soil where they form small balls of dung known as brood balls. The female lays eggs into those brood balls, where the larvae will feed. (C) Roller dung beetles sculpt a ball of dung and roll it away to safety, before burying it into the soil and laying eggs.

**BIODIVERSITY**

The variety of life on Earth.

**FRAGMENTATION**

The process of being split into smaller patches.

**RESILIENT**

Able to withstand changes in the environment.

<figure><img src="image_170.png" /><figcaption>Figure 2</figcaption></figure>

# DUNG BEETLES IN TROPICAL FOREST ECOSYSTEMS

Dung beetles form networks of interactions with the mammals whose dung they feed on. Both mammals and dung beetles have interactions with the fruiting plants whose seeds they disperse in the dung (Figure 3A). However, if some mammals become extinct, this may affect the dung beetles that feed on their dung and the distribution of the plants whose seeds they help disperse [1] (Figure 3B). We have been studying the interactions among mammals and dung beetles in the tropical forests of Brazil, Singapore, and Malaysia. These tropical forests have lost **biodiversity** because of deforestation, **fragmentation**, and hunting. We expect that as the forests become more disturbed and fragmented into smaller patches, the number of mammal species would decline, and we would also see fewer dung beetles. We predict that the complex interactions between mammals, dung beetles, and fruiting plants would be simpler in disturbed areas and isolated forest patches compared to large and healthy tropical forest.

To test our prediction, we set up traps baited with dung from different mammals in habitats ranging from forests to oil palm plantations. We looked at large areas of continuous forest and small forest patches. We then counted and identified the beetles that were attracted to each dung type in each habitat. We found that tropical forest dung beetle-mammal networks are fairly **resilient**, meaning they do not change very much in response to logging and fragmentation. We think this is because dung beetles are not very fussy about what kind of dung they eat. While many beetles have a preferred food type, few specialize only on a single dung type. So, if one mammal species is lost from an area, most dung beetles can simply switch to the dung of another mammal species for their next meal (Figure 3B). We discovered that some dung beetles even feed on python dung! However, although networks were resilient in moderately disturbed habitats, we found that in heavily disturbed sites, such as oil palm plantations and small isolated forest patches, the networks did become simplified, with
<caption>**Figure 3**</caption>

Simplified interactions among mammals, dung beetles, and seeds in (A) an undisturbed ecosystem, (B) a moderately disturbed or fragmented ecosystem, and (C) a heavily disturbed or fragmented ecosystem. The loss of species and interactions are shown in gray. Three types of interactions are possible: (1) mammal-seed-only the mammals disperse the seeds; (2) mammal-dung beetle—there is no seed dispersal; (3) mammal-dung beetle-seed—both mammals and dung beetles disperse seeds. In moderately disturbed or fragmented ecosystem (B), dung beetles can switch their feeding preferences to a different dung. This does not happen in heavily disturbed or fragmented ecosystem (C), so the beetle and its interactions are lost.

<figure><img src="image_171.png" /><figcaption>Figure 3</figcaption></figure>

fewer dung beetle species and fewer interactions between beetles and mammals [2] (Figure 3C).

Using traps baited with dung to catch beetles only tells us if the beetles are attracted to the dung, but not if those beetles actually feed on that dung type. We could only bait traps with dung from mammals that we could easily find, like animals kept in zoos. Luckily, new laboratory methods are allowing us to know exactly which mammal dung the beetles have been feeding on. So, in our current work, we are dissecting the guts of beetles to analyze the genetic material of the beetle gut contents. This method allows us to identify which mammal dung the beetles were feeding on before they were captured. We hope that this will allow us to document more complete networks, including rare or difficult-to-study mammal species, and the interactions between mammals and dung beetles that live in the forest canopy—yes, there are canopy dung beetles too!

## DUNG BEETLES IN AGRICULTURAL ECOSYSTEMS

Dung beetles are important members of agricultural **ecosystems**, and many researchers have explored how dung beetles help support the

### ECOSYSTEM

A community of animals, plants, bacteria, and fungi living in a particular location along with the non-living components of that environment.
production of food [3]. For example, flies that bite and disturb cows and other farm animals, lay their eggs in dung and the immature fly larvae then feed on dung when they hatch. Dung beetles help to keep farm animals like sheep, cows, and horses healthier by burying farm animal dung, so that it is not available for the flies to breed in.

Dung beetles also help to reduce parasite infections of farm animals. Parasitic nematodes, which are tiny worms, are eaten by animals grazing in the pastures. The nematodes then multiply in the animals and their eggs are excreted in the dung. When the eggs hatch, the larvae migrate to the grass and are ingested by grazing animals, like cows or sheep, which quickly increases the infection rates. When dung beetles tunnel through dung, they cause it to dry out. This kills the eggs, and reduces the numbers of parasitic nematodes in the pasture, which leads to fewer infected animals. This tunneling action also helps with the recycling and mixing of the soil, which aids the movement of nutrients through the soil so they can be available by plants. Because dung beetles are relatively small and often secretive, many farmers may not even recognize that dung beetles live on their farms. However, despite their small size, dung beetles save the cattle industry in the United Kingdom alone about £367 million per year [4]!

Dung beetles are sensitive to how pastures are managed. In one study, scientists collected beetles from a series of cattle farms in Ireland [5]. They compared conventional farms that used artificial fertilizers and insecticides to organically managed farms, which used neither. The researchers found that organic farms had a greater abundance of dung beetles and more dung beetle species than the other type of farms. We then discovered that having more dung beetle species increases plant growth, but has no benefit for improving the number of air pockets in the soil [6].

**Parasiticides** are also a serious threat to dung beetles living in agricultural ecosystems. These are chemicals given to farm animals to protect them against parasites, such as ticks, fleas, and nematodes. Parasites harm the animals by feeding on their blood and sometimes transmitting diseases. Parasiticides are generally excreted in animal dung, so the same chemicals that kill the parasites can also affect dung beetles when they then feed on the dung. Sadly, we discovered that a parasiticide commonly used to treat farm animals can kill dung beetles or stop them from breeding. This reduces the health and number of dung beetles and the amount of dung they bury [6]. Farmers can help protect their dung beetles by using fewer parasiticides, by treating only those animals with high numbers of parasites, or choosing treatments that are less toxic to dung beetles.

## NEMATODES

A group of worms, also known as roundworms, found in soil and aquatic environments that can be parasites in plants and animals.

## PARASITICIDES

Drugs given to animals to kill their parasites.
## THE PRECIOUS AND FASCINATING DUNG BEETLE

As you can see, dung beetles are precious. As you now know, they have many important ecological roles in natural and agricultural ecosystems, and can help us understand more about the health of ecosystems. For example, because dung beetles are linked to mammals, if we notice that some dung beetle species disappear from our forests, this suggests that the forest mammals might be disappearing too. Healthy soils and plants need nutrients, which dung beetles and other soil animals help provide. The disappearance of dung beetles and other soil animals due to chemical and environmental disturbances would lead to our soils becoming infertile, and the seeds of many plants would not be dispersed or grow. The many different behaviors that dung beetles exhibit have put them in the spotlight of numerous fascinating behavioral studies. Findings have included beetles navigating using the milky way, and others dispersing the seeds of a plant that have tricked the beetle by evolving to look and smell like antelope dung! Keep an eye open for these fascinating creatures wherever you find yourself, you never know what discoveries you might make.

## ACKNOWLEDGMENTS

We were grateful to Chien C. Lee for allowing us to use his photos of dung beetles taken in Sabah, Malaysia—his photography can be found at www.chienclee.com.

## REFERENCES

1. Raine, E. H., and Slade, E. M. 2019. Dung beetle–mammal associations: methods, research trends and future directions. *Proc. R. Soc. B* 286:20182002. doi: 10.1098/rspb.2018.2002

2. Ong, X. R., Slade, E. M., and Lim, M. L. M. 2020. Dung beetle-megafauna trophic networks in Singapore's fragmented forests. *Biotropica* 52:818–24. doi: 10.1111/btp.12840

3. Nichols, E., Spector, S., Louzada, J., Larsen, T., Amezquita, S., Favila, M., et al. 2008. Ecological functions and ecosystem services of Scarabaeinae dung beetles. *Biol. Conserv.* 141:1461–74. doi: 10.1016/j.biocon.2008.04.011

4. Beynon, S. A., Wainwright, W. A., and Christie, M. 2015. The application of an ecosystem services framework to estimate the economic value of dung beetles to the UK cattle industry. *Ecol. Entomol.* 40:124–35. doi: 10.1111/een.12240

5. Hutton, S. A., and Giller, P. S. 2003. The effects of the intensification of agriculture on northern temperate dung beetle communities. *J. Appl. Ecol.* 40:994–1007. doi: 10.1111/j.1365-2664.2003.00863.x

6. Manning, P., Slade, E. M., Beynon, S. A., and Lewis, O. T. 2017. Effect of dung beetle species richness and chemical perturbation on multiple ecosystem functions. *Ecol. Entomol.* 42:577–86. doi: 10.1111/een.12421
**SUBMITTED**: 15 July 2020; **ACCEPTED**: 25 March 2021;

**PUBLISHED ONLINE**: 22 April 2021.

**EDITED BY**: Helen Phillips, German Centre for Integrative Biodiversity Research (iDiv), Germany

**CITATION**: Manning P, Ong XR and Slade EM (2021) Dung Beetles Help Keep Ecosystems Healthy. Front. Young Minds 9:583675. doi: 10.3389/frym.2021.583675

**CONFLICT OF INTEREST**: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

**COPYRIGHT** © 2021 Manning, Ong and Slade. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

## YOUNG REVIEWERS

**FDR-HB_PERU IGEM TEAM, AGES: 14–17**

We are a synthetic biology team with the international Genetically Engineered Machine (iGEM) in Lima, Peru. We are the only high school team in Latin America and are proud of our work with creating a detector for cadmium using bacteria. Most of us are second language learners and the age range of our group is 14–17 years old. We love GMOs!

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## AUTHORS

### PAUL MANNING

Paul is a post-doctoral fellow at the Faculty of Agriculture at Dalhousie University. He holds a B.Sc. in agriculture from the Nova Scotia Agricultural College, and a D.Phil. in Zoology from the University of Oxford. He has been working on dung beetle ecology and toxicology since 2013. His research aims to understand how insect communities support ecosystem functions (like dung decomposition) in agricultural ecosystems. Paul is also interested in elevating public understanding and appreciation of insects through speaking to community groups, working with youth, and conducting research through participatory citizen science. *paul.manning@dal.ca

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### XIN RUI ONG

Xin Rui is a Ph.D. student at the Asian School of the Environment in Nanyang Technological University. She graduated from the National University of Singapore with a B.Sc. in Life Sciences, specializing in Environmental Biology. Xin Rui was first introduced to the fascinating world of dung beetles during her undergraduate years

<figure><img src="image_174.png" /></figure>
and is now studying the diversity of dung beetles and their interactions with mammal
communities in Southeast Asia.

ELEANOR M. SLADE

Eleanor is an Assistant Professor at the Asian School of the Environment. She holds a B.Sc. in Zoology from the University of Leeds, a M.Sc. in Ecology from Aberdeen University, and a D.Phil. in Zoology from the University of Oxford. Eleanor is an ecologist whose research focuses on the conservation, management, and restoration of tropical forest landscapes and agricultural systems. She is particularly interested in invertebrates and has been studying dung beetles and their importance for healthy ecosystems for 17 years. Eleanor is also interested in using science to help inform policy and best practices in the oil palm industry.

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# PLANT-EATING NEMATODES AND THE KEY TO FIGHTING THEM

Elisabeth Darling, Marisol Quintanilla-Tornel* and Henry Chung

Department of Entomology, Michigan State University, East Lansing, MI, United States

Plant roots interact with many bacteria, fungi, and microscopic organisms within the soil that can impact how well the plants grow. Some of these microscopic organisms are animals called nematodes, and they are an especially important part of the life in the soil. Nematodes can be good, bad, and neutral for plants. Some scientists called nematologists study nematodes and how to prevent the bad ones from damaging important crops, like carrots. Nematologists and other scientists partner up to help farmers manage these pests and grow healthy crops.

## LITTLE ORGANISMS CAN CAUSE MIGHTY PROBLEMS

A handful of soil contains thousands of animals that are so small we need a microscope to see them. Tiny as these animals may be, some of them have a worldwide impact on the successful growth of food crops. **Nematodes** (Figure 1) are tiny roundworms that live in the soils of our gardens, crop fields, and landscapes. While some nematodes are helpful for plants, others are enemies of the

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Figure 1

A male root-eating nematode of the *Pratylenchus* species, collected from carrot roots. For scale, 100 µm is 10 times smaller than 1 mm and is roughly the width of a human hair!

<figure><img src="image_182.png" /><figcaption>Figure 1</figcaption></figure>

NEMATODE

A group of animals (Phylum: Nematoda) that have a worm-like shape that is long and cylindrical. They can be found on all seven continents (even Antarctica!), as well as in oceans and lakes.

