ESTIMATING GROSS NITROGEN MINERALIZATION AND IMMOBILIZATION RATES IN ANAEROBIC AND AEROBIC SOIL SUSPENSIONS

Zero-order models for estimating gross rates of N mineralization (m) a nd immobilization (i) of NH4+ in soils were characterized by expressin g the analytical solutions in a standard format. Three models of m wer e identified out of seven evaluated. One model was based on the soluti on of differential equations, and two on arithmetic mean data of ratio s of labeled and total NH4+ pools. Data from well-mixed anaerobic and aerobic soil suspensions amended with rice straw and labeled with ((NH 4)-N-15)(2)SO4 were used to compare the three models of m and four mod els of i for 56 d, with measurements every 7 d. Estimated rates of m d eclined during incubation, particularly during the first 21 d. Correct ion of m for remineralization of immobilized N-15 in the anaerobic sys tem using the total organic N pool (OT) had little effect, but the cor rection was more pronounced when the estimated active organic N pool ( OT) was used. Remineralization did not account for the observed decli ne in m during anaerobic incubation. Numerical modeling and nonlinear parameter estimation were also used to estimate m and i, with values b eing similar to analytical estimates for Days 0 to 7 (m) and Days 7 to 14 (i). The OT comprised 10 to 11% of OT in both suspensions. Estima tes of OT were derived from the numerical analysis (anaerobic), or by assuming approximate isotopic equilibrium between the NH4+ (anaerobic ) or NO3- (aerobic) pools and the organic N pool following remineraliz ation of immobilized isotope.