Determination of potentially mineralizable nitrogen in agricultural soil

Potentially mineralizable soil N was determined after incubation for 2, 4, 8, 12, 16, 22, and 30 weeks, according to the Stanford and Smith method. A first-order kinetics model was used, and a simulation study was performed using three different statistical methods to estimate potentially mineralizable N and the rate constant k. The first method was based on the maximum-likelihood approach. The second one relied on nonlinear least square data fitting. The third method was based on linear of logarithmically transformed data. The results of the simulation study suggested that the non-linear least square method was preferable to the others. This method was then applied to real data from 30 different Italian soils. The values obtained for potentially mineralizable N were, on average, 10% of total N (mean standard error = 0.9), The estimated value of k was 0.050 (mean standard error = 0.005). Finally, from the values obtained for k and the results of the simulation, the results indicated that significantly less reliable estimates of potentially mineralizable N were obtained by using data for up to only 22 weeks of incubation.