PARASITES

An organism that relies on another host organism to steal food and nutrients.

ENZYMES

Biological molecules that induce chemical reactions within cells.

NEMATOLOGISTS

Scientists who study nematodes.

plant world. These dangerous nematodes feed on plant roots, which ultimately damages the plant and severely impacts plant growth. While feeding, nematodes create wounds on the roots, which can leave the plant's roots vulnerable to infection by other disease-causing organisms in the soil [1]. Nematodes that damage plant roots are often called **parasites**.

PLANT-EATING NEMATODES: A THREAT TO ROOT CROPS

Some garden plants have roots that we eat, like carrots, beets, parsnips, and potatoes. Nematodes can damage these important food crops along with many others. Nematodes possess a straw-like mouth part that injects a mixture of **enzymes** into the plant, which breaks down the plant cells into a plant-cell soup. After the cells are broken down, the nematodes eat up this soup [2]. When nematodes feed on carrot roots during the early stages of the plant's life, this can cause serious damage or even the plant's death. When some plant-parasitic nematodes feed on carrots and parsnips early in the growing season, they damage the roots so much that farmers cannot sell them. If certain nematodes of the genus *Pratylenchus* feed on very young carrots, the root damage causes the carrots to develop forked roots (Figure 2). When there are many of plant-parasitic nematodes in the soil, large crop losses can result.

**Nematologists** are scientists who study nematodes. Nematologists who research plant-parasitic nematodes can work with farmers to test
<caption>Figure 2</caption>

Carrots damaged by root-eating nematodes, resulting in forking of the main root.

<figure><img src="image_183.png" /><figcaption>Figure 2</figcaption></figure>

the effectiveness of products that can kill dangerous nematodes, to find the best ways to combat these pests. Nematodes that eat root vegetables are particularly difficult to manage because sometimes farmers cannot see the symptoms of nematode infestation until the end of the season, when the roots are harvested.

HOW IS A NEMATODE INFESTATION DIAGNOSED?

Unlike insect pests, nematodes cannot be seen by the naked eye, so confirming a nematode infestation requires laboratory testing. If a farmer is concerned that he has plant-parasitic nematode damage, he can send a soil sample collected from his field to a nematology laboratory. In the laboratory, the soil is mixed with water and shaken through mesh sieves, in a process like sifting flour. The goal is to remove any large chunks of sand or other debris from the soil and capture only the nematodes. The mixture is transferred to tubes, which are put into a centrifuge—a large machine that spins extremely quickly to collect denser particles at the bottom of the tubes. The nematodes float in the water while heavier sand and mud particles stay in the bottom of the tube. Then, a sugar solution is added to the tube that causes the nematodes to float to the top of the tube. Scientists can collect the tiny animals and view them under a light microscope and identify which species of nematodes are in the farmer's field. (Jenskins) However, some species of nematodes look so much like other species that they need to be identified by analyzing their **DNA**.

DNA

The instructions
required to make up
cells that is stored in
every organism.
### NEMATICIDE

A substance or organism that is effective at killing nematodes.

### BACTERIA

A group of organisms that are prokaryotic, meaning that they lack a nucleus and only have one cell.

### FUNGI

A group of organisms that are eukaryotic (cells contain a nucleus) and feed on organic matter. This group includes mushrooms and yeasts that we eat, as well as mildews and molds.

## HOW CAN FARMERS MANAGE NEMATODES?

Nematologists and other scientists work together to tackle the issues that farmers face with nematodes. There are some chemicals, called **nematicides**, that are currently used to treat nematode infestations. These chemicals are toxic to nematodes but can also be dangerous for the person applying them, and they are also expensive. Nematologists are researching other effective ways for farmers to manage these pests. Some types of **bacteria** and **fungi** [3] present in soil have nematocidal qualities, meaning they can kill or inhibit nematodes, and some are so specialized that they only attack the bad nematodes [4]. Many current studies are trying to identify and grow these beneficial species, so they can be applied to farmers' fields to lower the populations of bad nematodes in the soil. The most desirable nematicide would only be harmful to plant-parasitic nematodes, to ensure that soil resources and other helpful microscopic organisms in the soil are protected.

## FUNGI AND BACTERIA: A POTENTIAL KEY TO FIGHTING PLANT-PARASITIC NEMATODES

Plant-parasitic soil nematodes are dangerous pests that can damage plant roots and cause farmers to lose a lot of money due to unsellable crops. Thus, it is important to find effective ways to manage plant-parasitic nematodes. Ideally, these treatments should be safe for the environment and other soil-living species and should also be easily affordable for the farmer. This is a considerable challenge for nematologists, but the future is bright! The potential key solution is finding types of soil fungi or bacteria that can control plant-parasitic nematodes to develop new, effective, and environmentally friendly products for farmers to use. In addition, using nematode DNA to identify the different species of nematodes in soil has helped to advance the field of nematology. Sometimes molecular science like DNA analysis can be expensive but, by working together, scientists can make advances in molecular techniques that will make the process cheaper for future scientists and farmers. These techniques will pave the way for teams of nematologists and their partner scientists to help find the best ways to control these harmful pests.

## REFERENCES

1. Shah, M. M., and Mahamood, M. 2017. *Nematology: Concepts, Diagnosis and Control*. BoD – Books on Demand.

2. Zunke, U. 1990. Observations on the invasion and endoparasitic behavior of the root lesion nematode *Pratylenchus penetrans*. *J. Nematol.* 22:309–20.

3. Poveda, J., Abril-Urias, P., and Escobar, C. 2020. Biological control of plant-parasitic nematodes by filamentous fungi inducers of resistance:
Trichoderma, Mycorrhizal and Endophytic Fungi. *Front. Microbiol.* 11:992.
doi: 10.3389/fmicb.2020.00992

4. Topalović, O., Elhady, A., Hallmann, J., Richert-Pöggeler, K. R., and Heuer, H. 2019. Bacteria isolated from the cuticle of plant-parasitic nematodes attached to and antagonized the root-knot nematode *Meloidogyne hapla*. *Sci. Rep.* 9:11477. doi: 10.1038/s41598-019-47942-7

**SUBMITTED**: 08 September 2020; **ACCEPTED**: 29 September 2021;

**PUBLISHED ONLINE**: 27 October 2021.

**EDITED BY**: Malte Jochum, German Centre for Integrative Biodiversity Research (iDiv), Germany

**CITATION**: Darling E, Quintanilla-Tornel M and Chung H (2021) Plant-Eating Nematodes and the Key to Fighting Them. *Front. Young Minds* 9:604175. doi: 10. 3389/frym.2021.604175

**CONFLICT OF INTEREST**: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

**COPYRIGHT** © 2021 Darling, Quintanilla-Tornel and Chung. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

## YOUNG REVIEWERS

### MARIE, AGE: 13

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### MEHA, AGE: 15

Hey, I am a sophomore in high school, and looking forward to a career in medicine. My hobbies include drawing, tennis, and just hanging out with friends! I also love to volunteer and give back to my community. I am excited to be a part of Frontiers for Young Minds, as I want my peers and other students to be able to access these great scientific accomplishments made every day.

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### NIVEDITA, AGE: 14

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### SHREEYA, AGE: 11

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## AUTHORS

<figure><img src="image_189.png" /></figure>

### ELISABETH DARLING

I am Ph.D. student at Michigan State University in the Department of Entomology. My co-advisors are Drs. Marisol Quintanilla and Henry Chung. My research project focuses on how different plant-feeding nematode species impact carrot plants. These nematodes are called root lesion nematodes, and they are really damaging to many different crops! I am also really interested in learning about how and to what extent plant feeding nematodes impact other specialty crop systems, like parsnip and hop plants. I love learning about different aspects of nematology.

<figure><img src="image_190.png" /></figure>

### MARISOL QUINTANILLA-TORNEL

Marisol Quintanilla is the Nematologist in the Entomology department in Michigan State University. Her nematology helps to evaluate management practices to reduce plant parasitic nematodes in Michigan crops. She obtained Ph.D. from Michigan State University. *marisol@msu.edu

<figure><img src="image_191.png" /></figure>

### HENRY CHUNG

Henry is a biologist working at Michigan State University. He is fascinated by how life works at the molecular level. Henry studies insects and nematodes to understand how these organisms can adapt to different environments or feed on different host plants.
<figure><img src="image_192.png" /></figure>

# THE BIZARRE ROLE OF SOIL ANIMALS IN THE DECOMPOSITION OF DEAD LEAVES

François-Xavier Joly* and Jens-Arne Subke

Biological and Environmental Sciences, University of Stirling, Stirling, United Kingdom

YOUNG REVIEWER:

<figure><img src="image_193.png" /></figure>

JUAN
DIEGO

AGE: 15

When plant leaves die, they fall and accumulate on the soil where an important process occurs: they decompose. Decomposition is essential for recycling nutrients and returning them to the soil. It is mainly done by an army of creatures called microbes, invisible to the naked eye, that slowly make the dead leaves rot. But larger creatures, like millipede and snails, also eat dead leaves. These large creatures do not digest these leaves very well and return most of the leaf matter to the soil as poo, which is further decomposed by microbes. Does this transformation of dead leaves into poo affect the recycling process? By collecting poo from many soil animals feeding on dead leaves, we found that they decomposed faster than intact dead leaves. This means that soil animals help the decomposition of dead leaves, not by digesting them, but by transforming them into poo.
## PHOTOSYNTHESIS

The process by which plants capture the energy from sunlight with their leaves to transform carbon dioxide and water into sugars.

## DECOMPOSITION

The process by which complex plant or animal matter is broken down into a simpler form, producing carbon dioxide, and nutrients.

## MICROBES

Tiny living things, invisible to the naked eye, such as bacteria or fungi. Also called "microorganisms."

## ENZYME

Proteins that can break down large and complex molecules into smaller and simpler molecules.

## WHY STUDY DEAD LEAVES?

Plants are the basis of all life on Earth. Thanks to their leaves, plants can use the energy from the sun to capture carbon dioxide from the air and transform it into sugars. This process, called **photosynthesis**, is absolutely essential to life on Earth. The sugars are used to grow more plant parts (leaves, stems, roots), which can then be eaten by many different creatures. But a plant's leaves only live for several months to a few years. When the time comes for plants to shed them, dead leaves accumulate on the soil where another important process begins: dead leaves decompose. This **decomposition** process is just as critical as photosynthesis, as it allows the carbon that the leaves are made of to return to the atmosphere as carbon dioxide, which can be used again by other plants. Through decomposition, nutrients contained in the dead leaves also return to the soil, where they can be used again by plants to form new leaves. It is only thanks to this fragile balance between photosynthesis and decomposition that the cycle of life continues.

## MICROBES DECOMPOSE DEAD LEAVES

How does decomposition happen? This crucial process is mainly carried out by one group of soil organisms: **microbes**. Microbes consist of fungi—the same organisms that form mushrooms—and bacteria. Microbes are so tiny that they are mostly invisible to the naked eye, yet they are extremely abundant in the soil. Just 1 g of soil can contain 10 billion microbes. These microbes use dead leaves as their food. They digest leaves by releasing many different **enzymes** into the soil around them. These enzymes are like scissors, cutting the large dead leaves into microscopic bits. Microbes can then digest these leaf bits to get energy and grow. Then, the microbes release the carbon back to the atmosphere as carbon dioxide. The cycle of life can continue.

But microbes cannot decompose all dead leaves at the same speed. Some plants, like pines or beeches, form leaves that are sturdy, quite thick, and poor in nutrients. These leaves are not very good at photosynthesis, but they can survive even in difficult conditions. Consequently, when dead, these dead leaves decompose slowly because they offer few nutrients and little surface for microbes to grow on. On the other hand, plants like ash trees or clover form leaves that are much thinner and have more nutrients. These leaves are less resistant to damage but are better at photosynthesis. When these thinner, more nutritious leaves die, they offer a lot more nutrients and more surface area for microbes, and therefore they decompose faster.
Figure 1

(A–C) Examples of soil animals that eat dead leaves and transform them into feces. (D) Example of dead leaves that these animals can eat. (E) Examples of feces from animals that eat dead leaves. (F) The feces of leaf-eating animals are made up of many small leaf particles.

<figure><img src="image_194.png" /><figcaption>Figure 1</figcaption></figure>

FECES (FEE-SEEZ)

Remains of eaten food that is not digested by animals. In other words, poo.

ANIMALS HELP MICROBES WITH DECOMPOSITION

Microbes are not the only soil creatures that decompose dead leaves. Many larger soil creatures, such as millipedes, earthworms, woodlice, and snails, also feed on dead leaves (Figures 1A–C). In some types of forests, and even in deserts, the majority of the dead leaves that fall every year are eaten by these animals [1–4]. However, of all the leaves that these animals eat (e.g., Figure 1D), only a small part is actually digested and used by the animals. The majority is returned to the soil as **feces**— in other words, poo (Figure 1E). This means that, in ecosystems where these animals are abundant, the main sources of organic matter for bacteria and fungi are not dead leaves, but feces. Regardless of the type of dead leaves eaten, these feces are always much smaller than dead leaves, and they are made of tens of thousands of tiny leaf pieces (Figure 1F). This transformation from large, intact dead leaves into tiny pieces provides a lot more surface area for microbes to grow on and to decompose.

