Potential and effects of field-ditch-pond systems to mitigate N loss from paddy fields in China

Field-ditch-pond (FDP) systems can mitigate nitrogen (N) runoff loss from rice production by interception, impoundment and purification, but their regulation potential remains unclear across China. This study identified the scales of ditches and ponds and evaluated their N runoff mitigation efficiency, by combining image extraction of small water bodies and the newly developed FDP-NPS model. The four rice-growing regions varied in the scale of ditches and ponds: the Mid-lower Yangtze River Basin (MLYZ) exhibited the highest ditch-pond proportion (R_dp), followed by Northeast Plain, Southeast Coast, and Upper Yangtze River Basin. By jointly regulating water levels in paddies, ditches and ponds, the FDP system retained > 90% of runoff under light and moderate rainfall events and > 80% under heavy rainfall events, and further achieved notable N reduction efficiency (IR_Nload) of 14-90%. Compared with low- and medium-regulation intensities, a high-regulation intensity of FDP’ water levels enhanced IR_Nload. IR_Nload was primarily governed by R_dp, regulation intensity, and precipitation. Overall, the current ditch and pond scales exhibited acceptable N reduction potential, future efforts should prioritize the optimization of water management and ecological purification functions over the blind scale expansion of ditches and ponds, but differentiated optimization strategies are necessary for four rice-growing areas. This study provides decision support for implementing nature-based N loss reduction strategies.