The Nitrogen Cycle

Another important global process in which organic mat-ter plays a major role is the nitrogen cycle. It is of direct importance in agriculture, because there is frequently not enough available nitrogen in soils for plants to grow their best. Figure 2.9 shows the nitrogen cycle and how soil organic matter enters into the cycle. Some bacte-ria living in soils are able to “fix” nitrogen, converting nitrogen gas to forms that other organisms, including crop plants, can use. Inorganic forms of nitrogen, like ammonium and nitrate, exist in the atmosphere natu-rally, although air pollution causes higher amounts than normal. Rainfall and snow deposit inorganic nitrogen forms on the soil. Inorganic nitrogen also may be added in the form of commercial nitrogen fertilizers. These fer-tilizers are derived from nitrogen gas in the atmosphere through an industrial fixation process. Almost all of the nitrogen in soils exists as part of the organic matter, in forms that plants are not able to use as their main nitrogen source. Bacteria and fungi con-vert the organic forms of nitrogen into ammonium, and different bacteria convert ammonium into nitrate. Both nitrate and ammonium can be used by plants. Nitrogen can be lost from a soil in a number of ways. When crops are removed from fields, nitrogen and other nutrients also are removed. The nitrate (NO3–) form of nitrogen leaches readily from soils and may end up in groundwater at higher concentrations than may be safe for drinking. Organic forms of nitrate as well as nitrate and ammonium (NH4+) may be lost by runoff water and erosion. Once freed from soil organic matter, nitrogen may be converted to forms that end up back in the atmosphere. Bacteria convert nitrate to nitrogen (N2) and nitrous oxide (N2O) gases in a process called denitrification, which occurs in saturated soils. Nitrous oxide (also a “greenhouse gas”) contributes strongly to global warming. In addition, when it reaches the upper atmosphere, it decreases ozone levels that protect the earth’s surface from the harmful effects of ultraviolet (UV) radiation. So if you needed another reason not to The Nitrogen Cycle  apply excessive rates of nitrogen fertilizers or manures— in addition to the economic costs and the pollution of ground and surface waters—the possible formation of nitrous oxide should make you cautious The Nitrogen Cycle