N mineralization and nitrate leaching from vegetable crop residues under field conditions: A model evaluation

Six different vegetable crop residues were incorporated in the field and N mineralization from the residues and from an unamended plot was followed over 4 months by periodically monitoring mineral N contents of the soil. The crop residues were also fractionated according to a modified Stevenson chemical fractionation. Nitrogen mineralization parameters of the first order kinetic model N(t) = N-A(1 - e(-kt)) were derived from the chemical fractionation data. The first order model was used in combination with a model describing the temperature dependence of N-mineralization and a simple leaching model to predict N mineralization rates and nitrate redistribution after crop residue incorporation under field conditions. Comparison of predicted and measured mineral N contents in the upper soil layer (0-30 cm) before the start of leaching showed that the model was able to predict N mineralization from both soil organic matter and crop residues under field conditions. From the onset of leaching, mineral N contents were slightly overestimated in the upper layer and underestimated in the lower soil layers. Although the Burns leaching model underestimated the leaching rate, the general pattern of nitrate movement was simulated satisfactorily. Statistical analysis using the variance ratio test yielded small but significant F values, indicating that the model can still be improved. The modelling efficiency was rather high and the coefficient of residual mass very close to zero. Linear regression between measured and simulated nitrate contents over the whole profile (0-120 cm) for all samplings yielded Y = 9.6 + 0.876X (r - 0.94***) with all deviations smaller than 25 kg N ha(-1). Total N mineralization ranged from 48 kg N ha(-1) for the control plot to 136 kg N ha(-1) for the plots with cauliflower residues and cumulative leaching losses from 26-66 kg N ha(-1), with most of the mineral N left in the 60-120 cm layer. These results show that N losses by leaching in winter can be high when vegetable crop residues are incorporated, even when there is little mineral N in the soil at the time of incorporation. (C) 1998 Elsevier Science Ltd. All rights reserved.