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 ( R 2 > 0.87; normalized RMSE generally < 0.30), although nitrate and ammonium fits in the paddy validation period were lower ( R 2 = 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 NO 3 − 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.
Gong et al. (Sat,) studied this question.