Measurement and simulation of nitrogen leaching loss in hillslope cropland of purple soil

Nitrogen leaching is one of the most important processes in nitrogen biogeochemical cycling. It is a nondeductible and difficult to obtain accurate nitrogen leaching flux for N cycling in agro-ecosystem. There is a wide distribution of purple soil in the Sichuan Basin with shallow soil layers and low hydraulic permeable parent rocks. Seepage water reaching soil-bedrock interface gathers to form the interflow flowing out of soil. Nitrate accumulated at soil profile and dissolved by seepage water moves via interflow, and consequently leaches from the soil profile. Therefore, nitrogen leaching was observable by interflow monitoring from hillslope cropland of purple soil. DNDC is a process-based biogeochemical model used globally to simulate greenhouse gas emission. It had not been well documented for modeling tests of nitrogen leaching especially in China. Sloping lysimeter in purple soils with large area coverage (32 m^-2) were used to monitor seepage water discharge and nitrogen leaching loss flux. The DNDC model was validated and tested using observed data and used to simulate nitrogen leaching. Results showed that measured seepage water discharge was 323.6 mm with annual seepage coefficient of 33.3%. Nitrogen leaching flux was 36.93 kg·hm^-2, accounting for 13.2% of total fertilizer nitrogen applied in winter wheat and summer maize rotation system. The simulated seepage water and nitrogen leaching fluxes well agreed with observed values with correlation coefficients of 0.944 (P < 0.05) and 0.972 (P < 0.05), respectively. The Theil coefficients were 0.07 and 0.1 for simulated water seepage and nitrogen leaching, respectively. Rainfall, soil porosity and applied nitrogen amount were highly sensitive modeling parameters. The simulated seepage water discharge and leached nitrogen flux from the purple soil of hillslope cropland were therefore reliable. Results indicated that DNDC as a process-based biog-eochemical model was applicable in quantitative studies on nitrogen leaching in purple soil at field scale. Furthermore, DNDC model was suitable for describing the characteristic processes of nitrogen leaching, especially in continuous rain conditions. It was feasible to test the model in modeling nitrogen leaching processes and regional estimation of nitrogen leaching loss.