Fertilizer use responsible for increase in nitrous oxide in atmosphere | Berkeley News Fertilizer use responsible for increase in nitrous oxide in atmosphere By Robert Sanders , Media relations | April 2, 2012 Robert Sanders University of California, Berkeley, chemists have found a smoking gun proving that increased fertilizer use over the past 50 years is responsible for a dramatic rise in atmospheric nitrous oxide, which is a major greenhouse gas contributing to global climate change. The Cape Grim Baseline Air Pollution Station in Tasmania, where air samples have been collected since 1978. These samples show a long-term trend in isotopic composition that confirms that nitrogen-based fertilizer is largely responsible for the 20 percent increase in atmospheric nitrous oxide since the Industrial Revolution. Photo courtesy of CSIRO. Climate scientists have assumed that the cause of the increased nitrous oxide was nitrogen-based fertilizer, which stimulates microbes in the soil to convert nitrogen to nitrous oxide at a faster rate than normal. The new study, reported in the April issue of the journal Nature Geoscience, uses nitrogen isotope data to identify the unmistakable fingerprint of fertilizer use in archived air samples from Antarctica and Tasmania.

“Our study is the first to show empirically from the data at hand alone that the nitrogen isotope ratio in the atmosphere and how it has changed over time is a fingerprint of fertilizer use,” said study leader Kristie Boering, a UC Berkeley professor of chemistry and of earth and planetary science. “We are not vilifying fertilizer. We can’t just stop using fertilizer,” she added. “But we hope this study will contribute to changes in fertilizer use and agricultural practices that will help to mitigate the release of nitrous oxide into the atmosphere.” Since the year 1750, nitrous oxide levels have risen 20 percent – from below 270 parts per billion (ppb) to more than 320 ppb. After carbon dioxide and methane, nitrous oxide (N2O) is the most potent greenhouse gas, trapping heat and contributing to global warming. It also destroys stratospheric ozone, which protects the planet from harmful ultraviolet rays. Not surprisingly, a steep ramp-up in atmospheric nitrous oxide coincided with the green revolution that increased dramatically in the 1960s, when inexpensive, synthetic fertilizer and other developments boosted food production worldwide, feeding a burgeoning global population.

Tracking the origin of nitrous oxide in the atmosphere, however, is difficult because a molecule from a fertilized field looks identical to one from a natural forest or the ocean if you only measure total concentration. But a quirk of microbial metabolism affects the isotope ratio of the nitrogen the N2O microbes give off, producing a telltale fingerprint that can be detected with sensitive techniques. Archived air from Cape Grim Boering and her colleagues, including former UC Berkeley graduate students Sunyoung Park and Phillip Croteau, obtained air samples from Antarctic ice, called firn air, dating from 1940 to 2005, and from an atmospheric monitoring station at Cape Grim, Tasmania, which has archived air back to 1978. Law Dome, Antarctica. Air trapped in the consolidated snow from this region provides historical air samples going back to 1940. Analysis of N2O levels in the Cape Grim air samples revealed a seasonal cycle, which has been known before. But isotopic measurements by a very sensitive isotope ratio mass spectrometer also displayed a seasonal cycle, which had not been observed before.

At Cape Grim, the isotopes show that the seasonal cycle is due both to the circulation of air returning from the stratosphere, where N2O is destroyed after an average lifetime of 120 years, and to seasonal changes in the ocean, most likely upwelling that releases more N2O at some times of year than at others. “The fact that the isotopic composition of N2O shows a coherent signal in space and time is exciting, because now you have a way to differentiate agricultural N2O from natural ocean N2O from Amazon forest emissions from N2O returning from the stratosphere,” Boering said. “In addition, you also now have a way to check whether your international neighbors are abiding by agreements they’ve made to mitigate N2O emissions. It is a tool that, ultimately, we can use to verify whether N2O emissions by agriculture or biofuel production are in line with what they say they are.” Changes in fertilizer use can reduce N2O emissions Limiting nitrous oxide emissions could be part of a first step toward reducing all greenhouse gases and lessening global warming, Boering said, especially since immediately reducing global carbon dioxide emissions is proving difficult from a political standpoint.

In particular, reducing nitrous oxide emissions can initially offset more than its fair share of greenhouse gas emissions overall, since N2O traps heat at a different wavelength than CO2 and clogs a “window” that allows Earth to cool off independent of CO2 levels. “On a pound for pound basis, it is really worthwhile to figure how to limit our emissions of N2O and methane,” she said. “Limiting N2O emissions can buy us a little more time in figuring out how to reduce CO2 emissions.” Finding the fingerprint of fertilized microbes Boering was able to trace the source of N2O because bacteria in a nitrogen-rich environment, such as a freshly fertilized field, prefer to use nitrogen-14 (14N), the most common isotope, instead of nitrogen-15 (15N). “Microbes on a spa weekend can afford to discriminate against nitrogen-15, so the fingerprint of N2O from a fertilized field is a greater proportion of nitrogen-14,” Boering said.

“Our study is the first to show empirically from the data at hand alone that the nitrogen isotope ratio in the atmosphere and how it has changed over time is a fingerprint of fertilizer use.” Just as telling is the isotope ratio of the central nitrogen atom in the N-N-O molecule. By measuring the nitrogen isotope ratio overall, the isotope ratio in the central nitrogen atom, and contrasting these with the oxygen-18/oxygen-16 isotope ratio, which has not changed over the past 65 years, they were able to paint a consistent picture pointing at fertilizer as the major source of increased atmospheric N2O . The isotope ratios also revealed that fertilizer use has caused a shift in the way soil microbes produce N2O. The relative output of bacteria that produce N2O by nitrification grew from 13 to 23 percent worldwide, while the relative output of bacteria that produce N2O by denitrification – typically in the absence of oxygen – dropped from 87 to 77 percent. Although the numbers themselves are uncertain, these are the first numerical estimates of these global trends over time, made possible by the unique archived air dataset of this study.

