Turnover of organic nitrogen in soils and its availability to crops

Major known fractions of soil nitrogen are amino nitrogen (proteins, peptides), polymers of amino sugars, and NH4+ fixed in interlayers of 2:1 minerals. Only a small percentage of the total soil organic N is easily mineralizable and contributes to the pool of mineral soil N. Predominant sources of mineralization are amino-N and polymers of amino sugars present in the soil microbial biomass. Influx into this pool occurs with the application of organic matter (green manure, straw), organic carbon released by plant roots, N-2 assimilation by leguminous species and inorganic nitrogen. Microbial metabolization of green manure proteins results in a partial mineralization of the applied organic N, microbial metabolization of straw in the assimilation (immobilization) of inorganic nitrogen. Microbial biomass is characterized by a narrow C/N ratio (proteins, peptidoglycans, polymers of amino sugars). Its metabolization therefore is associated with a partial mineralization of the attacked organic nitrogen compounds. Nitrogen mineralization consists of a sequence of enzymatic processes for which the living microbial biomass provides the enzymes and the dead microbial biomass the substrate.