Fresh leguminous plant residues were incorporated into soil columns and incubated at 23°C for up to 20 weeks. The N released from specific fractions (foliage, stems, and roots) of each residue were monitored at specific time intervals. Relationships between organic carbon, total nitrogen, C∶N ratio, lipids, and lignin content of the plant materials and the cumulative amount of N mineralized in soil were investigated. Statistical analyses indicated that the rates of N mineralized were not significantly correlated with the organic C nor lipid content of the residues. However, the cumulative amount of N released was significantly correlated with the total N content of the plant material (r=0.93***). The percentage of organic N of the legumes mineralized in soil ranged from 15.9 to 76.0%. The relationship between the percentage of N released and the C∶N ratio of the plant material showed an inverse cuvilinear response (r= 0.88***). It was also evident that the composition of lignin in the residue influenced N mine-ralization rates of the leguminous organs incorporated into soil.
There was a curvilinear relationship between the cumulative amount of N released from the residues and time of incubation. Nitrogen mineralization rates were described by first-order kinetics to estimate the N mineralization potential (N0), mineralization rate constant (k), and the time of incubation required to mineralize one-half of N0 (t1/2). The kinetic parameters were calculated by both the linear least squares (LLS) and nonlinear least squares (NLLS) transformations. The N0 values among the crop residues varied from −35 to 510 μg Ng−1 soil. Statistical analyses revealed that the N0 values obtained by both LLS and NLLS methods were significantly correlated (r=0.93***). The mineralization rate constants (k) of the residues ranged from 0.045 to 0.325 week−1. The time of incubation required to mineralize one-half the nitrogen mineralization potential (t1/2) of the legumes incorporated into soil ranged from 2.1 to 15.4 weeks.
Critical C∶N ratio Critical N percentage Green manures Mineralizable N N transformations Organic N