One approach, for example, is to time fertilizer application to avoid rain, because wet and happy soil microbes can produce sudden bursts of nitrous oxide. Changes in the way fields are tilled, when they are fertilized and how much is used can reduce N2O production. Boering’s studies, which involve analyzing the isotopic fingerprints of nitrous oxide from different sources, could help farmers determine which strategies are most effective. It could also help assess the potential negative impacts of growing crops for biofuels, since some feedstocks may require fertilizer that will generate N2O that offsets their carbon neutrality. “This new evidence of the budget of nitrous oxide allows us to better predict its future changes– and therefore its impacts on climate and stratospheric ozone depletion – for different scenarios of fertilizer use in support of rising populations and increased production for bio-energy,” said coauthor David Etheridge of the Centre for Australian Weather and Climate Research in Aspendale, Victoria. Boering’s colleagues include D. M. Etheridge, P. J. Fraser, P. B. Krummel, R. L. Langenfelds, L. P. Steele and C. M.

Trudinger of the Centre for Australian Weather and Climate Research; D. Ferretti of the National Institute of Water and Atmospheric Research in Wellington, New Zealand; K-R. Kim of the School of Earth and Environmental Sciences at Seoul National University in Korea; and T. D. van Ommen of the Australian Antarctic Division in Tasmania. Park is now at Seoul National University, while Croteau is at Aerodyne Research, Inc., in Billerica, Mass. The work was supported by UC Berkeley’s Atmospheric Sciences Center, NASA’s Upper Atmosphere Research Program, the Camille Dreyfus Teacher-Scholar Award, the Brain 21 Korea Program, a Korean government research grant through Seoul National University, and the Australian government’s Cooperative Research Centres Programme. For more information: Soil Management Nitrogen gas N2 Although 78% of our atmosphere is nitrogen gas, this form of nitrogen must be transformed to usable forms before it is available for plant uptake. Ammonia NH3 Ammonia is a gas. Ammonium can escape from the surface of the soil under certain conditions and is harmful to plants in high quantities. Ammonium is the basic building block of commercial nitrogen fertilizers.

Ammonium NH4+ Soil particles attract and retain ammonium on cation exchange complexes. This form may be directly taken up by plants. Nitrate NO3- Nitrate is the second form of nitrogen which is available for plant uptake. In most soils, nitrate is highly mobile. However, in the highly weathered soils of Hawaii, nitrate is stored in soils with ‘anion exchange capacity’ and becomes less mobile. Nitrite NO2- Nitrite is an intermediate product in the conversion of ammonium to nitrate (nitrification). It is usually present in low quantities, but is toxic to plants. Organic Nitrogen Various compounds Organic nitrogen must be converted to ammonium before it is used by plants. This conversion occurs with time and is known as mineralization.

Though complex, the nitrogen cycle: Helps us to understand the complex relationships that exist between the many forms of nitrogen Provides us with insight pertaining to the availability of ammonium and nitrate, which are the only nitrogen forms usable by plants To understand the many ways in which N may be lost from the soil In this section, we will discuss eight major transformations of nitrogen in the soil: nitrogen fixation, mineralization, immobilization, nitrification, denitrification, volatilization, and leaching. Nitrogen fixation Although atmospheric nitrogen gas(N2) makes up approximately 78% of the air, it cannot be directly used by plants. Instead, atmospheric N2 only becomes available to plants through three unique processes. The final product of each of these processes is ammonium, which is then available for plant uptake.

The three processes which convert atmospheric nitrogen to ammonium biological nitrogen fixation Nitrogen is deposited onto the earth’s surface by: Rain In the form of ammonium, nitrate, nitrite Finely divided organic N swept along the earth’s surface Lightning Responsible for approximately 10-20 % of soil nitrate (114, Fertilizers) Industrial wastes In Hawaii, as compared to the major industrial regions of the Mainland, industrial wastes do not significantly contribute to atmospheric N. Table 7. This table presents estimates of the different sources of atmospherically fixed nitrogen that was deposited onto the earth in the latter half of the twentieth century. Biological sources account for around 20% of total nitrogen deposition. Source: Singleton et al., 1993. The importance of legume-based BNF in world agriculture - an examination of the major commercial and environmental issues, IFDC Muscle Shoals Alabama. Nitrogen Mineralization in Soils When absorbed by plants, ammonium and nitrate are incorporated into plant cells as organic, or living, forms of nitrogen. When plants die, microorganisms break down, or decompose, dead plant cells.

During the decomposition about plant matter, organic nitrogen is once again converted to inorganic ammonium and released into the soil. The process that converts organic N to ammonium is called mineralization and plays a significant role in the management of nitrogen. To calculate the amount of nitrogen which can potentially be mineralized from your organic fertilizer source, click on the link below: Conditions affecting N mineralization The amount of ammonium that is released to the soil through mineralization depends on several factors: Quantity of Organic Nitrogen: The amount of organic nitrogen originally present in the organic matter determines the amount of N that can ultimately be mineralized. Temperature: The optimal range for mineralization to occur is between 77-95 degrees Fahrenheit. Oxygen: Microorganisms need oxygen and since microorganisms mediate mineralization, sufficient oxygen must be available in the soil. Moisture content: Ideally, water should fill 15 – 70 % of pore space for maximum mineralization. This roughly corresponds to field capacity. Ratio of carbon to nitrogen (C:N): The C:N ratio is a term used to describe the relative amount of total carbon in comparison the amount of total nitrogen present in the soil and/or organic matter.

This ratio is very important in determining the rate of mineralization that should occur for a given type of organic matter. Since the microorganisms living in the soil need both carbon and nitrogen, net mineralization occurs when C:N ratio is less than 20:1. This means for every two parts of carbon, there should be 1 part nitrogen for net mineralization. If you are applying organic amendments to your soil, it is important to become familiar with the C:N ratio to ensure N availability. After mineralization of N occurs, ammonium can be: Taken up by the plants Consumed by other organisms Volatilized Immobilization Immobilization is process that converts inorganic nitrogen to organic nitrogen. It is the reverse reaction of mineralization. Immobilization occurs when decomposing organic matter contains low amounts of nitrogen. Thus, immobilization occurs if the source of organic matter has a high C:N ratio. Microorganisms, who also need nitrogen to live, scavenge the soil for nitrogen when plant residues contain inadequate amounts of nitrogen. As inorganic ammonium and nitrate are incorporated into the cells of living microorganisms, the total N levels in the soil are reduced. Immobilization can ultimately result in nitrogen deficiencies.