The importance of converting dead leaves into feces is not well-understood. Are feces more easily decomposed by microbes than intact dead leaves are? If so, is this effect more important for thick, sturdy leaves than for the thin leaves that are already easily degraded by microbes? Do all animals that feed on dead leaves help microbes to break the leaves down? In this study, we aimed to provide answers to these questions [5].

THE FECES FACTORY: STUDYING DECOMPOSITION IN THE
LAB

To study the importance of converting dead leaves into feces, we needed to find and collect tens of thousands of fresh feces from various soil animals that eat different kinds of dead leaves. This is an impossible task in the wild, because feces are very small and
<caption>Figure 2</caption>

Feces collected from six different soil animals that were feeding on six different types of dead leaves. Feces are drawn to scale but leaves and animals are not to scale. You can see that the color of feces depends on the dead leaves that were eaten, with light color when animal ate beech leaves and darker color when they ate lime leaves. Instead, the shape of feces depends on the animal, with oval feces for millipedes, rectangular feces for woodlice, and cylinders for snails (Image credit: adapted from [5]).

<figure><img src="image_195.png" /><figcaption>Figure 2</figcaption></figure>

it is impossible to determine the species of animals that produced the feces, or the type of dead leaves that the animal ate. So, we invented a new, special type of experiment: a feces factory. We started by wandering in various forests and grasslands of the Scottish Lowlands, where we collected thousands of soil animals from six species, including three millipede species, two woodlouse species, and one snail species. We also collected dead leaves from six tree species known to decompose at different speeds: oaks, beeches, hazelnuts, maples, horse chestnuts, and limes (Figure 2). We brought all the samples back to the laboratory and, using plastic boxes, we paired each species of animals with each type of dead leaves. In total, we had 36 leaf/animal combinations. We then let the animals eat and, twice a week for 1 month, we collected their feces. These feces were very diverse: they varied in color depending on the dead leaves that were eaten, and they varied in shape depending on the type of animal (Figure 2).

After the feces collection step, we measured how fast these feces decomposed, compared to the intact dead leaves. We wanted to measure this under conditions that other researchers across the world could repeat to verify our results or to compare our findings to those using different animal or leaf species. To do so, we created small, artificial soil systems in the laboratory. We filled small jars with soil from a local field and placed a small amount of feces or dead leaves on top of this soil, separated from the soil surface by a fine mesh. The mesh allowed us to retrieve the remaining pieces of feces or dead leaves easily, but it still allowed the microbes from the soil to move through it and decompose the feces or dead leaves. We placed the jars in a dark, warm, humid room for 6 months. Every week, we added water to the jars to keep the soil moisture optimum for microbes. After 6
<caption>Figure 3</caption>

By transforming dead leaves into feces, soil animals affect the speed at which dead leaves decompose. On average, the feces of soil animals decomposed 38% faster than the intact dead leaves did. This effect depends on the type of dead leaves. When animals ate dead leaves that were easy for microbes to decompose (shown in light yellow), their feces did not decompose much faster than dead leaves. However, when they ate dead leaves that were difficult for microbes to decompose (shown in dark brown), the resulting leaf fragments in the feces decomposed much faster.

<figure><img src="image_196.png" /><figcaption>Figure 3</figcaption></figure>

months, we retrieved the remaining feces and dead leaves. We dried them, weighed them, and compared the mass of the samples to their starting masses, to tell us how much of the leaves and feces had been decomposed by the microbes during those 6 months.

## WHAT DID WE FIND?

Our experiments led to two very interesting discoveries. First, for all the animal species we studied, we found that the feces decomposed 38% faster on average than the dead leaves did (Figure 3). These results were very consistent even though snails, millipedes, and woodlice are very different species. We think that feces decomposes faster because the animals transform the large dead leaves into thousands of small pieces, which are easier for microbes to grow on and decompose. Our second important discovery was that the increased speed of decomposition was not the same for all types of dead leaves. When animals ate dead leaves that were easy for microbes to decompose, their feces did not decompose much faster than the dead leaves themselves. However, when animals ate dead leaves that were difficult for microbes to decompose, their feces decomposed much faster than the uneaten leaves did. Because of this, the big differences in the speed of decomposition between various types of dead leaves nearly disappear once the leaves are eaten by animals and transformed into feces.
## SOIL ANIMALS CONTRIBUTE TO THE CYCLE OF LIFE

In summary, by transforming dead leaves into feces, soil animals speed up the decomposition carried out by microbes. This is the reason why we do not see dead leaves piling up on top of the soil, particularly under plants with slow-decomposing leaves. More importantly, by transforming dead leaves into feces, soil animals help the carbon from the leaves return to the atmosphere as carbon dioxide, and help the nutrients return to the soil, where these critical substances can be used again by plants. Thus, soil animals help to maintain the critical balance between photosynthesis and decomposition that sustains plants—and that therefore sustains all life on Earth!

## ORIGINAL SOURCE ARTICLE

Joly, F.-X., Coq, S., Coulis, M., David, J.-F., Hättenschwiler, S., Mueller, C. W., et al. 2020. Detritivore conversion of litter into faeces accelerates organic matter turnover. *Commun. Biol.* 3:660. doi: 10.1038/s42003-020-01392-4

## REFERENCES

1. Sagi, N., Grünzweig, J. M., and Hawlena, D. 2019. Burrowing detritivores regulate nutrient cycling in a desert ecosystem. *Proc. R. Soc. B Biol. Sci.* 286:20191647. doi: 10.1098/rspb.2019.1647

2. David, J. F., and Gillon, D. 2002. Annual feeding rate of the millipede *Glomeris marginata* on holm oak (*Quercus ilex*) leaf litter under Mediterranean conditions. *Pedobiologia*. 46:42–52. doi: 10.1078/0031-4056-00112

3. Coulis, M., Hättenschwiler, S., Coq, S., and David, J. F. 2016. Leaf litter consumption by macroarthropods and burial of their faeces enhance decomposition in a mediterranean ecosystem. *Ecosystems*. 19:1104–15. doi: 10.1007/s10021-016-9990-1

4. Cárcamo, H. A., Abe, T. A., Prescott, C. E., Holl, F. B., and Chanway, C. P. 2000. Influence of millipedes on litter decomposition, N mineralization, and microbial communities in a coastal forest in British Columbia, Canada. *Can. J. For. Res.* 30:817–26. doi: 10.1139/x00-014

5. Joly, F.-X., Coq, S., Coulis, M., David, J.-F., Hättenschwiler, S., Mueller, C. W., et al. 2020. Detritivore conversion of litter into faeces accelerates organic matter turnover. *Commun. Biol.* 3:660. doi: 10.1038/s42003-020-01392-4

**SUBMITTED**: 07 December 2020; **ACCEPTED**: 14 February 2022;

**PUBLISHED ONLINE**: 11 March 2022.

**EDITOR**: Rémy Beugnon, German Centre for Integrative Biodiversity Research (iDiv), Germany

**SCIENCE MENTOR**: Maria Claudia Segovia-Salcedo
**CITATION:** Joly F-X and Subke J-A (2022) The Bizarre Role of Soil Animals in the Decomposition of Dead Leaves. Front. Young Minds 10:638736. doi: 10.3389/frym. 2022.638736

**CONFLICT OF INTEREST:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

**COPYRIGHT** © 2022 Joly and Subke. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

## YOUNG REVIEWER

<figure><img src="image_197.png" /></figure>

### JUAN DIEGO, AGE: 15

Hi, my name is Juan Diego, I just turned 15. I am from Ecuador but I grew up in the United States. I have many hobbies. My favorite pass time is playing video games because it allows me to play with my friends now that I am locked in because of the pandemic. I enjoy animals and nature. That is why as part of a school project, currently, I am working on a small stickers book about endangered species of Ecuador.

## AUTHORS

<figure><img src="image_198.png" /></figure>

### FRANÇOIS-XAVIER JOLY

François-Xavier Joly is a French soil ecologist working at the University of Vienna, in Austria. His research focuses on understanding how soil organisms contribute to the decay of dead plants, and how this process may change due to global changes, such as biodiversity loss and climate change. *joly.fx@gmail.com

<figure><img src="image_199.png" /></figure>

### JENS-ARNE SUBKE

Jens-Arne Subke is an ecosystem ecologist at the University of Stirling, and is interested in the interactions of plants, soil, and the environment. He tries to answer questions about the way carbon taken from the atmosphere by plants can be stabilized in soils, to reduce atmospheric CO₂ concentrations.
<figure><img src="image_200.png" /></figure>

# DECOMPOSITION IN PEATLANDS: WHO ARE THE PLAYERS AND WHAT AFFECTS THEM?

Carlos Barreto *† and Zoë Lindo †

Soil Biodiversity and Ecosystem Function Laboratory, Department of Biology, Biotron Experimental Climate Change Research Centre, Western University, London, ON, Canada

## YOUNG REVIEWERS:

<figure><img src="image_201.png" /></figure>

ADAM

AGE: 14

<figure><img src="image_202.png" /></figure>

ALEXANDER

AGE: 12

All soils store carbon. As plants grow, they take up carbon from the atmosphere and this carbon enters the soil when they die. This dead plant material slowly decomposes as organisms, such as bacteria, fungi, and tiny animals called mites and springtails use this carbon as a food source. Decomposition is very slow in peatlands, and as a result, much of the carbon from dead plants remains in the soil, which can help slow climate warming. Decomposition in peatlands depends on how wet the soil is, and the different types of plants and soil organisms. We discovered that, in a peatland in northern Canada, dead plant material of different plant types decomposed at different rates, and more mites and springtails aiding in decomposition were found in wetter areas. Because peatlands are important for carbon storage, understanding who the players of decomposition are is important for understanding how to slow climate warming.
## DECOMPOSITION

Break-down of dead plants and animals which is measured as loss.

## ORGANISM

An individual plant, animal, bacteria, or fungi.

## PEATLANDS

Peatlands are a type of wetlands. The term “peatland” refers to the peat soil and the wetland habitat growing on its surface.

<caption>**Figure 1**</caption>

Example of simplified soil food web: Example of a predator (A) Predatory mite; Secondary decomposers. (B) Example of oribatid mites. (C) Example of springtail; Primary decomposers. (D) Fungi. (E) Fungi with bacteria at the bottom of the picture (ellipse). (F) Earthworms. (G) Vegetation in the peatland (only dead plants undergo decomposition). Arrows represent feeding links and point in the direction of the energy flow.

## INTRODUCTION

**Decomposition** is the natural process of breaking down dead plants and animals. During decomposition, the chemical composition of dead plants and animals changes, and carbon is released into the atmosphere. Decomposition results from the activities of different types of **organisms** like fungi (Figure 1D), bacteria (Figure 1E), worms (Figure 1F), oribatid mites (Figure 1B), and springtails (Figure 1C). For example, bacteria and fungi directly breakdown dead plant material and are considered primary decomposers. Fungi and bacteria (primary decomposers) are eaten by oribatid mites and springtails (secondary decomposers). In turn, predatory mites (Figure 1A) consume secondary decomposers. Therefore, oribatid mites and springtails indirectly affect how fast decomposition happens.

**Peatlands** are important ecosystems that accumulate partially decomposed vegetation (Figure 1G), and thus store carbon contained in the decomposing plant material [1] (Figure 2A). Their main plant type is mosses (Figure 2E). Mosses are small, slow growing plants that need a lot of water to survive because they do not have real roots. They also decompose very slowly in peatlands after they die.

<figure><img src="image_203.png" /></figure>
## Figure 2

**(A)** Peatland. **(B)**

Hummock in front of a
tree behind it. (C)
Hollow with litterbags
being deployed. (D)
Litterbags (10 × 7 cm
and the holes were
1 mm). (E) Moss
(species *Sphagnum*
sp.). (F) Shrub (species
*Chamaedaphne*
*calyculata*). (G) Sedge
(species *Carex* sp.). (H)
Tullgren funnel (fauna
extractor).

<figure><img src="image_204.png" /><figcaption>Figure 2</figcaption></figure>

## CLIMATE CHANGE

Climate change is the
warming of the entire
world, mostly caused
by increasing levels of
carbon dioxide in the
atmosphere. The main
source of carbon
dioxide is
human activities.

## COMMUNITY

A group of different
species living in the
same area and that
interact with
each other.

## SPECIES

Organisms that share
the same physical and
genetic features; for
example, all humans
comprise a species, as
do all dogs, all cats are
considered a
species too.

Peatlands are very wet, and decomposition is slow compared to other ecosystems like forests or grasslands which are drier. As a result, more dead plants accumulate, which means less carbon is released to the atmosphere from peatlands than from many other ecosystem types. In other words, more carbon enters and is held within peatland soils than is released back to the atmosphere as carbon dioxide. Carbon dioxide is a greenhouse gas that traps heat in the Earth's atmosphere, and therefore, peatlands can help slow or reverse climate warming. Peatlands could help reverse **climate change** by stocking more carbon in their soil.

