Modeling the effects of rainfall intensity on nitrogen leaching in a karst agroecosystem: a comparative study of...

Karst (a landscape formed from soluble limestone which often has sinkholes and underground rivers) wetlands are highly vulnerable to agricultural nitrogen losses, but field-scale vertical monitoring of soil solution is scarce. We combined in situ soil solution monitoring during two rainfall events with HYDRUS-1D simulations to quantify how single-application fertilizer rates affect vertical nitrate and ammonium dynamics in a paddy field and an orchard under heavy rainfall conditions in the Huixian karst wetland (Guilin, China). The model reproduced soil moisture and solute dynamics with high accuracy in most scenarios (R² > 0.87; normalized RMSE generally < 0.30), although nitrate and ammonium fits in the paddy validation period were lower (R² = 0.78 and 0.81). Under heavy events, orchard soils showed a transient ammonium spike at 100 cm and deeper nitrate accumulation in paddy soils. Increasing single-application N raised surface nitrate peaks by 31% (orchard, M2 = 420 kg N ha^-1 vs M1 = 225 kg N ha^-1) and by 191% in the paddy surface layer (M2 = 120 kg N ha^-1 vs M1 = 45 kg N ha^-1). Cumulative NO₃^- leaching fluxes averaged 23.7 → 24.4 kg ha^-1 (paddy, M1 → M2) and 21.6 → 25.4 kg ha^-1 (orchard, M1 → M2), indicating orchards were more sensitive to single-application increases. These results reveal land-use dependent, nonlinear responses of N transport to rainfall-fertilization interactions and support management measures (split applications, drainage, and irrigation adjustments) to reduce short-term leaching risk in karst agricultural landscapes.