When nitrogen is immobilized in the soil, there may be little nitrogen available for crop growth. As a result, plants can suffer from nitrogen deficiency and develop a yellow coloration. This is the reason why organic materials with a high C:N ratios, such as grass clippings and grain stover, are usually composted before they are incorporated into the soil or planting. This allows time for soil microorganisms to decompose the materials and begin to release nitrogen and other nutrients back into the soil Mineralization or Immobilization? The processes of mineralization and immobilization are constantly occurring simultaneously. As organic matter decomposes, inorganic nitrogen will be released into the soil. As both plants and microorganisms grow, they utilize the nitrogen in the soil. Once plants and microorganisms die, they decompose and release inorganic nitrogen to the soil through mineralization. Even though mineralization and immobilization are both occurring, we can determine which process, mineralization or immobilization, predominates. When mineralization occurs at a greater rate, we say that there is net mineralization. Likewise, when immobilization occurs to a greater extent, there is net immobilization. What determines whether there is net mineralization or net immobilization?

The answer is the C:N ratio of the decomposable organic matter. The C:N ratio is a characteristic of all organic matter, which includes: Crop residues Soil organic matter (including humus) Soil microorganisms (remember microorganism have both carbon and nitrogen) Rule of thumb When the C:N ratio of decomposing organic residues is between 20:1 and 30:1, mineralization and immobilization occur at fairly equal rates. Net mineralization occurs at C:N ratios less than 20:1. Net immobilization occurs at C:N ratios greater than above 30:1. Most well decomposed organic matter in soils have a C:N ration near 10:1 Management of organic residues If your program involves the addition of organic residues, it is important to know its C:N ratio. This knowledge allows you to predict whether net mineralization or net immobilization will occur. If the residue has a wide C:N ratio range, it may be necessary to apply additional amounts of nitrogen to your soil or choose a residue with a narrower range. Nitrification In most aerobic soils under optimal soil conditions, ammonium is rapidly converted to nitrate by soil bacteria through a process known as nitrification.

Nitrification involves two steps: First, ammonium is converted to nitrite Then, nitrite is converted to nitrate. As you can see from the outline of steps above, the intermediate product of nitrification is nitrite. If conditions are unfavorable to undergo the second step of nitrification, nitrite can leach into the ground water and pose as a health risk. The process of nitrification produces hydrogen ions. When large quantities of ammonium-containing fertilizers are applied to soil over time, this process can acidify the soil. See figure below for a simplified presentation of the nitrification process. Figure 10. Basic process that causes soil acidity by ammonium fertilizers. Source: Singleton, P. Nutrient Management Concepts: pH & Nutrient Formulation, University of Hawaii Cooperative Extension Service, Hilo Jul 25 2006. Factors affecting nitrification There are many factors that affect nitrification. Since nitrification is mediated by microorganisms, environmental factors that affect biological life will also influence nitrification.

In general, the optimal conditions for most plant growth are also the optimal conditions for nitrification: Presence of ammonium in the soil: In order for nitrification to occur, there must be a source of ammonium in the soil. Sources include mineralized ammonium or additions of ammonium-containing synthetic fertilizers Presence of microorganisms: Microorganisms that carry out nitrification must be present in the soil. Soil pH: The optimal pH for nitrification is 8.5, but it may occur over a fairly wide pH range. However, acidity (less than 5.5) has a detrimental effect on the nitrifying bacteria, thus reducing nitrification. Soil moisture: Nitrification is optimal at the field capacity of the soil. Nitrification is reduced at moisture levels greater and below field capacity. Field capacity is the amount of water that remains in the soil after free drainage in a saturated soil ceases. Field capacity is also the optimal soil moisture for most plant growth. Soil aeration: Nitrification requires oxygen. Any management factor that improves soil aeration, such as adding organic matter, will help optimize nitrification. Soil temperature: Nitrifying bacteria are sensitive to temperature.

The optimal temperature range for nitrification is between 77 and 95 degrees Fahrenheit. However, nitrification can occur between 41 and 95 degrees Fahrenheit. Environmental Considerations Nitrate is generally a very mobile in most soils. Excessive amounts of nitrate that are not taken up by plants is subject to leaching. Nitrate leaching can have an adverse effect on the environment. Nitrate, which moves through the soil profile during periods of rain, pollutes ground water reserves. Surface runoff of nitrate is a source of eutrophication, or algal blooms, in lakes and estuaries. To learn more about the possible negative effects associated with nitrate/nitrite and other elemental leaching on our water systems, click on the links below: Losses of ammonium due to leaching are minimal In contrast, nitrate is not retained by cation exchange capacity. Nitrate is highly immobile Losses of nitrate due to leaching can potentially be high Not only is nitrate leaching an economic loss to the farmer, it is also an environmental concern.

The conditions that lead to nitrate leaching follow: High rainfall intensity and distribution Highly irrigated fields Coarsely textured soils Nitrate Leaching in Maui soils In Hawaii, there are soils that can have a high anion exchange capacity under acidic conditions, which reduces nitrate leaching. This phenomenon primarily occurs in the acidic subsurface soil layers that have the greatest anion exchange capacity. As a result, researchers have found that subsurface soil layers in Hawaii can retain nitrate and may prevent it from leaching into the ground water. Acidification: Management of Nitrogen Fertilizers The fertilizer formulation can alter the pH of soil. Generally, fertilizers with high proportions of total nitrogen and are derived from ammonium sources (such as urea, ammonium sulfate, ammonium phosphate or ammonium nitrate) can acidify soils with repeated applications. Most fertilizers provide the “Lime Equivalent” on the bag’s label. The lime equivalent is the amount of limestone (calcium carbonate) it takes to neutralize the acidifying effects of using one ton of a particular fertilizer. In contrast, other fertilizers can increase soil pH. These fertilizers are usually low in ammonium, but high in nitrate.