Different factors can influence decomposition in peatlands; for instance, how wet the soil is, the different types of dead plant material present, and the soil **community**-the types of different organisms found in soil. Because we wanted to know what influences decomposition in peatlands, we went to the Boreal forest in northern Ontario, Canada to study the mites and springtails living in a beautiful peatland. We studied mite and springtail communities living in different areas of that peatland for two reasons: first because it is still not well-known what **species** of mites and springtails are found in peatland communities, and second because we also wanted to know how much they could help decompose leaves.
## WHAT DID WE DO?

A group of researchers from Western University (London, Ontario, Canada) have been working in a peatland in northern Ontario (Canada) in collaboration with Ontario Provincial government scientists at the Ontario Forest Research Institute. We are trying to answer different questions about peatlands by studying the plants, mites, insects, mercury, carbon, and water at this site. This peatland site is mostly covered in *Sphagnum* mosses that create raised areas called hummocks (Figure 2B) and depressions called hollows (Figure 2C). Hummocks are an accumulation of mosses and other plants, and they are dry at the surface. On the other hand, hollows, as depressions on the ground, are generally very wet at the surface. For this study, we wanted to know whether mite and springtail communities (secondary decomposers), as well as decomposition rates (how fast dead plants decompose) differed between hummocks and hollows at our peatland site.

### What Did We Do in the Peatland?

One way to study decomposition is to use litterbags [2]. Litterbags are small bags made of a mesh material that can be filled with dead plants; the holes in the mesh allow tiny organisms to come and go. We filled the litterbags (Figure 2D) with leaves of three different plants: moss (Figure 2E), shrub (Figure 2F) (these are small bushes), or sedge leaves (Figure 2G) (these are grass-like plants), and we weighed them to know the initial amount of dry leaves in each litterbag.

In June 2015, we placed one litterbag of each plant type (three litterbags) on five different hummocks (dry raised areas) and in five different hollows (wet depressions). Litterbags were pinned to the soil surface and left there for a whole year so that organisms had enough time to colonize the litterbags and help decompose the leaves. After 1 year, we went back to the peatland site, collected the litterbags, and brought them back to our laboratory at Western University, in southern Ontario (Canada).

### What Did We Do in the Laboratory?

In the laboratory, we placed each litterbag onto a special piece of equipment called a Tullgren funnel (Figure 2H) that has a light bulb inside of it, and when it is on, it warms the whole litterbag so that it forces the organisms to move out of the litterbags into a small vial where we can observe them. Afterwards, we cut the litterbags open and dried the leaves in an oven, and then weighed them with a scale. We observed the organisms we collected from the litterbags using a microscope and separated them into different species based on what they looked like. We also counted how many of each species we found in each litterbag. This part of the work took two researchers about 5 days working all day long. Finally, we compared the weight of the plant leaves left at our peatland site for 1 year, to the initial weight of the
<caption>Figure 3</caption>

(A) Different levels of decomposition on different leaf types; sedges decomposed more within the 1 year experiment. (B) Hollows had more species and individuals of oribatid mites and springtails. Pictures are not to scale.

<figure><img src="image_205.png" /></figure>

leaves before they went to our peatland site. The difference in weight told us how much of the leaves had decomposed over the course of 1 year, in other words, how much the animals had eaten.

## WHAT DID WE FIND?

Leaves from the different plants decomposed at different rates. The sedge leaves, that resemble grasses (reduction of 42.5%), decomposed more than the shrub leaves (reduction of 30.5%), which in turn decomposed more than the moss (reduction of 20.3%) in the litterbags (Figure 3A). However, it did not matter whether the litterbags were on the hummock surface or in the hollow, as we found a similar amount of decomposition in both cases. This was because even though hollows are wetter than hummocks, neither were fully saturated in a way that decomposition would slow significantly in our sites, compared to other submerged parts of the peatland.

A single litterbag had between zero and 203 individual mites and between zero and 123 individual springtails. We found more individual mites and springtails from our litterbags placed in hollows (wet depressions) than on hummocks (dry raised areas) (Figure 3B). But the tiny animals did not have a preferred leaf type, meaning they were found in similar numbers in litterbags that contained sedges,
shrubs, or mosses. Certain mites called oribatid mites (Figure 1B) were the dominant group in the litterbags (53.6% of the total number of individuals) followed by springtails, representing 40%. Other groups of mites that were not numerous, a few spiders, and a few insect larvae, were also collected, but altogether were only 6.4% of the animals in the litterbags. Besides having more individuals, hollows (wet depressions) also had more species collected. In total, we found 20 species from 506 individual oribatid mites and seven species from 378 individual springtails (Figure 1C). The communities of oribatid mites were more similar to one another in hollows and had the most species present. The species of oribatid mites found in hummocks seemed to be random.

## WHY IS THIS IMPORTANT?

Few studies have been done for mites and springtails in peatlands, so the first reason we did this study was to gain information on what species of oribatid mites lived in our peatland. Also, understanding how the tiny animal communities differ provides us with an idea on how fast or slow decomposition naturally happens on leaves in peatlands.

Although in most cases, oribatid mites and springtails are considered secondary decomposers [3] because they feed on fungi and bacteria, understanding who they are, where they live, and how much they contribute to decomposition processes is important for predicting the amount of carbon released to the atmosphere from soils. Peatland soils are a special case because peatlands only occupy a small portion of the world, but their slow decomposition means peatlands can store very high amounts of carbon [1].

Increases in global temperatures due to climate change are expected to change the plant types we observe in peatlands. Specifically, warmer temperatures will allow sedges to take over where mosses were previously found [4]. In our study, we found that sedges decomposed faster than mosses, and this means that a change in the type of plants in peatlands from mosses to sedges may increase the amount of carbon being released through decomposition.

Even though we did not collect bacteria and fungi in this specific study, other studies by our research group in the same peatland found that communities of fungi [5] and bacteria [6] also varied between hummocks and hollows. Mites and springtails are understudied, so we chose to focus on them. Results for fungi and bacteria also suggest that climate change may change the carbon storage ability of peatlands. In other words, increases in temperature may speed up the overall decomposition of leaves, releasing more carbon dioxide into the atmosphere, and making climate change even more pronounced. Nonetheless, we must engage in the conservation of peatlands, as these are important ecosystems for our life in a future where more
carbon dioxide will likely be present in the atmosphere. The time for conservation actions is now!

## AUTHOR CONTRIBUTIONS

CB and ZL wrote the manuscript and CB created the figures.

## ACKNOWLEDGMENTS

We were grateful for funding from the Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant program (ZL #418241-2012) and the Ontario Ministry of Research, Innovation and Science, Early Researcher Award (ZL). We thank Dr. J. McLaughlin (Ontario Ministry of Natural Resources and Forestry) for access to the White River, ON field site and Dr. Brian Branfireun for their continued support with our research program. We thank Dr. Greg Thorn and Dr. Branfireun for some of the pictures. Special thanks to Caitlyn Lyons, who helped us with making the language appropriate for children, and to our great young reviewers and their Science mentors. We also thank Dr. Malte Jochum for the invite to be part of this great initiative.

## ORIGINAL SOURCE ARTICLE

Barreto, C., and Lindo, Z. 2018. Drivers of decomposition and the detrital invertebrate community differ across a hummock-hollow microtopology in Boreal peatlands. *Ecoscience* 25:39–48. doi: 10.1080/011956860.2017.1412282

## REFERENCES

1. Gorham, E. 1991. Northern peatlands: role in the carbon cycle and probable responses to climatic warming. *Ecol. Appl.* 1:182–95.

2. Moore, T. R., Trofymow, J. A., Prescott, C. E., and Titus, B. D. 2017. Can short-term litter-bag measurements predict long-term decomposition in northern forests? *Plant Soil* 416:419–26. doi: 10.1007/s11104-017-3228-7

3. Lehmitz, R., and Maraun, M. 2016. Small-scale spatial heterogeneity of stable isotopes signatures (δ¹⁵N, δ¹³C) in *Sphagnum* sp. transfers to all trophic levels in oribatid mites. *Soil Biol. Biochem.* 100:242–51. doi: 10.1016/j.soilbio.2016.06.005

4. Dieleman, C. M., Branfireun, B. A., McLaughlin, J. W., and Lindo, Z. 2015. Climate change drives a shift in peatland ecosystem plant community: implications for ecosystem function and stability. *Glob. Change Biol.* 21:388–95. doi: 10.1111/gcb.12643
5. Asemaninejad, A., Thorn, R. G., and Lindo, Z. 2017. Vertical distribution of fungi in hollows and hummocks of boreal peatlands. *Fungal Ecol.* 27:59–68.
doi: 10.1016/j.funeco.2017.02.002

6. Asemaninejad, A., Thorn, R. G., Branfireun, B. A., and Lindo, Z. 2019. Vertical stratification of peatland microbial communities follows a gradient of functional types across hummock-hollow microtopographies. *Ecoscience* 26:249–58.
doi: 10.1080/11956860.2019.1595932

**SUBMITTED**: 19 January 2020; **ACCEPTED**: 13 July 2020;

**PUBLISHED ONLINE**: 21 August 2020.

**EDITED BY**: Malte Jochum, German Centre for Integrative Biodiversity Research (iDiv), Germany

**CITATION**: Barreto C and Lindo Z (2020) Decomposition in Peatlands: Who Are the Players and What Affects Them? Front. Young Minds 8:107. doi: 10.3389/frym.2020.00107

**CONFLICT OF INTEREST**: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

**COPYRIGHT** © 2020 Barreto and Lindo. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

## YOUNG REVIEWERS

### ADAM, AGE: 14

Hi, my name is Adam. I live with my parents, older brother, dog, fish, and two birds. I am a big fan of Science and History. I like to draw, write, and read. My favorite sport is soccer (or football). I enjoy swimming in the ocean and playing video games.

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### ALEXANDER, AGE: 12

I am a 12 years old boy in the seventh grade and my favorite subject is mathematics. I like to play the guitar and am learning how to play drums.

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## AUTHORS

### CARLOS BARRETO

Since a very young age I realized that I liked animals, maybe too much. Science in school was always my favorite discipline, all the way through to high school. It is then that I decided that I wanted to do something that involved science and animals. I tried to be a vet; it did not work out. No regrets. So, I became an Ecologist a few years later,

<figure><img src="image_208.png" /></figure>
and since then, I have worked with little animals (mostly insects and mites) in tropical forests, iron ore and limestone caves, boreal forests, urban fields, and peatlands on three continents; South America, North America, and Europe. *cbarreto@uwo.ca †orcid.org/0000-0003-2859-021X

## ZOË LINDO

Dr. Zoë Lindo is an expert on soil biodiversity and ecosystem function. She has worked extensively in Canadian forests including the mixed-wood boreal of Alberta, the subarctic taiga of Quebec, the coastal temperate rainforest of British Columbia, and the black spruce/peatlands of Ontario. "The overall focus of my research aims to mitigate biodiversity loss in association with human-induced environmental change and maintain ecosystem functioning in Canadian forest and soil ecosystems. I describe myself as a biodiversity scientist to encompass the breadth of my research in the areas of community ecology, soil ecology, and taxonomy." †orcid.org/0000-0001-9942-7204

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# DOES PLANT BIODIVERSITY INFLUENCE NUTRIENT CYCLES?

Eva Koller-France<fnref n="1" />*, Wolfgang Wilcke<fnref n="2" /> and Yvonne Oelmann<fnref n="1" />

## YOUNG REVIEWERS:

<figure><img src="image_211.png" /></figure>

MACKENZIE

AGE: 14

<figure><img src="image_212.png" /></figure>

ROSE

AGE: 14

All living things, like humans, animals, plants, and even microbes, need to take up the same nutrient elements to live, most importantly nitrogen and phosphorus. Understanding the cycling of these elements through the ecosystem is one key to understanding why ecosystems work the way they do. One of the questions we are asking is if the diversity of organisms, like plants or insects, is related to these nutrient cycles. When plant communities are made up of many different plant species, they seem to make better use of the available soil nutrients than plant communities made up of fewer species. This may be because of something called complementarity, which means different plant species access the available nutrients in different ways, for example from different soil depths. In this article, we will describe the connections between plant biodiversity and soil nutrient cycling and discuss the implications for the functioning of the whole ecosystem.

<footnote n="1">¹Department of Geography/Geoecology, University of Tübingen, Tübingen, Germany</footnote>

<footnote n="2">²Institute for Geography and Geoecology, Karlsruhe Institute of Technology, Karlsruhe, Germany</footnote>
## WHY DO WE CARE ABOUT BIODIVERSITY EFFECTS ON NUTRIENT CYCLES?

All living things on earth need certain nutrient elements. In natural ecosystems, these nutrients, most importantly nitrogen and phosphorus, are taken up by plants from the soil. Plants may then be eaten by animals or people. The nutrients are returned to the soil through animal droppings and when plants and animals die, and then can be taken up again by new plants. Because everything repeats on and on, we call this the nutrient cycle.

