Nitrogen is unique among the essential nutrient elements of higher plants in terms of its roles in biological systems and its complex cycling. In addition, it is the element most commonly limiting to crop production and the one most demanding of management skills. Soil organic matter (SOM) has a pivotal place in cycling N contents of crop residues and animal manures to mineral forms that are used by higher plants. In Chapter 7, we found that amount of SOM reflects the relative rates of C and N inputs to decomposition on the one hand and mineralization on the other. Nitrogen available for plant uptake comes from two sources: (1) indigenous N supply from SOM mineralization, biological N fixation, and atmospheric deposition, and (2) applied N in manure, compost, and mineral fertilizers. This chapter is concerned with several microbiological and physical processes important in N cycling and management of N in agriculture.

The nitrogen cycle

Nitrogen cycles between the atmosphere, soils, and plants through biologically mediated transformations that cover a wide range of chemical states. All N forms other than dinitrogen gas (N2) are “reactive” in the sense they have a large impact on ecosystem structure and function. Too much reactive N causes eutrophication with detrimental effects on the composition and health of plant and animal communities in ecosystems. Too little N results in low net primary production and low crop yields.