Nitrate losses through leaching were studied in a crop field in the central Sichuan Basin, southwestern China, during 2003 to 2006. Nitrate accumulations and losses via leaching were measured using two methods: a simulation with sampled soil profiles and field monitoring in situ at a 7% slope. The results showed that NO3 was accumulated in the dry season and leached in the rainy season. No spatial dependence of soil NO3 distribution could be distinguished along the hillslope due to complex water flow. The soil interflow was the major driver of NO3 leaching losses from the experimental plots. The average annual discharge of the interflows was 148 mm, accounting for 63% of total runoff in the rainy season in 2003 to 2006. The interflow water contained high NO3–N concentrations. The highest NO3–N concentration and loss flux were detected in storm runoff after a long drought in 2006. The NO3–N leaching through interflow showed clear annual and seasonal patterns, and largely occurred in the stages from stamen elongation to maturity of maize (Zea mays L.) with a flux of 2.2 g m−2, about 69% of the total annual loss flux. Annual estimated losses of NO3–N through interflow ranged from 20 to 53, with an average of 36 kg ha−1 yr−1 This study indicates that croplands on hillslopes with regosols are vulnerable to NO3 leaching. The interflow discharge monitoring method together with effluent NO3 content analysis used in this study proved to be useful and effective for quantifying NO3 leaching losses at the field scale in hillslope areas.

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