The Wonderful Critters Alice saw down the Rabbit Hole
By: Peter Miles
Tags: Science Education, Soil Health, Bacteria, Agriculture, Climate Change

The Wonderful Critters Alice saw down the Rabbit Hole

Image of White Rabbit from Wikimedia Commons. Alice scene create by author on Doodley.

In Lewis Carroll’s 1865 book Alice in Wonderland, when Alice and the White Rabbit, who was wearing a waist coat and running very late, were about to jump down the rabbit hole, they may have been standing on a layer of grasses and mulch that was full of small critters. These are the micro and macro-organisms that live in the top layer above the mineral component of soil, what soil scientists call the 0 layer of a soil horizon.

Amongst the grass and leaf litter, that is the dead leaves, twigs, floral parts and seeds, live many macro-organisms such as termites, earthworms, beetles, millipedes, slugs, snails, ants, cockroaches, cicada nymphs, spiders, pseudoscorpions, slaters, nematodes, together with soil fungi and fungi fruiting bodies, all eating the organic material in the mulch and taking it underground to release nutrients into the soil. The micro-organisms are mainly bacteria and algae but Alice will meet many more micro-organisms in the next soil layer.

Macro-organisms feeding on the mulch physically mix particles of organic matter with the mineral soil or they eat the organic matter, digest it and excrete it into the soil at lower levels. Or the micro-organisms partly decompose the organic matter first, for macro-organisms to eat it. In both cases it is then available to other micro-organisms to further decompose the organic matter.

The decomposition of mulch is of vital importance to soil fertility as many soils have low levels of nutrients, such as nitrogen and phosphorus. These nutrients are present in the dead plant material and are recycled back into the soil by the organisms feeding on the mulch and as such made available to plants again.

The leaf litter also protects the soil from wind and water erosion and enables good soil structure by binding soil mineral particles together, these larger aggregates then hold more air and water (Land for Wildlife, 2011).

The White Rabbit led Alice a little further down the rabbit burrow into the first mineral layer of the soil, the A horizon. The soil here is generally darker in colour than further down because of all the organic matter and there are lots of micro-organisms busily decomposing dead plant material as it is moist and still quite warm.

Soil profile. Image — Wikimedia Commons.

The micro-organisms generally can’t be seen with your eyes but Alice found a little bottle labelled Drink Me, which she did and shrunk down to a really small size so she could see the tiny micro-organisms. White rabbits wearing waist coats, reading watches and deciding that they are late, can generally find it easy to do simple tasks like changing size, so he to made himself really small as well. Together they met the bacteria, which are one celled but there are so many it is estimated that the bacteria in a hectare of soil would be the equivalent of 5 cows on the surface.

Soil bacteria are divided into 4 groups, most are decomposers, those that consume the carbon in the organic matter and also retain nutrients such as nitrogen in their cell preventing it from being leached away by soil water. Mutualists make up the second group and they have relationships with plants that benefit both bacteria and plants, such as the nitrogen fixing bacteria that live in root nodules grown by the plant, also the plant provides carbon, the bacteria fix atmospheric nitrogen into a form that can be used by the plant. A third group are the pathogens that can cause disease in plants. The fourth group is the chemoautotrophs which live on nitrogen, hydrogen, sulphur and iron; they are important in the nitrogen cycle and for breaking down pollutants, such as when there is an oil spill.

One large group of decomposer bacteria are the actinomycetes which look like fungal hyphae and are important for breaking down the hard to decompose material like cellulose and lignin and are more common at high pH levels. The actinomycetes bacteria give off the earthy smell when one digs into healthy soil (USDA, 2020a).

The white rabbit tapped Alice on her elbow, the only place that rabbits should touch girls if they need to get their attention without saying anything, he didn’t want to interrupt as Alice was saying hello to the bacteria. She was saying polite things like ‘nice to meet you’ and ‘my compliments to your family’ but he knew there were millions of bacteria and that she would be there all-day saying hello to everyone, so he had better interrupt.

He then introduced Alice to the fungi, these are a little larger than bacteria and often are in the shape of hyphae, long narrow threads that can push their way into soil particles to access nutrients such as phosphorous, present in soil as phosphate.

Some fungi are decomposers, these occur more in soils with low pH whereas the bacterial decomposers occur more with high pH, some are disease pathogens and others have a mutualistic relationship with plants on the plant roots. A common one is the Vesicular Arbuscular Mycorrhizal VAM mutualistic fungus that penetrates the roots of a vascular plant in order to help them to capture nutrients from the soil by accessing soil particles which the larger plant root hairs can’t access, the fungi get carbon from the plant in return. Fungi are vitally important to soil health and structure, plant roots together with VAM fungal hyphae hold soil particles together to make large aggregates, imagine a sticky string bag holding particles together. These larger aggregates make for good soil structure. VAM also produce a glycoprotein called glomalin which acts as a glue between soil particles and is hydrophobic, stable in water (Miller & Jastrow, 2000; Rillig & Mummey, 2006).