Additionally, these fertilizers sometimes contain calcium from calcium nitrate. The lime equivalent is also given for these fertilizers, but it indicates the equivalent liming effect rather than the lime needed to offset acidity. The lime equivalent is only an estimate. The actual acidifying effect of the fertilizer is influenced, to some degree, by soil conditions that affect the transformation of the ammonium to nitrate and also by how much ammonium the plant assimilates before this transformation occurs. If growers know the history of their fertilizer applications over time, they can use the lime equivalent to predict when lime additions are likely needed. See the table below for some fertilizer formulations and note the relationship between the percentage of nitrogen in the fertilizer that is derived from ammonium and the lime equivalent needed to counter this acidifying effect. Table 8.

Fertilizer composition can change the pH of soil Fertilizer Google+ Sponsor Message <a href="" target="_top"><img style="border:0px;" src="" alt="i" width="300" height="250"/></a> With an overload of fertilizer, soil microbes on farms may belch unexpectedly high levels of nitrous oxide, a greenhouse gas with 300 times as much heat-trapping power as carbon dioxide. The finding may help explain why agricultural nitrous oxide emissions are much higher than some scientists had predicted and could give clues for how to curb farm pollution. Soil microbes have long been known to convert nitrogen-rich crop fertilizers, including manure and synthetic fertilizers, into nitrous oxide. After more than 1,000 field experiments, climate scientists calculated in the mid-2000s that the dirt dwellers spew about one kilogram of the greenhouse gas for every 100 kilograms of fertilizer, or roughly 1 percent. Researchers generally thought that emissions would scale up linearly: doubling fertilizer would double the emissions of gas. But the predictions didn’t match up with real-world numbers.

Estimating regional and global fluxes of atmospheric nitrous oxide levels a few years ago, researchers pegged the microbial conversion of fertilizer to gas at somewhere between 1.75 and 5 percent. Either the initial calculations were off or there were unknown sources of nitrous oxide, says biogeochemist Phil Robertson of Michigan State University in East Lansing. The latter was unlikely, he adds. From cornfield experiments in Michigan in 2005, Robertson and colleagues found that the relationship between the amount of fertilizer and production of greenhouse gas is not always linear. When farmers apply larger amounts of fertilizer than initially tested, the relationship appears exponential — applying 200 kilograms of fertilizer could result in four kilograms of gas, for instance. Most of the original field experiments, he says, didn’t look at microbial gas conversion with excess fertilizer, far more than crops need. In such situations, the crops take up as much nitrogen as they can, and the rest goes to soil microbes, Robertson says. “They effectively go to town on that nitrogen,” he says.

To see if the finding held up on a global scale, Robertson and colleagues reanalyzed more than 200 experiments, each examining nitrous oxide emissions of multiple fertilizer levels, across 84 locations worldwide. The results confirm that excess fertilizer can exponentially boost the emissions of microbes, the team reports June 9 in the Proceedings of the National Academy of Sciences. From this finding, Robertson infers that some farmers are using far more fertilizer than they need to, yielding more nitrous oxide than scientists had expected. The good news is that the study provides a clear path to lower nitrous oxide emissions, says soil scientist Timothy Griffis of the University of Minnesota in St. Paul. There are many easy methods for assessing how much nitrogen-based fertilizer is right for a crop field, so addressing overuse could be simple, he says. But the exponential relationship between fertilizer and gas still doesn’t entirely account for the emission discrepancy, says soil scientist Rod Venterea of the U.S. Department of Agriculture in St. Paul. There must be other factors upping the nitrous oxide in the atmosphere, he says, including unknown emissions from nitrogen fertilizer after it leaves farms via streams and erosion.

Citations From Salon to Salad: Human Hair Makes Good Plant Fertilizer | WIRED From Salon to Salad: Human Hair Makes Good Plant Fertilizer subscribe 6 months for $5 - plus a FREE Portable Phone Charger. On Twitter 5 hours A comedy about the apocalypse—penned by @neilhimself —is slated for 2018. So there's that to look forward to at least bit.ly/2jcWNvp Author: Alexis Madrigal. Alexis Madrigal Science Date of Publication: 12.29.08. Time of Publication: 12:50 pm. 12:50 pm From Salon to Salad: Human Hair Makes Good Plant Fertilizer Human hair could be used instead of chemical fertilizers for some plants like lettuce, new research in a horticultural journal suggests. The hair, which is manufactured into cubes from barbershop and hair-salon waste, provides nitrogen for plants as it decomposes, just as natural-gas-derived sources like ammonia do.

"Once the degradation and mineralization of hair waste starts, it can provide sufficient nutrients to container-grown plants and ensure similar yields to those obtained with the commonly used fertilizers in horticulture," said horticulturalist Vlatcho Zheljazkov of Mississippi State University. All plants need nitrogen to grow. These plants form the basis of the proteins which eventually make their way into our bodies either directly through the consumption either of the plants themselves or of animals raised on plants. Our bodies turn those proteins into all sorts of useful things — like muscles — and some less useful things, like hair. In fact, studies carried out in the 1960s found that human hair contains about 15 percent nitrogen [pdf]. Nitrogen is ridiculously abundant. It composes about 75 percent of the atmosphere, but plants can’t use it in its inert atmospheric form. Luckily, some microbes that grow with legumes pull this N2 out of the air and "fix it" into the reactive nitrogen that all living things use. Until the development of the Haber-Bosch process for cooking natural gas with more natural gas into nitrogen-rich ammonia, humans depended entirely on these microbes for the production of nitrogen.