In different ecosystems and under different environmental conditions, the cycling of nutrients can work faster or slower, and nutrients can be used and recycled by different parts of the system in more or less complete ways, which may cause imbalances. For example, sometimes there are more available nutrients than are needed because farmers add too much fertilizer to the soil, or because there is a warm day in winter when tiny organisms in soil recycle and release nutrients from dead material which are not needed by plants during their inactive phase. If there are excess nutrients in the soil, those nutrients may be washed out into the groundwater or into lakes and streams. From there, they are transported to bigger rivers and finally to the sea. If these water bodies receive too great an amount of nutrients, there can be rapid growth of algae, which damages freshwater ecosystems. In this case, too much of a good thing can definitely be a big problem. This is why studying the nutrient cycles of ecosystems under different conditions is not just a good way of learning how ecosystems work, but also helps us with practical considerations, such as how to protect our supply of clean water.

We know that the **biodiversity**, the richness of species, of an ecosystem plays a role in many of its functions, and we also know that biodiversity is decreasing on a global scale. For example, some species of bees and rare flowers are going extinct, and therefore many ecosystems are now less diverse than they previously were. This is one of the reasons we are interested in how the nutrient cycle responds to changes in biodiversity.

## WHAT EFFECT DOES BIODIVERSITY HAVE ON NITROGEN IN SOIL?

A connection between biodiversity and nitrogen (in the form of nitrate, one form of nitrogen that is taken up by plants) in soil has been fairly well-established in experiments studying the effects of biodiversity on ecosystems [1]. In these experiments, plant diversity is studied by creating small model ecosystems (often grasslands, where this is easiest to do) with a known number of species growing under the same environmental conditions, for example in the same field. This is done by sowing a specific mixture of seeds into a square of ground, called

BIODIVERSITY

Simply put, the number of species in an ecosystem.
the experimental plot. These small plots are regularly checked for plants that were not sown into them, which are removed. Results from experimental plots with higher or lower diversity can be compared to each other quite well, since the only difference between plots should be the number of species growing on them.

In these experiments in grasslands, we find that the higher the number of plant species, the lower the concentration of nitrogen in soil, which is fairly easy to explain. If plants take up more nitrogen, this means that less is "left over" in the soil. In ecosystems that are reasonably rich in nutrients, this also means less nitrogen is washed out into ground water, which protects ground water quality and freshwater ecosystems.

To understand these results, we must consider one of the other important effects of plant biodiversity on unfertilized ecosystems, which is an increase in plant growth. When there is higher plant biodiversity, there is generally more plant **biomass**, for example more hay on meadows, being produced. More nitrogen is needed to build this greater amount of biomass. Of course, another way of looking at it is that this greater biomass can only be built if the plants can access more nitrogen (and all other necessary nutrients). This is where something called complementarity comes into play.

## DIFFERENT SPECIES WORK TOGETHER TO ACCESS NUTRIENTS

Complementarity describes a mechanism by which different parts of an ecosystem (such as different species) use different essential (and limited) resources from different locations or at different times. The use of this resource by one species "complements" that of the other species. In this way, the plant community uses the available resources more completely. In our example, the resource used is soil available nitrogen. You probably know that plants take up nutrients from the soil with their roots. But not all roots are the same. Some plants have strong, long roots that can access deeper parts of the soil, but do not branch out much on their way there. Others have roots that only reach shallower parts of the soil. If you combine even just these two types, you can see that one plant species takes water and nutrients out of the shallow soil, and the other takes the same resources out of the deeper soil (Figure 1). The two types of root systems complement each other, and this means that the nutrients that would have gone unused in a system that only contained one or the other of these plants are now being used to produce more plant biomass, which serves as food for microbes and animals. These two plants use different niches in space, which we call spatial niches. Similarly, not all plants develop and grow at the same time. If one species develops early in the spring and another only starts growing in summer, then these two species will not take up most of their nutrients at the same time. They use

## BIOMASS

The total amount of mass present in components of the ecosystem, such as plants or animals. For example, plant biomass, which we talked about in this article, can be defined as all the living matter contained in the plant roots, shoots, leaves, flowers, and fruits. In temperate climate, biomass is not constant but usually increases from spring to late summer and decreases in autumn.
<caption>**Figure 1**</caption>

Complementarity between rooting systems in soil systems under higher biodiversity leads to more efficient nutrient cycling. Broad arrows represent greater uptake of nitrate or higher phosphatase activity in more diverse ecosystems; narrow arrows represent less uptake of nitrate or lower phosphatase activity in less diverse ecosystems. While nitrate is being taken up by the roots and transported into the above-ground parts of the plant, phosphatase is released downwards into the soil to make phosphate available for uptake by roots.

<figure><img src="image_213.png" /></figure>

two temporal niches, or niches in time, and they too access nutrients and other resources much more completely together than they would alone. So, when not only two but many plants grow together using different spatial and temporal niches, the nitrogen in soil is used more completely, and therefore, less is left in the soil for us to measure.

## PLANT BIODIVERSITY AND SOIL PHOSPHORUS

It would be logical to assume that the effect of biodiversity we have just described for soil nitrogen is the same for soil phosphorus. Both are essential nutrient elements, and both can be limiting to biomass production. However, perhaps surprisingly at first, this is not something we find in biodiversity experiments, in which we control the species richness of single ecosystems to study biodiversity effects on these ecosystems. Often, concentrations of readily available phosphate, the chemical form of phosphorus that is taken up by plants, are so low in the soils of the systems we study that there simply cannot be any "leftovers" as is sometimes the case with nitrogen. So, does plant diversity have any effect on phosphorus cycling at all?

The short answer is yes, probably. We know that there is more phosphorus in the plant biomass of more diverse systems and this effect—similar to nitrogen—is caused by the greater amount of biomass that comes from a greater phosphorus uptake by plants [2]. The question is how more diverse ecosystems can take up more phosphate, even though we cannot see the results of this in the soil.
## ENZYME

Small molecules that speed up a (bio-)chemical reaction in or outside of cells.

To access phosphate in the soil, both plants and microbes use **enzymes** (substances that facilitate certain chemical reactions) to split phosphate from more complex chemical molecules that exist in soil humus, the organic part of the soil you likely know as compost. We can measure the speed and function of phosphatase, the enzyme responsible for making phosphate accessible in this way, which allows us to estimate how much phosphate is being released from the soil for plant or microbial use. In ecosystems where plant biodiversity is higher, we find more activity in soil phosphatases (Figure 1) [3]. This indicates that, while we cannot see the higher uptake of phosphorus from soils with higher plant biodiversity the same way we can for nitrogen, we can see that there is more efficient access to phosphorus in soils through higher phosphatase activity. This is one way plant biodiversity can influence phosphorus cycling through the ecosystem.

## ECOSYSTEM PRODUCTIVITY

The amount of organic material, such as plant biomass, produced by the ecosystem in a given time. A good example for this would be how much wheat or hay is harvested in the course of 1 year from a field.

## THE IMPORTANCE OF BIODIVERSITY FOR ECOSYSTEM FUNCTION

So, what does all this mean? The general assumption is that with continued global changes, more species will be lost from ecosystems and biodiversity will continue to decline. With the decline of biodiversity, it is likely that both nitrogen and phosphorus cycling will become less efficient, that is, ecosystems will be less capable of keeping and recycling nitrogen and phosphorus than they are now. This is a big change in the ecosystem and may be one factor leading to a reduction of **ecosystem productivity**. Declining biodiversity may also lead to nutrients being lost from the system, such as nitrate being washed out into the groundwater. Excess nitrate is a pollutant if it gets into our drinking water and it can also have negative effects on the aquatic ecosystems it is transported to, for example through the excessive growth of algae. And at the other end, these nutrients are then not available to plants, microbes or animals in the original ecosystem, leaving a system that is likely poorer in nutrients and less capable of sustaining the organisms living in it.

## REFERENCES

1. Oelmann, Y., Buchmann, N., Gleixner, G., Habekost, M., Roscher, C., Rosenkranz, S., et al. 2011. Plant diversity effects on aboveground and belowground N pools in temperate grassland ecosystems: development in the first 5 years after establishment. *Glob. Biogeochem. Cycles* 25. doi: 10.1029/2010gb003869

2. Oelmann, Y., Richter, A. K., Roscher, C., Rosenkranz, S., Temperton, V. M., Weisser, W. W., et al. 2011. Does plant diversity influence phosphorus cycling in experimental grasslands? *Geoderma* 167:178–87. doi: 10.1016/j.geoderma. 2011.09.012

3. Hacker, N., Ebeling, A., Gessler, A., Gleixner, G., Macé, O. G., Kroon, H., et al. 2015. Plant diversity shapes microbe-rhizosphere effects on P mobilisation from organic matter in soil. *Ecol. Lett.* 18:1356–65. doi: 10.1111/ele.12530
**SUBMITTED**: 30 April 2020; **ACCEPTED**: 02 March 2021;

**PUBLISHED ONLINE**: 14 April 2021.

**EDITED BY**: Malte Jochum, German Centre for Integrative Biodiversity Research (iDiv), Germany

**CITATION**: Koller-France E, Wilcke W and Oelmann Y (2021) Does Plant Biodiversity Influence Nutrient Cycles? Front. Young Minds 9:557532. doi: 10.3389/frym.2021.557532

**CONFLICT OF INTEREST**: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

**COPYRIGHT** © 2021 Koller-France, Wilcke and Oelmann. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

## YOUNG REVIEWERS

### MACKENZIE, AGE: 14

My name is Mackenzie, and I enjoy music (both playing and listening), books (fantasy in particular), and sports (my favorite is tennis). I also enjoy science, math, and language, but the thing I enjoy most is backpack camping.

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### ROSE, AGE: 14

Hello. I am 14 years old and live in Canada. I like knitting, crocheting, and reading.

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## AUTHORS

### EVA KOLLER-FRANCE

Eva is an ecosystem ecologist interested in the effects of all kinds of global changes on ecosystem carbon and nutrient cycling. She spent her formative Ph.D. years wandering the Arctic to study the effects of environmental change on the links between carbon and nutrient cycles, and she is now a post-doctoral researcher for the Jena Experiment (http://www.the-jena-experiment.de/), studying the long-term effects of plant species richness on nitrogen and phosphorus cycling.

*ekoller@gmail.com

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WOLFGANG WILCKE

Wolfgang Wilcke studied Geoecology at the University of Bayreuth and is now Professor of Geomorphology and Soil Science at the Karlsruhe Institute of Technology (KIT) after research and teaching stations at the TU Berlin, the Johannes Gutenberg University Mainz and the University of Berne. His research interests focus on the effects of environmental change, including climate change, land-use change, nutrient deposition, pollution, and biodiversity loss, on the element cycling between soils and plants. He uses soil chemical analyses, long-term observations of element fluxes, and stable isotope approaches.

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YVONNE OELMANN

Yvonne is a soil scientist working on ecosystem carbon and nutrient cycling. She did her Ph.D. at the TU Berlin on the effects of plant diversity on nutrient cycling in grassland soils (http://www.the-jena-experiment.de/). As a Post-Doc, she broadened her perspective on this issue by focusing on complex forest ecosystems and by including the impact of mankind. She was appointed a professorship at the University of Tübingen in 2011 and since then has been working on carbon and nutrient cycling in grasslands and forests around the globe.
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# CAN METHANE-EATING_BACTERIA_IN_DRYLANDS HELP_US_REDUCE GREENHOUSE_GASES?

Angela Lafuente<fnref n="1" />,<fnref n="2" />* and Concha Cano-Díaz<fnref n="1" />

YOUNG REVIEWER:

<figure><img src="image_220.png" /></figure>

SEBASTIAN

AGE: 10

What is a dryland? The first thing that may come to your mind is a desert-like place where nothing can live or grow. Despite the scarcity of water, dryland ecosystems are diverse and will expand due to global climate change. The main cause of global warming is the increase of greenhouse gases in our atmosphere. To solve this, we obviously need to reduce the emission of greenhouse gases, but the study of microorganisms in nature also gives us exciting clues for how to address the problem of global warming. Microorganisms live in all possible Earth environments, and luckily some of them can even take greenhouse gases from the air as their food! In this article, we describe our search of the global soils for bacteria that can consume one of the most powerful greenhouse gases, methane (CH₄). Contrary to what was expected, we found that these bacteria live in drylands all over the world!

<footnote n="1">¹Departamento de Biología y Geología, Física y Química Inorgánica, Escuela Superior de Ciencias Experimentales y Tecnología, Universidad Rey Juan Carlos, Móstoles, Spain</footnote>

<footnote n="2">²College of Forest Resources and Environmental Science, Michigan Technological University, Houghton, MI, United States</footnote>
Figure 1

Methods that we follow to find and study soil methanotrophs. We selected drylands all around the world and took soil samples (1). We analyzed the properties of those soils, such as organic matter content and pH (2). We extracted the genetic information (DNA) of the bacteria present in the soil (3). Studying the DNA, we obtained information about the abundance, richness, and community structure of the methanotrophs from each soil sample (4,5). Then we used mathematics to figure out which are the most relevant soil or climate conditions for methanotrophs (6).

