Organic Forms of Soil Nitrogen.

The chemical forms of soil organic N are thought to influence the cycling rates of soil N. Limited evidence for this relationship is available, though, because until recently technical constraints have not allowed adequate elucidation of the chemical forms.  Recent developments in spectroscopic analyses now indicate that as much as 80% of soil organic N is in amino acids and amino sugars, which are considered relatively labile.  Much of the remaining soil N is in recalcitrant forms such as heterocyclic molecules. Hence the chemical structure of soil N by itself cannot explain the stability of most soil N.  Alternative explanations lie in the (i) binding (H- or covalent) of organic N within the soil organic matter matrix, (ii) binding of organic N to soil mineral surfaces, and (iii) macromolecular characteristics of soil organic matter.  Information on the stabilization agents, cycling rates and chemical forms of soil organic N has been gained through each of the major procedures used for extracting fractions of soil organic matter, including physical (particle size, density, or aggregate size) and chemical extractions.  That information is summarized here.