Nitrate budget of a terrestrial-to-marine continuum in South China: Insights from isotopes and a Markov chain Monte Carlo model

Anthropogenic nitrate (NO3−) production has been increasing and is exported to the ocean via river networks, causing eutrophication and ecological damage. While studies have focused on river NO3− pollution, what has been lacking is the quantification of the sources of NO3− in coastal rivers. This study applied the dual isotopes (δ15N/δ18O-NO3−) to quantify the sources and their fluxes of NO3− in two inflow rivers of the Qinzhou Bay. By adding our results to the NO3− source apportionment in Qinzhou Bay, we, for the first time, established the NO3− budgets of the terrestrial-to-marine continuum in both high- and low-flow seasons. We quantitatively showed the direct and indirect roles (e.g., the stimulation of nitrification by sewage ammonium-NH4+) of terrestrial sources in driving the high NO3− loading in the estuary. The results highlighted the necessity to consider coastal rivers and estuary as a whole, which could shed light on the effective reduction of NO3− pollution in coastal environments. •NO3− sources in two coastal rivers were revealed by isotopes.•The fluxes of the sources were estimated by combining discharge data.•WWTP was a critical yet previously underestimated NO3− source.•Coastal rivers and estuary were considered as a whole to establish NO3− budget.