ABIOTIC

Non-living abiotic factors in an environment include temperature, water, and light.

ARIDITY

Mathematical relationship between the amount of precipitation (rain, fog, or snow) and the evaporation of water. It describes how deficient water is in an ecosystem.

<figure><img src="image_221.png" /></figure>

THE LARGEST TERRESTRIAL ECOSYSTEM ON EARTH:
DRYLANDS

Drylands are characterized by scarce rainfalls and consequently do not have lush vegetation. However, drylands cover a whole range of different ecosystems, from the driest place on Earth, the hot desert of Atacama in Chile, to the leafy eucalyptus forests in Australia where Koalas live (Figure 1.1). Dryland ecosystems also contain huge numbers of organisms, many of which are plants and animals that live only in drylands and have adapted to the harsh conditions. Drylands are the biggest terrestrial ecosystem, occupying almost half of the Earth's land surface (45%) and are home to more than 40% of the human population. So, you can see why drylands are extremely important areas of the earth to research.

Living beings and non-living substances in the environment, like plants and water, are intimately connected in by cycles in nature. These non-living substances are called **abiotic** factors. Water is crucial for all processes that are related to life, from plant growth to the development of communities of soil microorganisms. Therefore, water is the most important abiotic factor in an ecosystem. We measure the availability of water in an ecosystem with a measurement called **aridity**, a mathematical relationship between the amount of precipitation (rain, fog, or snow) and the evaporation of water. The less available water there, the more arid a place is (Figure 1.1).
## ABUNDANCE

The number of individuals of a certain type present in an environment.

## METHANOGENS

Group of microorganisms that do not need oxygen to survive and therefore can live in oxygen-free environments. They produce methane while decomposing organic matter, such as leaves or wood fragments.

## METHANOTROPHS

Group of microorganisms that are capable of using up methane as their source of carbon and energy. They are methane eaters.

In drylands, where water is not always available, the natural cycles between living beings and non-living substances are hugely affected. When there is no water from rain and the humidity goes down, this affects the carbon (C) and nitrogen (N) cycles, reducing the **abundance** of these elements in soil, which impacts plants, animals, and microorganisms. All this makes drylands extremely vulnerable to ongoing climate change.

## SOIL BACTERIA AND METHANE

The Earth is surrounded by a gaseous layer called the atmosphere, which protects us from solar radiation and helps maintain the overall temperature of the Earth. The principal components of the atmosphere are nitrogen (78%) and oxygen (21%), but there are many other gases in the atmosphere as well. Some atmospheric gases, such as carbon dioxide (CO₂) and water vapor, are the greenhouse gases, so called because they trap the Sun's heat, working like the glass in a greenhouse. The greenhouse gases let the Sun's light reach the Earth's surface but prevent heat from leaving the atmosphere. This trapping of heat contributes to global warming.

The most abundant human-produced greenhouse gas in the atmosphere is CO₂, released from burning fossil fuels. However, the second most important gas contributing to global warming is methane (CH₄). Methane is a simple molecule formed by one atom of carbon (C) and four atoms of hydrogen (H). The warming effect of one molecule of methane is equivalent to 25 molecules of CO₂, making it a super-powerful greenhouse gas. Methane is produced by **methanogens**, a group of microorganisms that do not need oxygen to survive and therefore can live in oxygen-free environments like rice fields, lake sediments, and wetlands. Methanogens also live in the digestive tracts of animals, such as the stomachs of cattle and even in humans! Methanogens are responsible for animal burps and farts! Methanogens also produce methane while decomposing organic matter, such as leaves or wood fragments. In addition to agriculture, other human activities, such as the oil and gas industries also release large amounts of methane into our atmosphere [1] (Figure 2).

The methane released into the atmosphere is greatly contributing to climate change and there is only one group of organisms that can consume it, the **methanotrophs**. This group of microorganisms is capable of using up methane as their source of carbon and energy. These microorganisms basically eat methane (Figure 2)! In drylands, methane production is low because of the scarcity of water (remember, methanogens typically live in flooded soils and other oxygen-free environments). However, due to the large extent of drylands and the global increase of methane in the atmosphere, dryland ecosystems could be of great interest if methanotrophs are also present and abundant there.
<figure><img src="image_222.png" /><figcaption>**Figure 2**
Main methane (CH₄) sources and sinks.</figcaption></figure>

## HOW TO FIND AND STUDY METHANOTROPHS

In our research, we were interested in learning if methanotrophs are common in the soils of drylands worldwide, and if they are sensitive to climate conditions and soil properties, like most soil microorganisms. First, we selected 80 dryland sites all around the world (Figure 1.1). At each site, we collected climate information, such as mean annual temperature, annual precipitation, and aridity. We also took soil samples and analyzed properties, such as the amount of organic matter (Organic Carbon), pH, and sand content (Figure 1.2). High soil organic matter indicates the soil is fertile, meaning it has the nutrients that plants, soil animals and microorganisms need to grow. pH analysis tells us how acidic the soil is. pH is one of the most important factors regulating soil bacteria growth. For example, when soils are very acidic like vinegar, only specific acid-tolerant bacteria can live there. Soil grains are very close to each other but also leave spaces for air and water to enter. The amount of sand, the largest type of particle in the soil, tells us how big these soil spaces are. This way, a high sand content means there are big spaces, so that air can enter the soils easily, but water and nutrients can also drain easily.

To study the methanotrophs in our soil samples, we need the genetic information (DNA) of these bacteria [2]. First, we obtain all the DNA present in our soil samples, through a process called **DNA extraction** (Figure 1.3). This process is done in the laboratory using powerful enzymes that break the cells open without damaging the genetic information. We then analyse the extracted DNA for a specific region that is present only in methanotrophs. This stretch of DNA is a gene called *pmoA*. The *pmoA* gene contains the instructions for the protein that allows methanotrophs to eat atmospheric methane. Knowing the concentration of the *pmoA* gene in each soil sample tells us how many methanotrophs were living in that sample (Figure 1.4). There are several closely related species of methanotrophs that all have similar

## DNA EXTRACTION

Laboratory procedure in which cells are broken open to release the genetic material (DNA) they contain inside, without damaging the DNA.
**Figure 3**

Microbial communities can be described using three properties. Abundance is the total number of bacteria of a certain type present. Richness is the number of different kinds of bacteria present in an environment. Community structure describes how many different kinds of bacteria there are and how abundant each kind is.

**RICHNESS**

The number of species (different kinds) of organisms present in an environment.

**COMMUNITY STRUCTURE**

The combined richness and abundance in the community.

<figure><img src="image_223.png" /><figcaption>Figure 3</figcaption></figure>

Abundance: 17 bacteria

Richness: 4 species

Community structure

<figure><img src="image_224.png" /></figure>

<figure><img src="image_225.png" /></figure>

<figure><img src="image_226.png" /></figure>

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DNA information, but different species have tiny genetic differences in their DNA, so we can use DNA to identify different methanotrophs, like a fingerprint (Figure 1.5).

Our DNA studies help us to obtain information about the abundance (total number of bacteria of a certain type present), **richness** (the number of different kinds of bacteria present), and **community structure** (the different kinds of bacteria and the abundance of each kind) of the methanotrophs from each soil sample (Figure 3). Then, we use mathematics to figure out which soil or climate conditions are the most important for methanotrophs (Figure 1.6).

**WHERE THE METHANOTROPHS LIVE**

We were not sure if we would find methanotrophs in drylands, since these microorganisms need methane to live and drylands are not the typical ecosystem for methane production. So, finding methanotrophs in all our dryland soil samples was an extraordinary finding! We can now say that methanotrophs are widely distributed in global drylands. Surprisingly, we even found some methanotrophs that are usually found in humid places, such as Denmark, Scotland, or New Zealand.

We also found that, in drylands, the average annual temperature and aridity are not the main conditions influencing the abundance and richness of methanotrophs. Abundance and richness may be driven by other factors, such as rainfall. However, climate conditions like mean annual temperature, rainfall, aridity, and soil properties, such as organic matter, pH, and sand content did affect the community structure of methanotrophs. For example, higher temperatures increased the abundance of certain methanotrophs that are heat-resistant. In other words, those drylands with higher temperatures, the methanotroph communities may contain more heat-resistant methanotrophs. Climate conditions can also affect soil properties, for example by favoring the breakdown of rocks, which increases sand content, or by modifying soil pH and organic matter. These soil properties affect the amount of air that can get into the soil, which we found to be very important for the community structure of the methanotrophs.
## WHAT WE LEARNT FROM METHANOTROPHS IN DRYLANDS?

As we found, methanotrophs are abundant and widely distributed in drylands worldwide. Both climate and soil affect the communities of methanotrophs. Moreover, we found the community structure of methane eating bacteria was highly dependent on climate conditions, such as amount of rainfall and temperature, and soil characteristics as soil organic content.

Because we found that climate influences the methanotrophs, we expect that ongoing climate change will modify methanotroph communities in the coming years, affecting the consumption of atmospheric methane. To date we knew methanotrophs lived on cold and humid places, which will be surely affected by climate change. The vast amount of land that drylands cover and the many methanotrophs they contain may make these areas extremely important for consuming atmospheric methane in the future. In other words, dryland bacteria can help us reduce greenhouse gases! Taking good care of drylands now and continue studying the hidden marvels they contain is important to deal with our future warmer planet. Methane-Eating Bacteria in Drylands can help us!

## ORIGINAL SOURCE ARTICLE

Lafuente, A., Bowker, M. A., Delgado-Baquerizo, M., Durán, J., Singh, B. K., and Maestre, F. T. 2019. Global drivers of methane oxidation and denitrifying gene distribution in drylands. *Glob. Ecol. Biogeogr.* 28:1230–43. doi: 10.1111/geb.12928

## REFERENCES

1. Cadena, S., Cervantes, F., Falcón, L., and García-Maldonado, J. 2019. The role of microorganisms in the methane cycle. *Front. Young Minds* 7:133. doi: 10.3389/frym.2019.00133

2. Schallenberg, L., Wood, S., Pochon, X., and Pearman, J. 2020. What can DNA in the environment tell us about an ecosystem? *Front. Young Minds* 8:150. doi: 10.3389/frym.2019.00150

**SUBMITTED**: 27 April 2020; **ACCEPTED**: 20 May 2021;

**PUBLISHED ONLINE**: 16 June 2021.

**EDITED BY**: Rémy Beugnon, German Centre for Integrative Biodiversity Research (iDiv), Germany

**CITATION**: Lafuente A and Cano-Díaz C (2021) Can Methane-Eating Bacteria in Drylands Help Us Reduce Greenhouse Gases? *Front. Young Minds* 9:556361. doi: 10.3389/frym.2021.556361
**CONFLICT OF INTEREST:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

**COPYRIGHT** © 2021 Lafuente and Cano-Díaz. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

## YOUNG REVIEWER

### SEBASTIAN, AGE: 10

I like sports, reading, math, and animals.

<figure><img src="image_228.png" /></figure>

## AUTHORS

### ANGELA LAFUENTE

I am currently a post-doc at Michigan Technological University, working on carbon cycling in tropical peatlands. I am an ecologist interested in understanding how global change affects soil microorganisms and greenhouse gas fluxes. In my free time, I enjoy the nature going on a hike, cycling, or skiing. *ellyon.diebrunnen@gmail.com

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### CONCHA CANO-DÍAZ

I am a biologist finishing my Ph.D. at Universidad Rey Juan Carlos (Spain). My research is focused on the distribution and ecological preferences of soil cyanobacteria. I am currently studying the effects of climate change and soil formation processes on cyanobacterial communities around the world. I love to make scientific illustrations and in my free time I enjoy playing music with the ukulele and singing in the choir.

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# EARTHWORMS AND THEIR ROLE IN GREENHOUSE GAS EMISSIONS

Pierre Ganault<fnref n="1" />*, Sacha Delmotte<fnref n="2" />, Agnès Duhamet<fnref n="2" />, Gaëlle Lextrait<fnref n="2" /> and Yvan Capowiez<fnref n="3" />

## YOUNG REVIEWER:

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GWEN

AGE: 13

The mass of all earthworms living on our planet is greater than the mass of any other terrestrial animal species. There are over 7,000 species of earthworms, and they are involved in many processes that keep soils healthy and help plants to grow, which makes them extremely important organisms to study. The activity of earthworms also stimulates the growth of bacteria, both in the soil and in their guts. Some studies have suggested that these bacteria might increase greenhouse gas emissions, particularly the gases carbon dioxide and nitrous oxide that contribute to global warming. So, are earthworms good or bad for the environment, overall? This article will describe the experiments that can be used to study the links between earthworms and greenhouse gas production, as well as the limitations of these experiments. The effects of earthworms on soil processes are very complex and therefore scientifically challenging, important, and exciting.

