Effect of ditch-pond system on nutrient runoff loss from rice fields during stormy rain season

Agricultural non-point source pollution, particularly nitrogen (N) and phosphorus (P) loss through surface runoff, poses a significant environmental threat. Ditch-pond systems are recognised as an integral component of agricultural ecosystems for mitigating nutrient loss. However, their climatic resilience under extreme hydrological conditions remains unclear. Functioning as a 'stress test' for Nature-based Solutions, this study monitored 12 agricultural drainage systems (6 ditch-pond vs. 6 single ditch) using paired inlet-outlet sampling in Pinghu City, eastern China, during the 'worst-case' hydraulic conditions of the stormy rain season (August to October 2024). Results demonstrated that ditch-pond systems achieved significantly higher reduction rates for ammonium-N (53.0%) and dissolved P (45.3%) compared to single ditches (P < 0.05), representing a relative efficiency improvement of 59.2% and 76.1%, respectively. While total N and total P reductions were 37.9% and 24.6%, the ditch-pond systems increased the interception load per unit area by 40.8% for total N and 19.7% for total P compared to ditch systems. The ditch-pond systems were estimated to intercept 2.32 kg ha⁻¹ of total N and 0.16 kg ha⁻¹ of total P, yielding ecological compensation benefits (34.52 $ ha⁻¹) significantly greater than those of the ditch systems (24.79 $ ha⁻¹). Correlation analyses further indicated that water quality parameters such as temperature, dissolved oxygen, electrical conductivity, and turbidity influenced nutrient interception efficiency. This study highlights ditch-pond systems as a cost-effective solution for mitigating agricultural nutrient losses, providing valuable insights for managing non-point source pollution.