Surface and Subsurface Losses of N and P from Sloping Karst Farmland in Southwest China

Non-point source pollution has become one of the most widespread environmental degradation problems in recent years. This study aimed to investigate how hydrological processes regulate nitrogen and phosphorus losses under simulated rainfall conditions through in situ rainfall experiments in karst farmland. We conducted a field-scale plot experiment, recorded rainfall and runoff, and measured the nutrient concentration in the runoff of nine experimental plots on the slope toe, middle slope and upper slope. Simulated rainfall intensity was 90 mm/h for 60 min. The results showed nitrogen losses were dominated by subsurface flow in small-scale studies, which accounted for 55.19% (2.50 kg/ha), 71.35% (3.88 kg/ha), and 93.85% (1.39 kg/ha) of TN losses at the toe, middle, and upper slope positions, respectively. The middle slope exhibited the highest losses of N mainly due to its larger subsurface runoff volume. NH4+ dominated TN in surface flow, contributing up to 97.5% (0.0092 kg/ha) at the slope toe, whereas NO3− was the dominant N form in subsurface flow, with little variation across the three slope positions, averaging 0.062 kg/ha. In contrast, phosphorus losses are primarily associated with surface flow, with TP concentrations in surface flow being 5–60 times higher than those in subsurface flow, with average surface TP losses of approximately 0.04 kg/ha. These results imply that nutrient management in karst farmland should adopt differentiated control strategies, with greater emphasis on reducing subsurface nitrogen leaching while limiting surface runoff and erosion to mitigate phosphorus losses. However, the conclusions are based solely on small-scale rainfall simulation experiments, and nutrient loss may also be influenced by factors such as karst terrain heterogeneity, prior soil moisture content, soil properties, and rainfall characteristics.