<footnote n="1">¹CEFE, CNRS, EPHE, IRD, Université de Montpellier, Université Paul-Valéry Montpellier, Montpellier, France</footnote>

<footnote n="2">²Biology and Ecology Departement, Université de Montpellier, Montpellier, France</footnote>

<footnote n="3">³INRAE, UMR 1114 EMMAH, INRAE/Université d'Avignon, Avignon, France</footnote>
## Figure 1

(A) The three main groups of earthworms, epigeic, anecic, and endogeic. Illustration credits: www.lesbullesdemo.fr.

(B) X-ray 3D reconstruction of the burrow systems of one endogeic species called gray worm (Aporrectodea icterica) and one anecic species called nightcrawler (Lumbricus terrestris). X-ray image credits: Yvan Capowiez.

<figure><img src="image_233.png" /></figure>

<figure><img src="image_234.png" /><figcaption>Figure 1</figcaption></figure>

## ECOLOGICAL GROUP

Earthworms differ regarding where they live in the soil, what they eat, and what color is their skin. There are three main groups: **epigeics** (pronounced “ep-i-jEE-ik”), **endogeic** (pronounced “en-d-oh-jEE-ik”), and **anevic** earthworms (pronounced “an-e-c-ik”).

## ORGANIC MATTER

It is matter composed of organic compounds that comes from the remains of organisms, such as plants and animals and their waste products in the environment.

## CASTS

It is earthworm poo. Depending on the earthworm ecological group, they can be deposited at the soil surface or within the soil, in their burrow.

## EARTHWORMS, THE UNDERGROUND ENGINEERS

Under our feet, thousands of animals live in the soil, including the famous earthworms. The term “earthworms” actually refers to many species. Scientists have described around 7,000 earthworm species worldwide, but some areas are poorly studied, and scientists expect that there are more than 30,000 earthworm species yet to be described [1]. Earthworms are invertebrates, so they do not have bones. Unlike insects, earthworms also lack an external skeleton and they do not have eyes, but they have strong muscles. Earthworms can move through the soil, and even eat it, along with some dead leaves. Although most earthworm species look fairly similar, they have various lifestyles, which fall into three main **ecological groups** (Figure 1A) [2].

The first group, called **epigeics**, are small earthworms (3–10 cm) with a red color, and can be used in vermicomposting. Epigeic earthworms live in dead leaves. Their color protects them from UV radiation and camouflages them from surface predators. Without digging into the soil, they eat dead leaves and transform them into small pieces of **organic matter** in their feces, which are called **casts**. **Endogeic** earthworms are larger (5–15 cm) and completely unpigmented. They only live in soils and create numerous burrows (Figure 1B). In an experimental pot, four endogeic earthworms dug 2.2 km of burrows of 3.5 mm width per cubic meter of soil in only 6 weeks [3]! As they dig, they also eat many very small pieces of dead leaves that are in the soil, and they mix organic matter within the soil (Figure 2). The third group is called **anevic** earthworms. They are the largest, they can
<caption>Figure 2</caption>

Organic matter passes through the earthworm's digestive tract, is broken into smaller pieces, digested, and the remainder exits as feces called casts. Casts then help to feed bacteria. Bacteria are also present within the digestive tract of earthworms. Bacteria need the right mix of organic matter, water, and air to be active. Image inspired by Drake and Horn [4].

<figure><img src="image_235.png" /><figcaption>Figure 2</figcaption></figure>

## ECOSYSTEM ENGINEER

They are organisms that modulate the availability of resources to other species. Termites, ants, and earthworms are the major ecosystem engineers.

grow as big as 10 cm and up to 1 m! They dig deep vertical burrows (Figure 1), that can exceed 1 meter in depth. During the night, they stick their heads out to grab dead leaves on the surface and bring them to deeper soil layers. Since only their heads leave the soil, only their heads are pigmented.

Eating or burying dead leaves and moving through the soil by creating burrows are the two main actions of earthworms. These actions are good for the soil, the other soil organisms, and the entire **ecosystem**, which has earned earthworms the name "ecosystem engineers."

## HOW EARTHWORMS ARE CHANGING THE SOIL AND GROWING BACTERIA

Earthworm burrows profoundly change soil structure by creating large spaces in the compact soil. The burrows are the habitat of many organisms like small invertebrates, bacteria, and plant roots. Burrows also act as pipes that increase water and oxygen flow between the surface and deeper soil layers. Earthworms of different ecological group build burrows that affect water and gas fluxes differently. In the experiment of Capowiez et al. [3], done in PVC tube (16 cm diameter, 30 cm height, Figure 1B), the endogeic earthworms' burrows allowed a water infiltration rate of 5.2 L per minute, while this rate reached 12.4 L per minute in the anecic earthworm' burrows as they are larger, more continuous, and vertical.
## OXYGEN

This is a gas constituting 21% of the air we breathe. Plants produce oxygen from carbon dioxide, water and sunlight while animals use oxygen and produce carbon dioxide.

## CARBON DIOXIDE

It is a colorless gas composed of one atom of carbon and two of oxygen. Its atmospheric concentration raised from 0.028 to 0.042% since 1850 causing an increase of 1° of global temperature.

## NITROUS OXIDE

It is a colorless gas composed of two atom of nitrogen and one of oxygen. It is in very low concentration but one nitrous oxide molecule warms the atmosphere as 270 carbon dioxide molecules.

## GREENHOUSE GASES

It is a gas that absorbs and emits solar energy causing the greenhouse effect, i.e., warming the atmosphere.

In ecosystems where earthworms are abundant, dead leaves disappear quite quickly and do not accumulate on the soil surface. Epigeic earthworms transform dead leaves into smaller pieces repacked in their casts and anecic earthworms bury dead leaves in deeper soil layers. Endogeic earthworms then eat small dead leaves or root particles along with the soil and excrete it behind them wherever they go. These actions of earthworms result in the redistribution of organic matter throughout the soil. Instead of accumulating at the soil surface, the organic matter is more spread out and available for plant roots and other soil inhabitants.

The changes of the soil made by earthworms affect another important group of soil organisms, like bacteria. Bacteria need the right balance of food, water, and air to live. They transform the small pieces of organic matter into even smaller particles, breaking them down into carbon and nitrogen. These particles are so small that plant roots can easily absorb them and use them to grow. To break down their food, bacteria use **oxygen** (they breath, even without having lungs) and produce **carbon dioxide** as a waste product. If there is too much water around, such as during a flood or in rice fields, the bacteria instead produce **nitrous oxide** as a waste product. Carbon dioxide and nitrous oxide are **greenhouse gases** that increase the atmosphere's temperature, so contributing to climate change.

In some soils, bacteria may lack organic matter, air, or water, and be less active. Earthworms can "wake up" bacteria by making organic matter, water, and air more available. This effect is even stronger for the bacteria living in the earthworm's gut (Figure 2). In the gut, organic matter and soil are perfectly mixed in an environment saturated with water. This is heaven for bacteria that produce nitrous oxide [4]. Since earthworms stimulate bacteria that produce carbon dioxide and nitrous oxide, this makes us wonder if earthworms serve to increase or decrease greenhouse gases emissions.

## STUDYING THE IMPACT OF EARTHWORMS ON GREENHOUSE GASES EMISSIONS

To study how earthworms affect bacteria and the greenhouse gases they produce, scientists can run experiments. In one type of laboratory experiment, scientists use pots filled with soil that is sifted to remove rocks, all animals, and roots. Then earthworms are added, usually a few individuals of the same species, ideally in numbers close to what would be found in nature. Some pots are kept without earthworms, for comparison. Then, greenhouse gases emissions are measured at the soil surface and the bacteria in the pots are studied, to see whether greenhouse gases emissions are higher in the presence or absence of earthworms.
<caption>**Figure 3**</caption>

(A) An experimental chamber can be used to measure GHG emissions in natural environments. Gases produced by bacteria accumulate in the sealed chamber, and then are sampled with a syringe through the latex plug to measure the levels of carbon dioxide (CO₂) and nitrous oxide (NO₂). (B) Example of a chamber placed on the soil surface of an experimental pot with earthworm and plants. Credits: Pierre Ganault.

<figure><img src="image_236.png" /><figcaption>Figure 3</figcaption></figure>

Another method scientists use is to measure greenhouse gases in nature. In this case, cylindrical chambers are pushed into the ground to measure gases like carbon dioxide and nitrous oxide (Figure 3). The earthworms in the soil are also studied so that scientists can try to relate greenhouse gases emissions to the abundance and number of species of earthworms present. Scientists can also measure other soil characteristics important for bacterial activity, including water content, availability of organic matter, and pH.

Another way to know the effect of earthworms on greenhouse gases emissions is to gather information from all the existing studies. So, it was found that, on average, earthworms increase carbon dioxide emissions by 33% and nitrous oxide emissions by 42% [5]. This seems to tell us that, although they are beneficial for soil health, earthworms may be detrimental for the environment because they increase bacterial activity and related greenhouse emissions.

## pH

In chemistry, pH is a scale used to specify the acidity or basicity of an liquid solution. Acidic solutions are measured to have lower pH values than basic or alkaline solutions.

## THESE EXPERIMENTS HAVE LIMITS

This seems like a real dilemma: earthworms improve soil health, but at the same time they appear to increase greenhouse gases emissions! Before we draw this conclusion, however, it is important to recognize that the experiments we have described all have drawbacks that make it difficult to be completely certain of the role earthworms play in greenhouse gases emissions. The interactions between earthworms, bacteria, soil, plants, and water that result in greenhouse gases emissions are extremely complex. These factors vary tremendously in the natural environment and are very difficult to recreate accurately in scientific experiments.

The first important factor limiting our full understanding of the role of earthworms in greenhouse gases emissions is the great diversity
of soil properties, such as sand content. Most earthworms generally prefer soils with a low sand content because sandy soils dry faster, and sand particles can be abrasive for their skin. Soil pH can also strongly affect earthworms, and many may not survive in soils with a pH below 4.5. It would be extremely difficult to make experimental pots for the thousands of different soil types that exist in nature, so our knowledge is currently limited to certain common types of soil.

A second limitation is that very few studies included plants in the experiments. Plants absorb water and nutrients with their roots, reducing the availability of water and nutrients for earthworms and bacteria. However, plants and bacteria also help each other. Plant roots produce sugar in the surrounding soil that bacteria can eat in exchange for providing minerals that plants need. Unfortunately, it is very difficult to set up an experiment that could test all the possible positive and negative interactions happening at the same time in the soil.

The third limitation is that most studies kept the soil water content constant. This is generally done to optimize earthworm activity. In nature, soils are constantly drying out and being remoistened by rainfall. Earthworms can be completely inactive if the soil gets too dry. This means that experiments in which the soil has a constant water content might overestimate the negative effects of earthworms on greenhouse gases emissions. In a lab experiment in which scientists used more realistic drying-rewetting cycles, the presence of earthworms actually reduced nitrous oxide emissions [6]. The scientists reasoned that earthworm burrows increased water flow to lower soil layers and aerated the soil, which sped up soil drying and reduced bacteria activity. The effects of drying-rewetting cycles are very important to study, especially since these cycles are expected to be more frequent and extreme with the on-going climate change.

## CLIMATE CHANGE—DO NOT BLAME THE EARTHWORMS

We showed you just how complex it is to study greenhouse gases emissions from the soil. Earthworms modify the distribution of organic matter and the availability of water and air in the soil. All these change the activity of soil bacteria. However, soil bacteria also depend upon the properties of the soil, on drying-rewetting cycles, and on the plants that grow there. We are far from having enough studies with realistic experiments to understand the true role of earthworms in greenhouse gases emissions. On the other hand, human activities, notably agriculture, produce large amount of greenhouse gases and we need to keep thinking in innovative ways to improve the health of our planet and all living creatures.
ACKNOWLEDGMENTS

The authors thank the young reviewer for his/her thorough work that
improved the manuscript. The authors thank the TEBIS consortium
and the different NGO, such as CARABES (https://assocarabes.com)
with which the authors are working with to increase citizen awareness
and encourage protection of soils and their biodiversity. The authors
also thank Morgane Arietta Ganault (www.lesbullesdemo.fr) for the
quality of the detailed drawings.

REFERENCES

1. Orgiazzi, A., Bardgett, R. D., Barrios, E., Behan-Pelletier, V., Briones, M. J. I., Chotte, J. L., et al. 2016. *Global Soil Diversity Atlas*. Luxembourg: European Union. Available online at: http://esdac.jrc.ec.europa.eu/public_path/JRC_global_soilbio_atlas_online.pdf (accessed April 28, 2020).

