Calculation of net nitrogen mineralization from the decomposable soil organic matter pool

The calculation of Net Nitrogen Mineralization (NNM) presented in this pape r was based on the assumption that soil organic matter carbon may be divide d into a stable, texture dependent pool (C-stab) and a labile, decomposable pool (C-dec) Because the C-stab pool is biologically inert due to its inte raction with fine-grained mineral soil particles, only the C-dec pool refle cts man made changes in the soil organic matter content. Consequently, to e stimate the rate constant of soil organic matter carbon, the CO2-C producti on of soils was related to the corresponding amounts of C-dec Because this rate constant k(dec) was derived from incubation experiments, the influence of actual decomposition conditions in the field was calculated according t o the concept of "Effective Mineralization Time" (EMT). Setting the C/N rat io of C-dec = 10, the NNM from the C-dec pool was calculated according to t he Equation: NNM = 0.1 * C-dec * k(dec) * EMT. To evaluate the calculation method of NNM, the nitrogen uptake of plants was related to nitrogen supply . It was estimated that the mean recovery of N by different vegetable crops grown on plots without mineral nitrogen fertilizer was about 70% of the to tal nitrogen supply (including the NNM and the N-min values at the start an d the end of the cultivation period).