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20.4: The Nitrogen Cycle

  • Page ID
    69925
    • Boundless
    • Boundless
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    Learning Objectives

    • Describe the nitrogen cycle

    Key Points

    • Bacteria, such as cyanobacteria, convert nitrogen into nitrogen gas via nitrogen fixation.
    • Nitrogen fixation occurs in three steps: ammonification, nitrification, and denitrification.
    • Human activity can release nitrogen into the environment by the combustion of fossil fuels and by the use of artificial fertilizers in agriculture.
    • Atmospheric nitrogen is responsible for acid rain, the release of greenhouse gasses, and eutrophication.
    • Nitrogen fixation can be performed by marine bacteria; nitrogen falls to the ocean floor as sediment and is then moved to land, becoming incorporated into terrestrial rock.

    Key Terms

    • denitrification: process of converting nitrates into nitrogen gas, especially by the action of bacteria
    • nitrification: the conversion of ammonium into nitrites (NO2−) by nitrifying bacteria
    • ammonification: the formation of ammonia or its compounds from nitrogenous compounds, especially as a result of bacterial decomposition

    The Nitrogen Cycle

    All organisms require nitrogen because it is an important component of nucleic acids, proteins, and other organic molecules. Getting nitrogen into the living world is difficult. Plants and phytoplankton are not equipped to incorporate nitrogen from the atmosphere (which exists as tightly-bonded, triple-covalent N2), even though this molecule comprises approximately 78 percent of the atmosphere. Nitrogen enters the living world through nitrogen fixation (Figure \(\PageIndex{1-2}\), the process of converting nitrogen gas into ammonia (NH3), which spontaneously becomes ammonium (NH4+). Ammonium is found in bodies of water and in the soil (figure \(\PageIndex{1-2}\)).   

    The illustration shows the nitrogen cycle. Nitrogen gas from the atmosphere is fixed into organic nitrogen by nitrogen fixing bacteria. This organic nitrogen enters terrestrial food webs. It leaves the food webs as nitrogenous wastes in the soil. Ammonification of this nitrogenous waste by bacteria and fungi in the soil converts the organic nitrogen to ammonium ion (NH4 plus). Ammonium is converted to nitrite (NO2 minus), then to nitrate (NO3 minus) by nitrifying bacteria. Denitrifying bacteria convert the nitrate back into nitrogen gas, which reenters the atmosphere. Nitrogen from runoff and fertilizers enters the ocean, where it enters marine food webs. Some organic nitrogen falls to the ocean floor as sediment. Other organic nitrogen in the ocean is converted to nitrite and nitrate ions, which is then converted to nitrogen gas in a process analogous to the one that occurs on land.
    Figure \(\PageIndex{1}\): Nitrogen enters the living world from the atmosphere through nitrogen-fixing bacteria. This nitrogen and nitrogenous waste from animals is then processed back into gaseous nitrogen by soil bacteria, which also supply terrestrial food webs with the organic nitrogen they need. (credit: modification of work by John M. Evans and Howard Perlman, USGS)

    A section of soil with plants and animals on the surface shows each step of the nitrogen cycle.

    Figure \(\PageIndex{2}\): In the nitrogen cycle, nitrogen-fixing bacteria in the soil or legume root nodules convert nitrogen gas (N2) from the atmosphere to ammonium (NH4+). Nitrification occurs when bacteria convert ammonium to nitrites (NO2-) and then to nitrates (NO3-). Nitrates re-enter the atmosphere as nitrogen gas through denitrification by bacteria. Plants assimilate ammonium and nitrates, producing organic nitrogen, which is available to consumers. Decomposers, including aerobic and anaerobic bacteria and fungi, break down organic nitrogen and release ammonium through ammonification. (credit: “Nitrogen cycle” by Johann Dréo & Raeky is licensed under CC BY-SA 3.0)

    Three processes are responsible for most of the nitrogen fixation in the biosphere. The first is atmospheric fixation by lightning. The enormous energy of lightning breaks nitrogen molecules and enables their atoms to combine with oxygen in the air forming nitrogen oxides. These dissolve in rain, forming nitrates, that are carried to the earth. Atmospheric nitrogen fixation probably contributes some 5-8% of the total nitrogen fixed. The second process is industrial fixation. Under great pressure, at a temperature of 600°C (1112°F), and with the use of a catalyst (which facilitates chemical reactions), atmospheric nitrogen and hydrogen can be combined to form ammonia (NH3). Ammonia can be used directly as fertilizer, but most of it is further processed to urea and ammonium nitrate (NH4NO3).

    The third process is biological fixation by certain free-living or symbiotic bacteria, which incorporate nitrogen into their macromolecules. Cyanobacteria live in most aquatic ecosystems where sunlight is present; they play a key role in nitrogen fixation. Nitrogen-fixing cyanobacteria are essential to maintaining the fertility of semi-aquatic environments like rice paddies. Free-living bacteria, such as Azotobacter, are also important nitrogen fixers. Some nitrogen fixing bacteria form a symbiotic relationship with plants in the legume family, which includes beans, peas, soybeans, alfalfa, and clovers (figure \(\PageIndex{3}\)). Rhizobium bacteria live symbiotically in the root nodules of legumes (such as peas, beans, and peanuts) and provide them with the organic nitrogen they need. Some nitrogen-fixing bacteria even establish symbiotic relationships with animals, e.g., termites and "shipworms" (wood-eating bivalves). Although the first stable product of the process is ammonia, this is quickly incorporated into protein and other organic nitrogen compounds.

    A dirty soybean root with spherical root nodules. Secondary roots branch off the primary roots.

