Modelling the release and loss of nitrogen after the incorporation of vegetable crop residues

A computer model is described that is able to simulate the mineralization-immobilization turnover of nitrogen derived from vegetable crop residues added to soil. Once mineralized, N is subject to loss from soil in the model by leaching and denitrification. That the mineralization part of the model works well is demonstrated with reference to some pot experiments in which residues from Brussels sprouts, leeks, cabbage or spinach were mixed with either a sandy or clay soil and incubated at 20 degrees C under optimal moisture conditions for 48 weeks. The release of mineral N was measured at intervals during the experiment and was strongly dependent upon the amount of N added in the crop residues: spinach (C:N = 6) released most nitrogen and most quickly, the other residues (C:N = 13-15) released N more slowly. With adaptations for field conditions, the model was then used to elucidate the fate of nitrogen remaining after field vegetables. The dynamics of both mineral and organic N remaining in soil were traced with this model. After spinach, much nitrate leaches to groundwater; sprouts, however, appear able to immobilize or denitrify what little mineral N remains at harvest reducing the loading of N in percolating water. The model suggests that during the last 40 years over winter losses of nitrate after cabbage almost always exceeded the EC drinking water limit of 11.3 mg NO3-N/litre: in some years by a factor of four. Since mineral N remaining in soil at harvest is shown to have most influence on leaching losses, measures taken to reduce unused mineral N will probably benefit groundwater quality most