Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: October 9, 2002
Publication Date: June 9, 2008
Citation: Olk, D.C. 2008. Organic forms of soil nitrogen. In: Schepers, J.S., Raun, W.R., editors. Nitrogen in Agricultural Systems. Agronomy Monograph 49. ASA-CSSA-SSSA, Madison, WI. p. 57-100. Technical Abstract: This review summarizes recent progress in elucidating the chemical nature of soil organic nitrogen (N). A primary research objective has been to improve our understanding why most soil organic N is resistant in the short-term to mineralization by soil microorganisms. Current technology, most notably **15N nuclear magnetic resonance spectroscopy, indicates that the bulk of soil organic N is in polypeptide forms, a labile form. Evidence exists for modest levels of more recalcitrant forms, including heterocyclic N and aromatic N. The general recalcitrance of soil N may result from any of several factors other than its chemical nature, including type of bonding of the nitrogenous molecule (hydrogen, covalent, mineral, or cross-linkage), macromolecular structure of organic N, general stabilization of soil organic matter, and the random chemical structure of organic matter. Available literature on each of these factors is reviewed. Their respective significances appear to vary by setting, and none is primarily responsible for soil N recalcitrance in all settings. Leading methods for fractionating soil N are also reviewed; their mixed performances again illustrate the complexity of N binding in the soil.