    Figure \(\PageIndex{3}\): Nitrogen-fixing bacteria live in the spherical nodules of this soybean root. Image by United Soybean Board (CC-BY).

    Organic nitrogen is especially important to the study of ecosystem dynamics since many ecosystem processes, such as primary production and decomposition, are limited by the available supply of nitrogen. Plants and other producers directly use ammonium and nitrates to make organic molecules through the process of assimilation (Figure \(\PageIndex{2}\)). This nitrogen is now available to consumers. Consumers excrete organic nitrogen compounds that return to the environment. Additionally dead organisms at each trophic level contain organic nitrogen.

    As shown in Figure \(\PageIndex{2}\), the nitrogen that enters living systems by nitrogen fixation is eventually converted from organic nitrogen back into nitrogen gas by bacteria. This process occurs in three steps in terrestrial systems: ammonification, nitrification, and denitrification. First, the ammonification or nitrogen mineralization process converts nitrogenous waste from living organisims or the remains of dead organisms into ammonium (NH4+ ) by certain bacteria and fungi. Second, this ammonium is then converted to nitrites (NO2) and then nitrates (NO3) by nitrifying bacteria and archaea, such as Nitrosomonas or Nitrobacter, through the process of nitrification. In addition, both soil and the ocean contain archaeal microbes, assigned to the Crenarchaeota, that convert ammonia to nitrites. They are more abundant than the nitrifying bacteria and may turn out to play an important role in the nitrogen cycle.. Like ammonium, nitrites and nitrates are found in water and the soil. Some nitrates are converted back into nitrogen gas, which is released into the atmosphere. The process, called denitrification, is conducted by bacteria, such as Pseudomonas and Clostridium, which use nitrate when decomposing organic matter in the absence of oxygen. In the process of denitrification several intermediates are formed and may be released to the atmosphere including nitric oxide (NO) and nitrous oxide (N2O, a greenhouse gas). Under anaerobic conditions in marine and freshwater systems other species of bacteria are able to oxidize ammonia with nitrite forming nitrogen gas in a process called anammox (anaerobic ammonia oxidation).

    In marine ecosystems, nitrogen compounds created by bacteria, or through decomposition, collects in ocean floor sediments. It can then be moved to land in geologic time by uplift of Earth’s crust and thereby incorporated into terrestrial rock. Although the movement of nitrogen from rock directly into living systems has been traditionally seen as insignificant compared with nitrogen fixed from the atmosphere, a recent study showed that this process may indeed be significant and should be included in any study of the global nitrogen cycle.1

    Human Alteration of the Nitrogen Cycle

    Human activity can release nitrogen into the environment by two primary means: the combustion of fossil fuels, which releases different nitrogen oxides, and by the use of artificial fertilizers (which contain nitrogen and phosphorus compounds) in agriculture, which are then washed into lakes, streams, and rivers by surface runoff. Humans are also increasing the amount of reactive nitrogen in the environment by the cultivation of nitrogen fixing crops, such as soybeans. If the nitrogen fixation from leguminous crops (e.g. beans, alfalfa) is included, then the anthropogenic flux of nitrogen from the atmosphere to the land exceeds natural fluxes to the land. Atmospheric nitrogen (other than N2) is associated with several effects on Earth’s ecosystems including the production of acid deposition (as nitric acid, HNO3), also known as acid rain. Acid deposition damages healthy trees, destroys aquatic systems and erodes building materials such as marble and limestone. Like carbon dioxide, nitrous oxide (N2O) is a greenhouse gas, potentially causing climate change when released during denitrification.

    Humans are primarily dependent on the nitrogen cycle as a supporting ecosystem service for crop and forest productivity. Nitrogen fertilizers are added to enhance the growth of many crops and plantations (figure \(\PageIndex{4}\)). The enhanced use of fertilizers in agriculture was a key feature of the green revolution that boosted global crop yields in the 1970s. The industrial production of nitrogen-rich fertilizers has increased substantially over time and now matches more than half of the input to the land from biological nitrogen fixation (90 megatons = 1 million tons of nitrogen each year). If the nitrogen fixation from legume crops is included, then the anthropogenic flux of nitrogen from the atmosphere to the land exceeds natural fluxes to the land. Fertilizers are washed into lakes, streams, and rivers by surface runoff, resulting in saltwater and freshwater eutrophication, a process whereby nutrient runoff causes the overgrowth of algae, the depletion of oxygen, and death of aquatic fauna. Excess nitrates in water supplies have also been linked to human health problems.

    Farming equipment sprays a fine mist over crops.

    Figure \(\PageIndex{4}\): Fertilizer containing nitrogen is conventionally applied at large scales in agriculture. Image by Bob Nichols, USDA Natural Resources Conservation Service (public domain).

    Efforts to reduce nitrogen pollution focus on increasing the efficiency of synthetic fertilizer use, altering feeding of animals to reduce nitrogen content in their excreta, and better processing of livestock waste and sewage sludge to reduce ammonia release. At the same time, increasing demand for food production from a growing global population with a greater appetite for meat is driving greater total fertilizer use, so there is no guarantee that better practices will lead to a reduction in the overall amount of nitrogen pollution.

    Footnotes

    1. 1 Scott L. Morford, Benjamin Z. Houlton, and Randy A. Dahlgren, “Increased Forest Ecosystem Carbon and Nitrogen Storage from Nitrogen Rich Bedrock,” Nature 477, no. 7362 (2011): 78–81.

      Contributors and Attributions

      Modified by Kyle Whittinghill (University of Pittsburgh) and Melissa Ha from the following sources:


    This page titled 20.4: The Nitrogen Cycle is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by Boundless.