High resolution nitrogen source apportionment and critical area identification in the Yangtze River Basin

Nitrogen pollution in China’s Yangtze River Basin (YRB) poses a critical threat to ecological integrity, yet effective mitigation is often hindered by the coarse spatial resolution of existing source-apportionment models. This study addresses this limitation by implementing a high-fidelity SPARROW model, discretized into 303 sub-basins, to quantify Total Nitrogen (TN) fluxes and source composition for the 2021-2022 period. Results reveal pronounced spatial heterogeneity in nitrogen yields that macro-scale assessments frequently obscure. While agricultural fertilizer remains the predominant basin-wide driver (38%), our refined resolution uncovers distinct source-dominance shifts-ranging from atmospheric deposition in headwaters to point sources in urbanized deltas. Crucially, we identified 15 Critical Source Areas (CSAs) which, despite occupying only 11% of the basin area, contribute over 20% of the total TN load. These findings demonstrate that effective pollution control requires a shift from uniform reduction targets to zoned, source-specific strategies: prioritizing fertilizer management in the middle reach agricultural plains and wastewater infrastructure upgrades in downstream urban clusters. • Refined SPARROW modeling with 303 sub-basins resolves local nitrogen yield heterogeneity. • Dominant sources shift from agricultural inputs in middle plains to urban point sources downstream. • Fifteen Critical Source Areas (11% of area) disproportionately contribute >20% of basin-wide loads.