We either rotated other crops with legumes or used the nitrogen left over from animal digestion as manure. The last 50 years, however, have seen enormous increases in the use of synthetic fertilizers using ammonia from the Haber-Bosch process. Vaclav Smil of the University of Manitoba, one of the world’s foremost experts on energy, estimates that 40 percent of the human population owes its existence [pdf] to synthetic fertilizers. But all that nitrogen takes a lot of energy to produce. Smil has also estimated that the Haber-Bosch process uses about 1 percent of the world’s total energy usage [xls], or about 150 gigawatts. That’s about as much coal power as the UNited States plans to add between now and 2030, so low-energy alternatives to traditional chemical fertilizers could help reduce energy usage. The new study, published in HortTechnology , shows that both lettuce and wormwood, the psychoactive ingredient in absinthe, grow about as well with hair as a fertilizer as they do with chemical fertilizers. The plants seem to be able to use about 50 percent of the nitrogen contained in the hair.

The only catch is that the hair takes a while to start decomposing and releasing nutrients into the soil, so it has to be paired with more fast-acting fertilizers. (And that some people might find decomposing hair fertilizing their salad greens a little gross.) Visionlearning | Earth Science | The Nitrogen Cycle The Nitrogen Cycle The Nitrogen Cycle: Of Microbes and Men by John Arthur Harrison, Ph.D. fertilizer : a substance added to soil to help plants grow microbial : relating to microorganisms such as bacteria species : a distinct type of plant or animal; a related group of living things that can produce offspring Glossary Terms N2 → NH4+ Nitrogen (N) is an essential component of DNA , RNA , and proteins , the building blocks of life. All organisms require nitrogen to live and grow. Although the majority of the air we breathe is N2, most of the nitrogen in the atmosphere is unavailable for use by organisms. This is because the strong triple bond between the N atoms in N2 molecules makes it relatively inert , or unreactive, whereas organisms need reactive nitrogen to be able to incorporate it into cells .

In order for plants and animals to be able to use nitrogen, N2 gas must first be converted to more a chemically available form such as ammonium (NH4+), nitrate (NO3-), or organic nitrogen (e.g., urea, which has the formula (NH2)2CO). The inert nature of N2 means that biologically available nitrogen is often in short supply in natural ecosystems , limiting plant growth. Nitrogen is an incredibly versatile element , existing in both inorganic and organic forms as well as many different oxidation states . The movement of nitrogen between the atmosphere , biosphere , and geosphere in different forms is called the nitrogen cycle (Figure 1), one of the major biogeochemical cycles. Similar to the carbon cycle, the nitrogen cycle consists of various reservoirs of nitrogen and processes by which those reservoirs exchange nitrogen (note the arrows in the figure). (See The Carbon Cycle module for more information.) Figure 1: The nitrogen cycle. Yellow arrows indicate human sources of nitrogen to the environment. Red arrows indicate processes in which microorganisms participate in the transformation of nitrogen. Blue arrows indicate physical forces acting on nitrogen. And green arrows indicate natural processes affecting the form and fate of nitrogen that do not involve microbes.

Processes in the nitrogen cycle Five main processes cycle nitrogen through the biosphere , atmosphere , and geosphere: nitrogen fixation , nitrogen uptake through organismal growth, nitrogen mineralization through decay , nitrification , and denitrification . Microorganisms, particularly bacteria , play major roles in all of the principal nitrogen transformations. Because these processes are microbially mediated, or controlled by microorganisms, these nitrogen transformations tend to occur faster than geological processes like plate motion, a very slow, purely physical process that is a part of the carbon cycle. Instead, rates are affected by environmental factors that influence microbial activity, such as temperature, moisture, and resource availability. Comprehension Checkpoint Processes within the nitrogen cycle progress at a _______ rate than geological processes like plate motion. a.faster b.slower Figure 2: Part of a clover root system bearing naturally occurring nodules of Rhizobium, bacteria that can transform atmospheric nitrogen through the process of nitrogen fixation. Each nodule is about 2-3 mm long. Image courtesy of image © The Microbial World Nitrogen fixation Nitrogen fixation is the process wherein N2 is converted to ammonium, or NH4+.

This is the only way that organisms can attain nitrogen directly from the atmosphere ; the few that can do this are called nitrogen-fixing organisms. Certain bacteria , including those among the genus Rhizobium, are able to fix nitrogen (or convert it to ammonium) through metabolic processes, analogous to the way mammals convert oxygen to CO2 when they breathe. Nitrogen-fixing bacteria often form symbiotic relationships with host plants. This symbiosis is well-known to occur in the legume family of plants (e.g., beans, peas, and clover). In this relationship, nitrogen-fixing bacteria inhabit legume root nodules (Figure 2) and receive carbohydrates and a favorable environment from their host plant in exchange for some of the nitrogen they fix. There are also nitrogen-fixing bacteria that exist without plant hosts, known as free-living nitrogen fixers. In aquatic environments, blue-green algae (really a bacteria called cyanobacteria) are an important free-living nitrogen fixer. In addition to nitrogen-fixing bacteria , high-energy natural events such as lightning, forest fires, and even hot lava flows can cause the fixation of smaller, but significant, amounts of nitrogen.

The high energy of these natural phenomena can break the triple bonds of N2 molecules , thereby making individual N atoms available for chemical transformation. Within the last century, humans have become as important a source of fixed nitrogen as all natural sources combined. Burning fossil fuels, using synthetic nitrogen fertilizers, and cultivating legumes all fix nitrogen. Through these activities, humans have more than doubled the amount of fixed nitrogen that is pumped into the biosphere every year (Figure 3), the consequences of which are discussed below. Figure 3: Recent increases in anthropogenic N fixation in relation to “natural” N fixation. Modified from Vitousek, P. M.. and Matson, P. A. (1993). Agriculture, the global nitrogen cycle, and trace gas flux. The Biogeochemistry of Global Change: Radiative Trace Gases. R. S. Oremland. New York, Chapman and Hall: 193-208. Nitrogen uptake NH4+ → Organic N The ammonium (NH4+) produced by nitrogen-fixing bacteria is usually quickly taken up by a host plant, the bacteria itself, or another soil organism and incorporated into proteins and other organic nitrogen compounds , like DNA .

