INFLUENCE OF CULTIVATION, CROPPING SYSTEMS, AND CROP RESIDUES ON THE CONTENT AND DISTRIBUTION OF NITROGEN FORMS IN SOILS (ORGANIC FRACTIONS, CARBON, INCUBATION, MINERALIZATION - IMMOBILIZATION, MICHIGAN)

Changes in organic C and total N, and in the distribution of N in fractions obtained by acid hydrolysis were used to evaluate effects of tillage, cropping systems and types of crop residues on N status in soils.;In a livestock operation on Kalamazoo sandy loam, total N was lower by 23% and organic C by 12% than in the virgin soil.;At another location on Charity clay, seven cash crop systems involving different sequences and proportions of corn, navy beans, sugar beets, oats and alfalfa were compared. After 11 years, total N had declined by 2 to 11% from levels found at the beginning of the experiment. Organic C had declined by 13 to 17%.;At all field locations, decreases in the proportion of nonhydrolyzable N and increases in the proportion of hydrolyzable unknown forms (HUN fraction) indicated that a long-term effect of tillage was to increase exposure and decrease stability of physically protected humic complexes. This increased the proportion of lower molecular weight fragments and precursors. An increase in exposed surface was indicated also by increases in hydrolyzable ammonium. Variable changes in the proportion of hydrolyzable amino acid and amino sugar fractions appeared related to current crop cover.;Under cash crop systems on Charity clay and Hodunk sandy loam, total N in the plow layer was lower by 66% than in surface layers of adjacent virgin soils. The corresponding decreases in organic C were 62% in clay and 80% in the sandy loam.;A Hodunk sandy loam amended with residues of corn, navy beans, sugar beets or alfalfa was incubated for 63 days. Both chemical and biological transformations may have contributed to observed patterns of mineralization-immobilization. The HUN fraction seemed to be a key intermediate in transfers of N among organic fractions and in exchanges with the mineral N pool. Amino acid and amino sugar fractions peaked after major respiratory losses of C has occurred and appeared related to size or activity of microbial populations. Lesser parallel changes in hydrolyzable ammonium indicated its association with active surface sites.