THE ENVIRONMENTAL IMPACT OF NITROGEN IN FIELD VEGETABLE PRODUCTION

Abstract

Many intensive systems of field vegetable production are not sustainable because they lose excessive amounts of nitrogen (N) to the environment. Processes in the N cycle of agricultural systems include assimilation, mineralization/immobilization, nitrification, denitrification, ammonia volatilization, nitrate leaching, runoff and erosion. Emission of N from agriculture may affect the quality of the atmosphere, ground and surface waters. This occurs through leaching, ammonia volatilization, denitrification, nitrification, runoff and erosion. In field vegetable production, nitrate leaching is the dominant process affecting the environment. Often, large amounts of nitrogen, including residual soil mineral nitrogen and the nitrogen present in crop residues remain in the soil after harvest of the crop. Both sources of nitrogen may affect groundwater quality through nitrate leaching. Residual soil mineral nitrogen levels after application of the recommended rates of nitrogen fertiliser to Brussels sprouts, white cabbage and onions are low to moderate (20-75 kg N ha-1). Application of the recommended rates to other field vegetables, however, may leave large amounts of residual soil mineral nitrogen, especially after crops that are harvested before maturing, e.g., spinach, where residual soil mineral nitrogen may even exceed 200 kg N ha-1. Obviously, large amounts of nitrate will then be at risk of leaching and denitrifying in the period after harvest. Crop residues of spinach and celeriac contain 25-60 kg N ha-1, cauliflower residues 80-120 kg N ha-1, and white cabbage and Brussels sprout residues as much as 150-250 kg N ha-1. If the residues decompose before winter, nitrogen from the decomposed plant material may leach or denitrify during the subsequent winter period. Realistic estimates of nitrogen losses through leaching and denitrification after harvest of field vegetables were generated with a simulation model. It was calculated that leaching losses may exceed 200 kg N ha-1 after spinach or leeks, but denitrification was low. Leaching and denitrification losses after Brussels sprouts and cabbage were much lower.