When organisms nearer the top of the food chain (like us!) eat, we are taking up nitrogen that has been fixed initially by nitrogen-fixing bacteria. Nitrogen mineralization Organic N → NH4+ After nitrogen is incorporated into organic matter , it is often converted back into inorganic nitrogen by a process called nitrogen mineralization , otherwise known as decay . When organisms die, decomposers (such as bacteria and fungi) consume the organic matter and lead to the process of decomposition. During this process, a significant amount of the nitrogen contained within the dead organism is converted to ammonium. Once in the form of ammonium, nitrogen is available for use by plants or for further transformation into nitrate (NO3-) through the process called nitrification . Nitrification NH4+ → NO3- Some of the ammonium produced by decomposition is converted to nitrate (NO3-) via a process called nitrification . The bacteria that carry out this reaction gain energy from it. Nitrification requires the presence of oxygen, so nitrification can happen only in oxygen-rich environments like circulating or flowing waters and the surface layers of soils and sediments . The process of nitrification has some important consequences.

Ammonium ions (NH4+) are positively charged and therefore stick (are sorbed) to negatively charged clay particles and soil organic matter . The positive charge prevents ammonium nitrogen from being washed out of the soil (or leached) by rainfall. In contrast, the negatively charged nitrate ion is not held by soil particles and so can be washed out of the soil, leading to decreased soil fertility and nitrate enrichment of downstream surface and groundwater . Denitrification NO3- → N2+ N2O Through denitrification , oxidized forms of nitrogen such as nitrate (NO3-) and nitrite (NO2-) are converted to dinitrogen (N2) and, to a lesser extent, nitrous oxide gas (NO2). Denitrification is an anaerobic process that is carried out by denitrifying bacteria , which convert nitrate to dinitrogen in the following sequence: NO3- → NO2- → NO → N2O → N2. Nitric oxide and nitrous oxide are gases that have environmental impacts. Nitric oxide (NO) contributes to smog , and nitrous oxide (N2O) is an important greenhouse gas , thereby contributing to global climate change.

Once converted to dinitrogen, nitrogen is unlikely to be reconverted to a biologically available form because it is a gas and is rapidly lost to the atmosphere . Denitrification is the only nitrogen transformation that removes nitrogen from ecosystems (essentially irreversibly), and it roughly balances the amount of nitrogen fixed by the nitrogen fixers described above. Comprehension Checkpoint Which process returns nitrogen gas to the atmosphere? a.nitrification The Nitrogen Cycle About Watch and Favorite Watch Watching this resources will notify you when proposed changes or new versions are created so you can keep track of improvements that have been made. Favorite Favoriting this resource allows you to save it in the “My Resources” tab of your account. There, you can easily access this resource later when you’re ready to customize it or assign it to your students. The Nitrogen Cycle The nitrogen cycle is the process by which nitrogen is converted from organic to inorganic forms; many steps are performed by microbes. Learning Objective Describe the nitrogen cycle and how it is affected by human activity Key Points Nitrogen is converted from atmospheric nitrogen (N2) into usable forms, such as NO2-, in a process known as fixation .

The majority of nitrogen is fixed by bacteria , most of which are symbiotic with plants. Recently fixed ammonia is then converted to biologically useful forms by specialized bacteria. This occurs in two steps: first, bacteria convert ammonia in to (nitrites) NO2-, and then other bacteria species convert it to NO3- (nitrate). Nitriates are a form of nitrogen that is usable by plants. It is assimilated into plant tissue as protein . The nitrogen is passed through the food chain by animals that consume the plants, and then released into the soil by decomposer bacteria when they die. De-nitrifying bacteria convert NO2- back into atmospheric nitrogen (N2), completing the cycle. Terms Register for FREE to remove ads and unlock more features! Learn more Full Text The nitrogen cycle describes the conversion of nitrogen between different chemical forms. The majority of the earth's atmosphere (about 78%) is composed of atmospheric nitrogen, but it is not in a form that is usable to living things. Complex species interactions allow organisms to convert nitrogen to usable forms and exchange it between themselves. Nitrogen is essential for the formation of amino acids and nucleotides . It is essential for all living things.

Fixation: In order for organisms to use atmospheric nitrogen (N2), it must be "fixed" or converted into ammonia (NH3). This can happen occasionally through a lightning strike, but the bulk of nitrogen fixation is done by free living or symbiotic bacteria. These bacteria have the nitrogenase enzyme that combines gaseous nitrogen with hydrogen to produce ammonia. It is then further converted by the bacteria to make their own organic compounds. Some nitrogen fixing bacteria live in the root nodules of legumes where they produce ammonia in exchange for sugars. Today, about 30% of the total fixed nitrogen is manufactured in chemical plants for fertilizer . BioLINCS | Nitrogen Cycling in the Open Ocean C-MORE Home | Cruises | BioLINCS Home | Nitrogen Cycling Nitrogen Cycling in the Open Ocean Nitrogen is one of the most important elements for biological systems. While we think of air as containing oxygen and maybe a little carbon dioxide, it’s mostly (80 %) nitrogen. Nitrogen is also one of the basic elements of life, along with carbon, hydrogen, phosphorus, and oxygen. Although nitrogen is incredibly abundant in the air we breathe, it is often a limiting nutrient for the growth of living organisms.

This is because the particular form of nitrogen found in air—nitrogen gas—cannot be assimilated by most organisms. Most plants, algae, and microbes require biologically available, or “fixed” forms of nitrogen, such as ammonia or ammonium. That’s why these compounds are a primary component of many plant fertilizers. In the open ocean, as on land, fixed nitrogen is one of the most important growth-limiting nutrients for photosynthetic organisms (primary producers) such as algae and marine bacteria. Nitrogen can also serve as an energy source or as an oxidant for marine bacteria and archaea. (Archaea are single celled organisms that look similar to bacteria, but which are in an entirely separate biological domain). The ocean absorbs nitrogen gas from the atmosphere. In open-ocean areas with low concentrations of nutrients (“oligotrophic” regions), some of this nitrogen is taken up by microbes and transformed into various chemical compounds. The key steps in this process are listed below and shown in the simplified diagram at right (click on it to see a larger version). Nitrogen gas (N2) from the atmosphere dissolves into seawater at the ocean surface.