2. Bottinelli, N., Hedde, M., Jouquet, P., and Capowiez, Y. 2020. An explicit definition of earthworm ecological categories – Marcel Bouché’s triangle revisited. *Geoderma* **372**:114361. doi: 10.1016/j.geoderma.2020.114361

3. Capowiez, Y., Bottinelli, N., Sammartino, S., Michel, E., and Jouquet, P. 2015. Morphological and functional characterisation of the burrow systems of six earthworm species (Lumbricidae). *Biol. Fertil. Soils* **51**:869–77. doi: 10.1007/s00374-015-1036-x

4. Drake, H. L., and Horn, M. A. 2006. Earthworms as a transient heaven for terrestrial denitrifying microbes: a review. *Eng. Life Sci.* **6**:261–5.
doi: 10.1002/elsc.200620126

5. Lubbers, I. M., van Groenigen, K. J., Fonte, S. J., Six, J., Brussaard, L., and van Groenigen, J. W. 2013. Greenhouse-gas emissions from soils increased by earthworms. *Nat. Clim. Change* **3**:187–94. doi: 10.1038/nclimate1692

6. Chen, C., Whalen, J. K., and Guo, X. 2014. Earthworms reduce soil nitrous oxide emissions during drying and rewetting cycles. *Soil Biol. Biochem.* **68**:117–24. doi: 10.1016/j.soilbio.2013.09.020

SUBMITTED: 15 May 2020; ACCEPTED: 06 April 2021;

PUBLISHED ONLINE: 06 May 2021.

EDITED BY: Malte Jochum, German Centre for Integrative Biodiversity Research (iDiv), Germany

**CITATION:** Ganault P, Delmotte S, Duhamet A, Lextrait G and Capowiez Y (2021) Earthworms and Their Role in Greenhouse Gas Emissions. Front. Young Minds 9:562583. doi: 10.3389/frym.2021.562583

**CONFLICT OF INTEREST:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

**COPYRIGHT** © 2021 Ganault, Delmotte, Duhamet, Lextrait and Capowiez. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums
is permitted, provided the original author(s) and the copyright owner(s) are credited
and that the original publication in this journal is cited, in accordance with accepted
academic practice. No use, distribution or reproduction is permitted which does not
comply with these terms.

YOUNG REVIEWER

GWEN, AGE: 13

Hi, my name is Gwen, I live in the U.S. and play piano and volleyball. I just finished seventh grade, and my favorite subjects are science, math, art, and Spanish. I love to read, particularly Sci-Fi novels and series (I am also a huge fan of Harry Potter). I am very excited to be working with Frontiers for Young Minds!

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AUTHORS

PIERRE GANAULT

At each walk in nature, I cannot help myself from flipping logs and rocks over or
searching into the dead leaves to see what wonderful animal I will find hiding there.
This curiosity led me to study soil biodiversity and do a Ph.D. on the effect of tree
species mixture on soil invertebrates and the role of these animals for soil processes.
I also work with associations to bridge the gap between scientists and citizens so we
can work all together to study, better understand, and protect the creatures living in
the soil. *pierre.ganault@gmail.com

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SACHA DELMOTTE

My passions for nature, humans, science, and the transmission of knowledge led me to pursue 7 years of university studies in ecology, biology, and geology. I am also an animator in the plains of nature for audiences ranging from 3 to 18 years old to awaken them to the things that fascinate me.

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AGNÈS DUHAMET

Agnès is a Ph.D. student in marine biology. She obtained her bachelor's degree in biology at the University of Avignon and her master's degree in ecology and evolutionary biology at the University of Montpellier. She is passionate about nature and likes to transmit knowledge about natural sciences.

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GAËLLE LEXTRAIT

I have always been passionate about microorganisms, whether pathogenic or mutualistic. I turned to the interactions between small animals living in soils (insects) and their symbionts. I am currently a Ph.D. student in microbiology at the CNRS of Gif-sur-Yvette (University Paris-Saclay), where I am continuing my observations on the symbiotic interactions between stinkbugs and their symbiotic bacteria.

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YVAN CAPOWIEZ

Yvan Capowiez is a Senior Research Scientist at the French Agronomy Institute (INRAE), Avignon, France. His research is focused on earthworm ecology and behavior. He has extensive experience studying how earthworms burrow in the soil and how the resulting burrows will influence important soil functions, such as water transport and organic matter burial.

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# SOIL ECOSYSTEMS CHANGE WITH TIME

Enrique Doblas-Miranda*

CREAF, Bellaterra (Cerdanyola del Vallès), Barcelona, Spain

## YOUNG REVIEWERS:

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RUTENDO

AGE: 14

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NOKUTENDA

AGE: 14

All the animals living below our feet are not still. They can move (to a lot of places because the soil is a 3D space) and to change (for example, from a cocoon to an active state). Therefore, the same soil below a given piece of field may not contain the same living communities in winter as in summer, or even during a sunny day compared to a cold night. For example, research on soil beetle larvae showed seasonal vertical movements, as the larvae searched for better living conditions. Moreover, the soil varies a lot during its formation, and consequently its inhabitants also change. In the case of oribatids, a minuscule but diverse group of soil mites, scientists observed changes in the community over dozen to hundreds of years! Many studies showed a basic but powerful principle: ecosystems are not still photographs, but instead are constantly changing environments.

## ECOSYSTEMS ARE NOT STATIC PHOTOGRAPHS

When we imagine ecosystem diversity, we often picture ecosystems as mostly stable and unchanging, like photographs in a book, with all the
## SOIL BIODIVERSITY

The total variety of living creatures inhabiting soils.

## INVERTEBRATES

Animals with no bones. In the soil, that means mainly worms and arthropods (centipedes, woodlice, insects, spiders...).

## SOIL PORES

Extremely small (<0.075 mm) spaces in the solid structure of the soil, filled mainly with air and water [1].

## VERTICAL MIGRATION

Vertical migration is typical of soil and aquatic environments, where mobile organisms are not limited to move over a (horizontal) surface. Like any other migration, it is normally guided to find resources or better environmental conditions.

## SOIL MESOFAUNA

Soil inhabitants, smaller than 2 mm, such as springtails, mites, and tiny worms [2].

## PASSIVE DISPERSAL

Mobile organism can move actively (using their legs or appendices to go through the territory) or let themselves "go with the flow" (of water, wind or even other animals), which is called passive movement or dispersal.

plants and animals existing in a frozen state of balance. In our heads (and in many book pictures), plants are readily available for herbivores to eat, and herbivores are waiting to be eaten by carnivores, all under wonderful daylight. But reality is not like that! Most of the animals in an ecosystem move during the day and some of them only appear at night. Plants produce different edible parts depending on the season. The entire ecosystem can even change due to catastrophes like forest fires. Not to mention, we rarely even imagine the diversity that happens in the soil under our feet.

## HOW CHANGE WORKS UNDER OUR FEET

Of course, **soil biodiversity** also changes with time, although not necessarily the same way changes happen aboveground. First, movement is certainly more difficult in the soil. Earthworms, insect larvae, mole crickets (also moles, but we are going to focus on small **invertebrates**), and many other tiny creatures must dig with their mouths, claws, or legs. Smaller creatures move throughout the soil mainly using tiny air-filled spaces called **soil pores**.

Soil inhabitants are not limited to the typical horizontal movements of surface animals. Soil invertebrates can also move up and down beneath the same surface area, which is called **vertical migration**. Vertical migrations can occur during a single day, or across seasons. Enchytraeids, very tiny worms, are one of the few types of soil-dwelling animals that have been observed to migrate during the day. Enchytraeids move deeper into the soil to escape from dry surface conditions at midday and return from the deep in the evening, when their favorite moist conditions are reestablished. This migration is the basis of one of the most-used methods to study **soil mesofauna**. This method consists of drying a soil sample in a funnel with a light bulb at the top, so the creatures "escape" by falling to a collection container in the bottom (Figure 1).

Many soil invertebrates can exist in resistant forms that allow them to survive harsh conditions for a long time. Ground pearls, small, rounded, and very interesting insects, are a perfect example. They can secrete a pearly covering around themselves, forming a spherical cyst or "resting" stage, in which they can remain for decades! But when delicious roots are available, the cysts develop and become voracious adults. If conditions are really good, many ground pearl species can clone themselves to profit as much as possible from favorable conditions. An unlucky vineyard farmer may not see the tiny ground pearls 1 year, but find his crops infested with adults the next.

At the soil surface, many small animals can be carried by the wind, water, and even by other animals. Some surface-dwelling creatures travel this way as well, but the so-called **passive dispersal** of soil
<caption>**Figure 1**</caption>

Isolating soil invertebrates for study. A typical Tullgren or Berlese funnel trap, named after its inventors. The soil is placed in the top of a funnel, held up by a layer of mesh. A light bulb heats and dries the soil, forcing the soil organisms to go down. When they drop from the sample, they fall into a collection container, usually filled with a substance that keeps them alive. The organisms can then be studied.

<figure><img src="image_246.png" /><figcaption>Figure 1</figcaption></figure>

fauna has gained a lot of research attention lately, as it may explain movements of soil organisms across large distances.

## SEASONAL CHANGES

During my early years as a researcher, movements of soil organisms were not as understood as they are today and every discovery was very exciting, including the discovery that some soil-living insect larvae perform seasonal vertical migrations [3]!

Soil was sampled in multiple locations every month for 2 years, in a desert-type shrubland in Southern Spain. Soil samples were collected at different depths, from the surface litter down to 50 cm deep. For each sample, all the **macroinvertebrates** were counted and identified. After analyzing all the samples, from every season and soil depth, scientists found that the larvae of one abundant beetle family called Tenebrionidae, which eat organic debris, made the same movement each year. They were more abundant at the soil surface in winter than they were in summer (Figure 2).

In the studied area, summers are very hot and dry. However, Tenebrionidae's favorite dish, litter debris, is found in exposed soil-surface "restaurants," like shrubs and ant mounds. Therefore, these beetle larvae prefer to devour surface litter debris during the winter's gentle weather, but enjoy other, deeper "restaurants," such
## Figure 2

Vertical migration of beetle larvae, depending on soil depth and season. In summer, the larvae are equally abundant at the surface and in the deeper levels of the soil, but in the winter they are much more abundant at the surface, where they can feed on leaf litter and not be damaged by the hot, dry conditions present in summer.

<figure><img src="image_247.png" /><figcaption>Figure 2</figcaption></figure>

as decaying roots, in summer. When the larvae perform this vertical migration, they are also doing a great service for the entire ecosystem. Like earthworms in more humid ecosystems, these tough larvae move the soil in arid environments, so air, water, and organic materials are mixed in the soil, which is highly beneficial for soil health.

## ECOSYSTEM SUCCESSION

The process by which ecosystems are "born" and "grow" after the creation of new surfaces, like a new coral island or the soil revealed after glaciers melt, or how ecosystems "regrow" after disturbances like forest fires.

## CHRONOSEQUENCE

A group of ecosystems studied at the same time, which are similar in origin, plant species, and geographical area, but have different ages. Studying ecosystems in chronosequence is necessary because we cannot wait decades to sample one ecosystem over the course of its development.

## ECOSYSTEMS CAN GROW... AND EVEN AGE!

Changes in weather conditions and food availability are not the only changing features of ecosystems. In fact, an entire ecosystem can change during a process called **ecosystem succession**. Scientists have studied how the diversity of soil animals changes during ecosystem succession, focusing on a group of soil mites called oribatids. Oribatids are tiny, abundant, and diverse, which means that you can find an entire community of them in a small sample of soil. There are also many resources available to help identify various oribatid species, so they are a perfect organism to observe to study changes in soil ecosystem diversity. Also, the mobility of oribatids is relatively limited, since they exist in the deep soil are restricted to moving through soil pores and can occasionally move by passive dispersal. Therefore, oribatid communities mainly develop through the process of ecosystem succession.

In a recent study, scientists carried out in a **chronosequence** of forests that are re-growing after cropland abandonment. They wanted to know if similar forests of different ages have the same soil communities. Scientists hypothesized that croplands probably had only a few oribatid species in low abundances, but that complex communities with high diversity would develop in older forests. Comparing current aerial photographs with others from the 1950s,
<caption>Figure 3</caption>

Ecosystems change and grow over time. In the 1950s (top), there were more abundant and rich communities of oribatids in the forest than in croplands. After crop abandonment (middle), individuals of some species from the forest arrived in the connected croplands mainly by passive dispersal (arrows). Finally, the disappearance (dashed arrow) of a few species, which probably needed a more developed soil, created the current differences among the three kinds of forests (bottom).

<figure><img src="image_248.png" /></figure>

scientists determined which areas had been forests in the 1950s (long-established forests), and which had been croplands (recent forests). Among the recent forests, we also distinguished between isolated forests (surrounded mainly by croplands and most likely showing oribatid communities similar to those found in croplands) and those connected to other forests (probably with oribatid communities similar to those found in old forests).

Scientists observed two important results. First, long-established and recent but connected forests showed similar numbers of oribatids and similar numbers of species, which were higher than what was seen in the isolated forests. Second, oribatid communities in isolated and connected recent forests shared more species than they did with long-established forests (Figure 3, bottom). Likely, oribatids arrive mainly by passive dispersal early in ecosystem development. That is probably why recent forests connected to long-established forests quickly establish oribatid communities similar to those in the long-established forests. But as the recent forest ecosystem continues to develop, lack of refuge availability and access to food may prevent some oribatid species from permanently settling there. This could explain why oribatid communities in recent and connected forests are more like those of isolated forests and thus to those of croplands (Figure 3).