The Pearl River-Estuary-Northern South China Sea (NSCS) continuum constitutes a critical land-ocean interface where massive anthropogenic nitrogen loading from one of China’s largest watersheds converge with oligotrophic marginal sea conditions; yet the biogeochemical controls on nitrate across the full salinity gradient remain poorly constrained. Here, we present nitrate dual-isotope (δ¹⁵N, δ¹⁸O) measurements along this transect, complemented by seasonal-interannual time-series from the Dongjiang River, one of the three major tributaries of the Pearl River System, and Bayesian mixing models (MixSIAR), to quantify nitrate sources, transformation pathways, and their modulation by climatic and anthropogenic factors.
In the Dongjiang River, soil nitrogen emerges as the dominant nitrate source, with manure/sewage and fertilizer playing secondary roles. Seasonally, wet-season rainfall enhances soil-nitrate flushing, whereas dry-season baseflow amplifies sewage-derived signals; transitional climatic events produce distinct mixtures not captured by dry/wet dichotomy. El Niño-driven heavy rainfall increases soil-derived nitrate export. Urbanization expansion and agricultural intensification increase sewage and fertilizer contributions, respectively; policy upgrades (i.e., wastewater treatment and fertilizer restrictions) are reflected in declining nitrate concentrations accompanied by increasing δ¹⁵N, indicating measurable mitigation. In the Pearl River Estuary, elevated nitrate with high δ¹⁵N values reflects a mixed provenance (including soil, fertilizer and manure/sewage). Isotopic deviations from conservative mixing delineate nearshore nitrate assimilation and nitrification, with estimated fractionation factors (εN = 4.0‰, εO = 5.4‰) for assimilation in the lower euphotic zone. In the offshore NSCS, nitrate becomes nearly depleted and exhibits moderate isotope signatures, and surface water δ¹⁵N versus δ¹⁸O relationship departs from the 1:1 assimilation line, revealing a rising atmospheric nitrate contribution seaward.
Collectively, the isotopic gradients demonstrate a systematic source transition from terrestrial anthropogenic nitrate in the river-estuary to upwelled deep-water nitrate and atmospheric deposition offshore, with estuarine assimilation and nitrification acting as primary biogeochemical filters. Climatic and anthropogenic drivers jointly regulate source partitioning and transformation intensities across multiple timescales. These dual-isotope constraints thus provide a robust quantitative framework for elucidating nitrogen budgets in river-dominated marginal seas and for guiding adaptive nutrient management strategies under changing environmental conditions.