Nitrogen gas is the most abundant form of nitrogen in the ocean, but is not useful to most living things. Dissolved nitrogen gas is taken up by just a few types microbes, which convert the nitrogen into a much more useable form, known as ammonium (NH4+). This process, known as “nitrogen fixation,” is vitally important. Without it, very little nitrogen would available for thousands of other organisms that live near the ocean surface. Ammonium is the form of nitrogen that is most easily consumed by microorganisms. For this reason, ammonium is consumed almost as fast as it is produced, a process called “assimilation.” The result is that the nitrogen becomes incorporated into the cells of living organisms. Some marine microbes consume nitrite and nitrate, another form of assimilation. When microbes (and other organisms) die, they decompose, releasing ammonium and tiny particles containing particulate organic nitrogen (PON), as well as dissolved organic nitrogen (DON) into the surrounding seawater. Some microbes convert ammonium to nitrite (NO2-) and then nitrite to nitrate (NO3-).

This two-step process is called “nitrification.” The result of this process is that nitrate is released into the ocean. A host of organisms consume particulate organic nitrogen and dissolved organic nitrogen, converting some of the nitrogen back to ammonium. This process is called “remineralization.” To complete this complex cycle (which begins to look more like a Celtic knot), some microbes convert nitrate and nitrite back to nitrogen gas through a process called “denitrification.” The open ocean is host to a wide variety of microbes, each adapted to a perform one or more steps in the nitrogen cycle. This enormous diversity of microbes has evolved to use every form of nitrogen in the ocean, either as a source of energy or as a nutrient. However, “reduced” nitrogen compounds, such as ammonium are easier for most microbes to use than “oxidized” nitrogen compounds such as nitrate. Many types of microbes extract energy from nitrogen compounds by “oxidizing” these compounds (for example, converting ammonium to nitrite or nitrite to nitrate). However, relatively few microbes can “reduce” nitrogen (for example converting nitrogen gas to ammonium), because this requires extra energy, which they must get from other sources (“food” and/or sunlight).

For more information on some the different types of nitrogen-processing microbes we’ll be studying during the BioLINCS cruise, see the marine microbes page. Slick Solution: How Microbes Will Clean Up the Deepwater Horizon Oil Spill - Scientific American Scientific American The Sciences Slick Solution: How Microbes Will Clean Up the Deepwater Horizon Oil Spill Bacteria and other microbes are the only thing that will ultimately clean up the ongoing oil spill in the Gulf of Mexico By David Biello on May 25, 2010 Credit: Courtesy of Heimholtz Center for Infection Research (HZI) Advertisement | Report Ad The last (and only) defense against the ongoing Deepwater Horizon oil spill in the Gulf of Mexico is tiny—billions of hydrocarbon-chewing microbes, such as Alcanivorax borkumensis . In fact, the primary motive for using the more than 830,000 gallons of chemical dispersants on the oil slick both above and below the surface of the sea is to break the oil into smaller droplets that bacteria can more easily consume.

"If the oil is in very small droplets, microbial degradation is much quicker," says microbial ecologist Kenneth Lee, director of the Center for Offshore Oil, Gas and Energy Research with Fisheries and Oceans Canada, who has been measuring the oil droplets in the Gulf of Mexico to determine the effectiveness of the dispersant use . "The dispersants can also stimulate microbial growth. Bacteria will chew on the dispersants as well as the oil." For decades scientists have pursued genetic modifications that might enhance these microbes' ability to chew up oil spills, whether on land or sea. Even geneticist Craig Venter forecast such an application last week during the unveiling of the world's first synthetic cell , and one of the first patents on a genetically engineered organism was a hydrocarbon-eating microbe, notes microbiologist Ronald Atlas of the University of Louisville. But there are no signs of such organisms put to work outside the lab. "Microbes are available now but they are not effective for the most part," says marine microbiologist Jay Grimes of the University of Southern Mississippi. At this point, there are no man-made microbes that are more effective than naturally occurring ones at utilizing hydrocarbons.

The natural world is replete with a host of organisms that combine as a community to decompose oil—and no single microbe, no matter how genetically enhanced, has proved better than this natural defense. "Every ocean we look at, from the Antarctic to the Arctic, there are oil-degrading bacteria," says Atlas, who evaluated genetically engineered microbes and other cleanup ideas in the wake of the Exxon-Valdez oil spill in Alaska. "Petroleum has thousands of compounds. It's complex and the communities that feed on it are complex. A superbug fails because it competes with this community that is adapted to the environment." Nor is it easy to help the existing communities of thousands of microbes, such as various species of Vibrio and Pseudomonads, to eat the oil faster—seeding experiments have generally failed. "Microbes are a lot like teenagers, they are hard to control," says marine chemist Chris Reddy of the Woods Hole Oceanographic Institution . "The concept that nature will eat it all up is not accurate, at least not on the time scale we're worried about." Just like your automobile, these marine-dwelling bacteria and fungi use the hydrocarbons as fuel—and emit the greenhouse gas carbon dioxide (CO2) as a result.

In essence, the microbes break down the ring structures of the hydrocarbons in seaborne oil using enzymes and oxygen in the seawater. The end result is ancient oil turned into modern-day bacterial biomass—populations can grow exponentially in days. "Down in the Gulf of Mexico there is an indigenous population [of microbes] adapted to oil from so much marine traffic and daily spills. Oil is not new," says Lee, who has also been monitoring the plumes of oil beneath the surface . "There are so many natural seeps around the world that if it wasn't for microbes we would have a lot of oil in the oceans." Already, measurements of oxygen depletion of as much as 30 percent in the Gulf of Mexico seawater suggest that the microbes are hard at work eating oil. "I take the 30 percent depletion of oxygen in water near the oil as indicating bacterial degradation," Atlas says. That happens best near the surface, whether at land or sea, where warm-water bacteria such as Thalassolituus oleivorans can thrive; colder, deeper waters inhibit microbial growth.

