A review on sustainable nitrogen management in intensive vegetable production systems

Many current intensive systems of field vegetable production are not sustainable because they cause severe environmental damage. Often, large amounts of nitrogen remain in the soil after harvest of the crop. This nitrogen includes residual soil mineral nitrogen and nitrogen present in crop residues. Both sources of nitrogen may affect groundwater quality through nitrate leaching and ail quality through nitrous oxide emission. 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 a value of 200 kg N ha(-1) Obviously, large amounts of nitrate will then be at risk of leaching and denitrification during the subsequent winter. 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 are decomposed 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. Losses after Brussels sprouts and cabbage were much lower. Two kinds of measures can be taken to maximise nitrogen use efficiency while minimising environmental losses. These are (1) matching nitrogen supply and demand, and (2) reducing losses outside the growing season.