Soil aggregates. Image — Imaggeo European Geosciences Union. Wikimedia Commons

Soil fungi can be quite extensive. Indeed, the largest organism in the world is a soil fungus Armillaria ostoyae in the Malheur National Forest of Oregon USA and covers an area of almost 9 square kilometres (Minter, 2015). This particular fungi is antagonistic to trees not mutualistic.

Fungi are aerobic, they need the oxygen present in aerated soils with good structure, if soil becomes anaerobic for long periods it is likely to lose its fungal component. Anaerobic conditions can occur in waterlogged soil and in compacted soils, compaction can result from mechanical cultivation such as ploughing and is one reason why this practice is decreasing or not practiced, as in regenerative farming (USDA, 2020b). Chemical fertilizers can temporarily increase pH, osmotic potential — the ability to move nutrients through a cell membrane, and nitrogen concentrations to levels that reduce micro-organism population levels (Geisseler & Scow, 2014).

There were still the protozoa to introduce to Alice but the rabbit kept looking at his watch and saying he was late. Protozoa are single celled but much larger than bacteria, they feed upon bacteria, thereby keeping the number of bacteria in check and also releasing the nitrogen stored in the bacteria cells, which the bacteria weren’t too happy about. It was a little socially awkward to move over to the protozoa as the bacteria were still watching, so the rabbit led Alice down to the B horizon where the plant roots are.

In the A horizon they had just left, there were plenty of plant roots, it was just easier to see them in the lighter colour soil of the B horizon. The roots looked huge to Alice being as small as she currently was, but she could see the root hairs which are elongated cells from the root that do most of the absorbing of soil moisture and nutrients. On the end of the roots was the hyphae of mycorrhizae fungi they had seen before but here when there were so many altogether, they are called a mycelium.

The White Rabbit was just about to introduce Alice to the plants roots when he noticed the bacteria along with the protozoa and nematodes all around the roots, this root zone is called the rhizosphere and is where much soil activity and absorption of nutrients occurs. As we know the protozoa eat the bacteria and so do the nematodes but they all seemed to be getting along together in their tiny ecosystem, so the rabbit thought it best to keep quiet and observe.

After a polite time, the White Rabbit and Alice moved a little further down the burrow but it was getting deeper into the C soil horizon and the soil was cold, with not so much micro-organic life because of the drop in temperature. The C horizon is often clay with some rocks mixed in with the soil. There were still a few plant roots absorbing the deep soil moisture that was mostly always available even in summer, except after a long drought and they were also absorbing the plant nutrients that had leached down with the water from above.

The White Rabbit said further down was more C horizon and then the R horizon which stood for bedrock and it was continuous rock that the soil was made from over a long period of time. He looked at his watch again and muttered something about being late and said let’s go back up. Alice was pleased to start walking up to the surface again, it was very interesting and nice here, in an earthy smelly kind of way but she liked the sunshine. When they reached the A horizon, Alice had another sip from the Drink Me bottle and grew back to normal size, while magically the White Rabbit grew back as well.

They climbed out onto the grass and mulch into the sunshine, where upon the White Rabbit checked his pocket watch and said I am very late and with a ‘nice to meet you’, he ran off and disappeared down another rabbit hole. Alice thought about how many critters she had just met, too many to count. She then looked up at the trees and their green leaves with the sun shining on them and the process occurring inside the leaves on which all life on earth depends, photosynthesis, but that’s another story.

Sunshine and photosynthesis. Red flowering gum, Corymbia ficifolia. Image by author.

References:

Geisseler, D., & Scow, K. M. (2014). Long-term effects of mineral fertilizers on soil microorganisms–A review. Soil Biology and Biochemistry, 75, 54–63.

Land for Wildlife. (2011). Note S1: Healthy Soil and Leaf Litter Layers. LFW-Note-2016_S1.pdf (lfwseq.org.au)

Miller, R. M., & Jastrow, J. D. (2000). Mycorrhizal fungi influence soil structure. In Arbuscular mycorrhizas: physiology and function (pp. 3–18). Springer, Dordrecht.

Minter, D. (2015). A Future for Fungi: The Orphans of Rio. International Society for Fungal Conservation. orphans-of-rio.pdf (fungal-conservation.org)

Rillig, M. C., & Mummey, D. L. (2006). Mycorrhizas and soil structure. New Phytologist, 171(1), 41–53.

USDA. (2020a). Soil Bacteria. Soil Bacteria | NRCS Soils (usda.gov)

USDA. (2020b). Soil Fungi. Soil Fungi | NRCS Soils (usda.gov)

USDA. (2020c). Soil Protozoa. https://www.nrcs.usda.gov/wps/portal/nrcs/detailfull/soils/health/biology/?cid=nrcs142p2_053867