"Metabolism slows by about a factor of two or three for every 10 degree[s] Celsius you drop in temperature," notes biogeochemist David Valentine of the University of California, Santa Barbara, who just received funding from the National Science Foundation to characterize the microbial response to the ongoing oil spill . "The deeper stuff, that's going to happen very slowly because the temperature is so low." Unfortunately, that's exactly where some of the Deepwater Horizon oil seems to be ending up. "They saw the oil at 800 to 1,400 meters depth," says microbial ecologist Andreas Teske of the University of North Carolina at Chapel Hill, whose graduate student Luke McKay was on the research vessel Pelican that first reported such subsurface plumes—as predicted by small-scale experiments, such as the U.S. Minerals Management Services Project "Deep Spill" . "It is either at the surface or hanging in the water column and possibly sinking down to the sediment." Yet, microbes are the only process to break down the oil deeper in the water, far away from physical processes on the surface such as evaporation or waves.

"The deep waters are dominantly microbial" when it comes to oil degradation , although these communities are not as well studied as those at the surface, notes microbial geochemist Samantha Joye of the University of Georgia. "As long as there is oxygen around, it will get chewed up." To understand how the microbes will work and how quickly, however, will require a better understanding of exactly how much oil is out there. "It's a function of size, and we don't know size," Joye says. "We need to know how much oil is leaking out. Without that information we can't begin to make any kind of calculation of potential oxygen demand or anything else." BP now admits that its original estimate of roughly 200,000 gallons per day was far too low without providing an alternative; independent experts have offered estimates as high as four million gallons per day. It is possible to add fertilizers, such as iron, nitrogen and phosphorus, to stimulate the growth of such bacteria, an approach used to speed up microbial activity in the sediment along the Alaska coast after the Exxon-Valdez spill .

"We saw a three to five times increase in rate of biodegradation," Atlas says, suggesting the technique might prove effective along the oil-inundated Louisiana coast as well. "It was hundreds of miles of shoreline, the largest bioremediation project ever." But that's strictly onshore. "In the ocean, how do you keep the nutrients with the oil?" Lee asks. "It's much easier to add to soil. That's why you don't see bioremediation in the open ocean." And aerating soils in wetlands can have its own problems; Lee tried tilling oil-soaked wetlands in Nova Scotia where there was limited oxygen to increase microbial activity. "That didn't work. We had large erosion as a result," he says. "If the oil reaches shore, our recommendation was to leave the oil alone and let nature do it." But sediment, whether the muck of Louisiana marshland or the deep ocean seafloor, suffers from a dearth of oxygen. That means it's up to anaerobic microbes— ancient organisms that live via sulfate rather than oxygen—to do the dirty work of consuming the spill.

"What occurred in 10 days aerobically, took 365 days to occur anaerobically," says Atlas of the breakdown of oil in the wake of the Amoco Cadiz spill off the coast of France in 1978. Adds Teske: "The heavy components are sinking to the sediment and forming an oily or tarry carpet or getting buried. Then they are much harder to degrade." Such anaerobic environments can develop locally in the seawater itself, thanks to a ready supply of oil and blooming microbes eager to devour it. In deepwater, where there's less mixing with the surface waters to provide fresh supplies of oxygen, a dead zone may result. "It's not exchanging with the atmosphere," Joye notes. "Once the oxygen is gone, how are you going to replace it? It's not going to get mixed up by winter storms." That's bad news for the speedy breakdown of oil as well as for the Lophelia coral and other sessile deepwater life.

At the same time, the addition of 130,000 gallons of dispersants deep beneath the surface is having uncertain effects; it may even end up killing the microbes it is meant to help thanks to the fact that Corexit 9527A contains the solvent 2-butoxyethanol, which is a known human carcinogen and toxic to animals and other life. But the U.S. Environmental Protection Agency, National Oceanic and Atmospheric Administration and others are monitoring whether adding such dispersants ends up boosting microbe-growth and hence dangerously depletes oxygen levels, among other potential environmental ill effects. Nor is it clear how fast the microbial community will respond. "Which microbial communities are the fastest responders?" Teske asks. "That would be interesting to know" and this oil spill may provide the real- world answer. Some research suggests that oil spills may actually feed themselves nitrogen by stimulating the growth of various bacteria that fix the vital nutrient , Joye notes. At the same time, microbial predators such as protozoa tend to dampen the efficiency of would-be oil-eating microbes.

Scientists are still working to deploy known oil-eaters, such as Alcanivorax, in the form of booms laced with slow-release fertilizer and the microbes. In experiments such microbial booms ate heavy fuel oil in two months and "the experimental waste water was clean enough to be released back to the sea," says environmental geneticist Peter Golyshin of Bangor University in Wales. But "in the Gulf of Mexico, the amount of oil is simply too big. The oil gets dispersed but there is not enough [nitrogen] and [phosphorus] to feed bacterial growth." Ultimately, it is only microbes that can remove the oil from the ocean. "In the long run, it's biodegradation that removes most of the oil from the environment in these situations," Lee says. Or, as Joye puts it, "They're clever, they're tough, they can basically eat nails…. The microbes have to save us again." Regardless, the oil will linger in the environment for a long time . The microbes break down hydrocarbons in "weeks to months to years, depending on the compounds and concentrations—not hours or days," Atlas notes. "Much of the real tar or asphalt compounds are not readily subject to microbial attack….

Tar tends to persist. Asphalt tends to persist." Adds Valentine: "We wouldn't make roads out of them if the bacteria ate them."

Question: Which gas in the atmosphere can be turned into fertilizer by some microbes?
Answer: Nitrogen