Optimizing Small Water Bodies as a Nature-Based Solution for Mitigating Nitrogen Pollution

Despite the widely acknowledged importance of small water bodies (SWBs), their large-scale capacity for nitrogen (N) removal in agricultural landscapes remains poorly understood. This study assessed the N removal efficiency and potential of 1.75 million SWBs (<0.33 ha) in China's rice-growing regions, using an N removal model incorporating key biogeochemical factors. Collectively, these SWBs potentially remove approximately 169.97 kt N y^-1 from paddy runoff, equivalent to 23.62% of national crop N emissions, yielding an estimated economic benefit of 1.68 billion USD. However, a spatial mismatch between SWB distribution and N emission hotspots hampers the current efficiency, as 23.04% of paddy fields have high N loads but limited SWBs. Increasing SWBs in these critical areas shows better N removal efficiency than a nationwide increase strategy under land resource constraints. Specifically, increasing SWBs from the current 1.08-1.23% of the rice region achieves the most cost-effective 20.96% increase in N removal. Increasing macrophyte coverage in these SWBs to 25-50% could further augment N removal by 5.98-10.58%. This study highlights SWB spatial optimization and macrophyte manipulation as viable strategies to maximize ecological and economic benefits under resource constraints, offering a nature-based solution for N pollution.