Excess nitrogen load from agricultural and livestock activities threatens groundwater quality. A watershed-scale assessment of current and alternative agricultural management practices is critical for effectively reducing potential water pollution. The objective of this study was to predict the spatial distribution of nitrate leaching to shallow aquifer at an agricultural watershed, based on a numerical simulation approach, under different nitrogen management scenarios.

The study watershed area is part of the Kanto region, Japan, composed of 42 agricultural villages with an area of 68 km². Mean annual air temperature and precipitation for the region are 13.3 °C and 1379 mm. Soil in the area is classified as volcanic ash soils, termed Andosols (FAO). Cropping systems are a single crop of melon and sweet potato-vegetables rotation. LEACHM (Leaching Estimation And solute transport CHemistry Model) (Hutson, 2003) was used to predict the nitrogen transport after appropriate modification and parameterization of the model particularly on the soil organic carbon and nitrogen transformation processes. The model performance was validated using 3-yr field lysimeter data on the nitrogen leaching from cropped Andosols and sand-dune Regosols soils without adjusting model parameters to fit measured data. The following three scenarios of agricultural management practices were simulated: 1) comply with the agricultural management practice standard (chemical fertilizer and manure compost were applied at the rates of 209 and 180 kg-N ha^–1 yr^–1 in average, respectively.), 2) introduce sorghum [Sorghum bicolor (L.)] as a green manure crop to the practice standard, and 3) reduce the use of chemical nitrogen fertilizer by 50 % of that of the practice standard and compensate for the reduced chemical fertilizer using manure compost.

In scenario 2, the cultivation of sorghum decreased the nitrogen load discharged to the shallow aquifer by 30 % of the practice standard (scenario 1). This appears to be the result of the increase in soil organic carbon and nitrogen contents by incorporating the sorghum residue with high nitrogen uptake of 215 kg-N ha^–1 and high C/N ratio of 31. In scenario 3, the nitrogen concentrations in drainage water from the agricultural watershed area were below the environmental standard of 10 mg-N L^–1 although there was little difference in nitrogen uptake by crops between scenarios. Results from this study suggest that shifting from the use of chemical nitrogen fertilizers to alternative organic nitrogen resources of crop residue and manure compost should be effective way to reduce the risk of groundwater nitrate pollution.