Modelling N mineralization of vegetable crop residues during laboratory incubations

A range of vegetable crop residues were subjected to a modified Stevenson chemical fractionation. N mineralization parameters were determined for the fractionated crop residue components from aerobic laboratory incubations. Fresh chopped crop residues were incubated with soil at fixed temperature and soil moisture content. N mineralization (both NH4+N and NO3−N) was measured over a 3–4 month period (depending on degradability) at regular intervals. The first order kinetic model N(t) = NA (1 − e−kt) was fitted to the mineralization data of total N (parameters NA and k) and of organic N (parameters NA,org and korg). Both the amount of mineralizable N (NA) and the rate constant k differed widely between the residues. The amount of mineralizable organic N (NA,org) was better correlated to chemical composition than the amount of mineralizable total N (NA). The parameter NA,org was best related to the C-to-N ratio of the lignin (78% of the total variance of NA,org explained). Good correlations were also observed with lignin content (r = 0.863) and with the water soluble fraction (r= 0.861). The rate constant korg was much less predictable. It was related negatively to the relative amount of organic N (relative to total N) contained in the residues. The model presented can be used to calculate the amount of N mineralized at any specified time after incorporation of the residues under the experimental conditions used. Prediction of N mineralization improves with time of incubation of the residues because the influence of the rate constant diminishes. The critical C-to-N ratio, i.e. the break point between net N mineralization and net N immobilization (NA = 0), was calculated to be at a C-to-N ratio of 44.