Reproduced, with permission, from:
U.S. Office of Technology Assessment (OTA). 1991. Changing by degrees: Steps to reduce greenhouse gases. Washington, D.C.: Government Printing Office.
Figure 8-4 shows how nitrogen flows through the environment. Molecular nitrogen (N2)--an element essential to all plant and animal life--makes up 78 percent of the atmosphere, but few organisms can use it until it has been "fixed" (i.e., made into usable compounds or ions). Much of the nitrogen stored in soil also is not readily available to plants. Therefore, even though nitrogen is contained in relatively large amounts in the atmosphere and soil, it is often the limiting nutrient in agricultural systems.
Some microorganisms take molecular nitrogen from the atmosphere (or the air spaces in soil) and convert (or fix) it into ammonium and related nitrogen-containing compounds. Many microorganisms can do this, but the most famous are bacteria that live in the root nodules of many legumes (e.g., peas, beans). Bacteria and fungi also decompose organic materials (e.g., manure, crop residues) in the soil and release ammonia or ammonium as part of their metabolic processes. Other bacteria then oxidize the ammonia or ammonium to nitrite, and another group of bacteria then oxidizes nitrite to nitrate. This process is called nitrification. Nitrate and ammonium ions can be directly taken up from the soil and used by plants. Animals then obtain nitrogen in the form of more complex organic compounds manufactured by plants.
Conversely, nitrogen compounds can be lost from the soil by leaching into ground and surface waters, and by soil erosion. Moreover, yet another group of bacteria can convert nitrate, in the absence of oxygen, into gaseous nitrogen compounds, including N2O, that are emitted into the atmosphere. This process is known as denitrification. Other nitrogen-based gases, such as nitrogen oxide (NO) and nitrogen dioxide (NO2) (both of which are involved in the formation of tropospheric ozone smog), are also emitted by these microbial processes. In fact, recent evidence indicates that in some soils the emission of NO far exceeds that of N2O (106).
SOURCE: Office of Technology Assessment, 1991.
