Nitrate subsurface transport and losses in response to its initial distributions in sloped soils: An experimental and modelling study

Abstract Transport and losses of nitrate from sloped soils are closely linked to nitrogen fertilizer management. Previous studies have always focused on different types of fertilizer applications and rarely analysed various initial nitrate distributions as a result of nitrogen fertilizer applications. Under certain conditions, both subsurface lateral saturated flow and vertical leaching dominate nitrate losses. Soil tank experiments and HYDRUS‐2D modelling were used to better understand the subsurface nitrate transport and losses through lateral saturated flow and vertical leaching under various initial nitrate distributions. Low (L: 180 mg L−1), normal (N: 350 mg L−1), and high (H: 500 mg L−1) nitrate concentrations were used in five different distributions (NNNN, NLLN, LHHL, LNLN, and HNHN) along the slope of the tank. The first two treatments (NNNN and NLLN) were analysed both experimentally and numerically. Experiments were conducted under 12 rainfall events at intervals of 3 days. The HYDRUS‐2D model was calibrated and validated against the experimental data and demonstrated good model performance. The other three treatments (LHHL, LNLN, and HNHN) were investigated using the calibrated model. Nitrate concentrations in purple sloped soils declined exponentially with time under intermittent rainfalls, predominantly in the upper soil layers. Non‐uniform initial nitrate distributions contributed to larger differences between four locations along the slope in deeper soil layers. The non‐uniform nitrate distribution either enhanced or reduced decreases in nitrate concentrations in areas with higher or lower initial nitrate concentrations, respectively. Higher nitrate concentrations at the slope foot and along the slope were reduced mainly by lateral flow and vertical leaching, respectively. Increasing nitrogen application rates increased subsurface nitrate losses. Mean subsurface lateral nitrate fluxes were twice as large as mean vertical leaching nitrate fluxes. However, due to longer leaching durations, total nitrate losses due to vertical leaching were comparable with those due to lateral flow, which indicated comparable environmental risks to surface waters and groundwater.