Abstract

The GLEAMS simulation model was used to determine the relative effects of climate (19 years of data), soil type (4 soils distinguished by effective soil depth), and farm management (6 sowing dates and 5 levels of nitrogen fertiliser) on leaching of nitrate under wheat production. All combinations of inputs were simulated and the effects of each input were estimated with sensitivity analysis software (SimLab). Soil type, climate, and sowing date explained about equal amounts of the variance in nitrate leaching, whereas fertiliser application explained only about one-third of the variance of the other inputs. The 2 most significant results were: (1) the importance of having plant uptake of nitrogen during autumn and winter to limit nitrate availability for leaching, and (2) the recognition that leaching of nitrate becomes increasingly sensitive to farm management practices with decreasing soil depth. The risk of nitrate leaching was found to be very low on deep soils when the crop was sowed in the autumn or winter. These results help with identifying areas where management changes might be effective in reducing the long-term risk of nitrate leaching. Crop growth over winter and judicious use of fertiliser are particularly recommended for cropping on shallow soils.