Soil Nutrient Cycling
Nitrogen Cycle
The nitrogen cycle consists of a sequence of biochemical changes wherein atmospheric nitrogen gas (N2) is transformed into various forms of nitrogen by a variety of soil-inhabiting organisms for plant use. The major transformations of nitrogen are: (1) nitrogen fixation, (2) mineralization, (3) immobilization, (4) denitrification, (5) ammonia volatilization, and (6) leaching (Figure 9.2). The transformation of nitrogen into its many oxidation states is highly dependent on the activities of a diverse assemblage of microorganisms, such as bacteria and fungi. Although the nitrogen-cycle is very complex, it is probably the most important nutrient cycle to understand. There are two reasons for this: (1) nitrogen is usually the most growth-limiting plant nutrient in terrestrial (land) ecosystems, so there is often a very large crop-yield response to additional nitrogen, and (2) nitrogen in the nitrate form is very soluble and one of the most mobile plant nutrients in soil, so it can easily be lost from farm fields and become a contaminant in surface or groundwater. Managing nitrogen is a critical part pf soil fertility management.
Nitrogen Fixation
Fixation refers to the conversion of atmospheric nitrogen to a plant available form. This occurs either through an industrial process, as in the production of commercial fertilizers, or a biological process, as with legumes such as alfalfa and clover.
Commercial Nitrogen Fertilizers
Commercial nitrogen fertilizers are produced through industrial nitrogen fixation. Atmospheric nitrogen is reacted with hydrogen gas (produced from the steam treatment of natural gas) under high pressure and heated to form ammonia, which can be used directly as a fertilizer (i.e., anhydrous ammonia) or can undergo additional processing to produce other forms of nitrogen fertilizer.
Biological Nitrogen Fixation
Biological nitrogen fixation is a critical process—most ecosystems depend on it to provide the nitrogen that sustains their primary productivity. In fact, because nitrogen is not an important constituent of rocks and soil minerals, nitrogen fixation is ultimately responsible for almost all of the organic nitrogen in the biomass of organisms and ecosystems throughout the biosphere.
Mineralization
Mineralization is important where organic nitrogen is converted to inorganic nitrogen. Unlike biological nitrogen fixation, which is carried out by specific groups of microbes, nitrogen mineralization is less specialized and may be completed by a range of heterotrophic microorganisms, mainly fungi and bacteria. Mineralization is influenced by environmental factors that affect biological activity such as temperature, moisture, aeration, and pH. Moisture is necessary for microbial function in mineralization processes.
Aminization
Aminization is the first step of mineralization in which microorganisms break down complex proteins into simpler organic compounds such as amino acids, amides, and amines.
Ammonification
Ammonification is the second step of mineralization. Ammonification refers to any chemical reaction in which the amino groups (NH2) associated with organic forms of nitrogen are converted into ammonia (NH3) or its ionic form, ammonium (NH4⁺), as an end product. Ammonium is then available for use in plants as a nutrient, or as a substrate for the nitrification processes.
Nitrification
Nitrification is the final step of mineralization. During the process of nitrification, ammonium ions are oxidized first to nitrite (NO2¯) and the oxidation of nitrite to nitrate (NO3¯). Nitrification is regulated by abiotic conditions, such as soil oxygen levels and pH, as well as soil ammonium concentration.
Immobilization
Immobilization is the reverse of mineralization. All living things require nitrogen; therefore, microorganisms in the soil compete with crops for nitrogen. Immobilization refers to the process in which nitrate and ammonium are taken up by soil organisms and therefore become unavailable to crops. Decomposing microorganisms have first priority for any mineralized nitrogen (nitrogen released in decomposition).
Denitrification
Nitrogen is commonly lost to the atmosphere when nitrate ions are converted to gaseous forms of nitrogen by a series of biochemical reduction reactions termed denitrification. Denitrification occurs only under anaerobic (without air) conditions when soil is saturated with water. Microbes generally use oxygen (O2) in the soil when they decompose organic matter, but if oxygen is not available due to saturation with water, they will use nitrate instead.
Ammonia Volatilization
Ammonia (NH3) loss to the atmosphere is called ammonia volatilization. Ammonia volatilization occurs when ammonium in the soil is converted to ammonia, which can be lost as a gas. The process of ammonia volatilization commonly takes place when any surface applied ammonia- and ammonium-based nitrogen fertilizer, including manure, can lose nitrogen to the atmosphere via ammonia volatilization (Section 12.2).
Leaching
Leaching losses refer to the flushing of nitrates down through the soil profile by rain or irrigation water. Ammonium nitrogen bears a positive charge. Negatively charged soil colloids attract the cation, protecting it from leaching. The nitrate ion, by contrast, moves freely in the soil because of its negative charge. Nitrate in a field may originate from many sources, including manures, composts, decaying plants, or from fertilizer.
Click on the following topics for more information on soil nutrient cycling.
