| Turnover of N through mineralization-immobilization leads to incorporation of fertilizer N into organic forms. In time, part of the immobilized N is converted into biologically resistant humus forms. Attempts to characterize the stabilized N have been unsuccessful, partly because the procedures normally used to fractionate soil humus make little distinction between N in the so-called active and passive phases.;Other studies dealt with the chemical distribution of soil and fertilizer-derived N, as influenced by type of carbon substrate and incubation time. Net mineralization of immobilized ('15)N varied according to substrate type and was affected by drying and rewetting the soil during incubation. Data for clay-fixed NH(,4)('+)-N supported the concept of a dynamic equilibrium between soluble, exchangeable, and fixed forms of NH(,4)('+).;Major differences were observed in the percentage distribution of immobilized ('15)N and native soil N. Lower percentages of the immobilized ('15)N were accounted for as acid-insoluble N and NH(,3)-N; higher percentages occurred in the amino acid and hydrolyzable unidentified (HUN) N fractions. Similar proportions of the native and fertilizer-derived N were estimated as amino sugar N. After 120 days of incubation, the percentage distribution of immobilized ('15)N (0.01M CaCl(,2)-extracted soils) was: amino acid N (31-38%), HUN (29-30%) > NH(,3)-N (11-18%) > amino sugar N (10-14%) > acid-insoluble N (7-10%). Comparable data for the native humus N were: acid-insoluble N (29%) > amino acid N (24%), HUN (24%) > NH(,3)-N (18%) > amino sugar N (13%).;The biomass of a typical Illinois Mollisol (Flanagan silt loam) was labeled with ('15)N, and several extractants were tested to determine their effectiveness in separating the immobilized ('15)N from the native soil N. Acidified permanganate and anhydrous formic acid were the most drastic extractants tested, and they removed from 9 to 13% of the total N, and about 14% of the immobilized ('15)N. From 3 to 5% of the total N, and from 7 to 11% of the ('15)N were removed by the other extractants (i.e., hot water, hot 0.01M CaCl(,2), hot 0.005M NaHCO(,3), and cold 0.01M NaHCO(,3)). An inverse relationship was observed between the amount of N extracted and selectivity of the extractants for removing the immobilized ('15)N. |
