Nutrient emission policies for multiple clean emissions goals based on water elemental balance

China has substantially improved wastewater collection and concentration-based control of total nitrogen and total phosphorus, yet nutrient stoichiometric imbalance remains a persistent pressure on river basins. This study developed a national-scale C:N:P stoichiometric assessment of nutrient generation, wastewater discharge, and receiving-water conditions within the coupled social-natural water cycle. Literature-derived wastewater data from 450 publications, provincial wastewater and socio-environmental statistics from 2001 to 2023, and 9.86 million receiving-water monitoring records from 1705 national control stations during 2015-2024 were harmonized under a unified C(COD_eq):N(TN):P(TP) framework. The C:N:P ratio shifted from [(354.07 ± 121.33):(10.84 ± 1.85):1] in wastewater influent to [(73.44 ± 26.52):(10.10 ± 2.53):1] in effluent, whereas receiving waters showed [(136.30 ± 51.24):(75.15 ± 48.15):1]. This pattern indicates stronger removal of COD-equivalent organic matter than nitrogen and phosphorus, resulting in carbon-limited effluent and residual nitrogen-phosphorus enrichment. Nitrogen imbalance was mainly associated with agricultural irrigation (32.4%) and water-resource quantity (25.2%), while phosphorus imbalance was dominated by water consumption (32.5%), population pressure (21.3%), and water-resource quantity (21.2%). These findings reveal a stoichiometric feedback pathway from the social water cycle to the natural water cycle, whereby effluent with treatment-altered C:N:P stoichiometry enters receiving waters and may reshape nutrient-limitation conditions not fully captured by individual pollutant concentrations. Incorporating stoichiometric assessment alongside concentration- and load-based regulation therefore provides an additional management dimension for aligning treated-effluent quality with receiving-water ecological conditions and supporting region-specific watershed nutrient management.