Spatial Targeting and Budget-Adaptive Optimization of Best Management Practices for Cost-Effective Nitrogen Reduction

This study developed a Soil and Water Assessment Tool (SWAT) model for the Fuzhou River Basin in China to quantify the spatial distribution, sources, and reduction potential of total nitrogen (TN) load. We comprehensively evaluated the effectiveness of eight Best Management Practices (BMPs) and 186 combinations thereof in reducing TN load. Our analysis demonstrated that adding more BMPs did not yield proportionally additive benefits but instead led to reduced cost-effectiveness (CE) once the number of BMPs exceeded three. Targeting BMPs to Critical Source Areas (CSAs) increased CE by an average of 15.6% compared to watershed-wide application, although the environmental benefit (EB) was lower (22.0% versus 32.8% on average). We identified a critical budget threshold of 70 million CNY. Below this threshold, CSA-targeting optimized BMPs delivered the most cost-effective TN reductions (123.0 kg/104 CNY per year). However, with a sufficient budget exceeding this threshold, our findings support implementing BMPs throughout the entire watershed, which maximized the TN reduction rate to over 40%. Overall, our findings highlight that spatial targeting and budget-adaptive implementation of BMPs are essential for maximizing both economic efficiency and environmental benefits, providing a practical decision approach for nutrient management in river basins.