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Publisher Summary The gaseous form of nitrogen makes up 78% of the troposphere. Its incorporation into terrestrial nitrogenous compounds takes place via different pathways, including microorganisms, plants, and human industrial and agricultural activities. Nitrogen taken from the air is converted to ammonia by nitrogen-fixing bacteria. In most soils, ammonium is rapidly oxidized to nitrite and, consequently, into nitrate in a nitrification process by the action of the aerobic bacteria, such as Nitrosomonas and Nitrobacter. The nitrate ion is the stable form of nitrogen for oxygenated systems. Nitrate is very soluble and, unless intercepted or taken up by plant roots, leaches down into the soil along with irrigation or rainwater or is carried away by runoff. Nitrate and nitrite occur in drinking water mainly as a result of intensive agricultural activities. Nitrate-containing compounds present in soil are generally soluble and readily migrate into groundwater. Contamination of soil with nitrogen-containing fertilizers, including anhydrous ammonia, as well as animal or human natural organic wastes, can raise the concentration of nitrate in water. As nitrite is easily oxidized to nitrate, nitrite levels in water are usually low, and nitrate is the compound predominantly found in groundwater and surface waters. Being an essential element for plant growth, nitrogen is absorbed by plants in the form of ammonium or nitrate from soil water. Nitrates accumulated in plants form a nitrogen reserve, which is needed for amino acid and protein synthesis. All nitrates absorbed by plant roots are reduced once inside the plant to form ammonia, which serves as a precursor for protein synthesis. Every reduction step is catalyzed by a certain enzyme-reductase. |
