Nitrogen Transformations

No other element essential for life takes as many forms in soil as nitrogen (N), and transformations among these forms are mostly mediated by microbes. Soil microbiology thus plays yet another crucial role in ecosystem function: in most terrestrial ecosystems N limits plant growth, and thus net primary production—the productive capacity of the ecosystem—can be regulated by the rates at which soil microbes transform N to plant-usable forms. However, several forms of N are also pollutants, so soil microbial transformations of N also affect human and environmental health, sometimes far away from the microbes that performed the transformation. Understanding N transformations and the soil microbes that perform them is thus essential for understanding and managing ecosystem health and productivity. Nitrogen takes nine different forms in soil corresponding to different oxidative states (Table 13.1). Dinitrogen gas (N2) is by far the most abundant form of N in the biosphere but is unusable by most organisms, including plants. Biological N2 fixation, whereby N2 is transformed to organic N (described in detail in Chap. 14), is the dominant process by which N first enters soil biological pools. All subsequent transformations are covered in this chapter: N mineralization, which is the conversion of organic N to inorganic forms; N immobilization, which is the uptake