Coupling geochemical and microbial molecular techniques to reveal catchment-scale nitrate yield and fluvial export...

The disturbance of reactive nitrogen (N) on ecosystems and biogeochemical cycles is now one of the most severe environmental problems worldwide. Nitrate (NO₃^-) is usually a dominant reactive N species in river ecosystems. Excessive NO₃^- concentrations in rivers have led to eutrophication and consequent ecological and environmental damages. Quantifying catchment-scale NO₃^- yield and export dynamics is crucial for effective remediation of river NO₃^- pollution. Frequently, natural abundance isotopes of NO₃^- in a river (δ¹⁵N/δ¹⁸O-NO₃^-) are applied to identify sources and potential transformations of NO₃^- at a catchment scale, while microbial molecular techniques and ¹⁵N pairing experiments are employed to reveal the NO₃^- production and removal processes and their underlying mechanisms in microenvironments (e.g., sediments and soils). In this study, we developed a novel protocol that couples these complementary geochemical and molecular techniques to quantify catchment-scale NO₃^- yield and fluvial export dynamics. The protocol links microscopic processes with catchment-scale geochemical characteristics to explicitly describe the NO₃^- cycling processes and their underlying abiotic and biotic mechanisms within a catchment. We applied the protocol to the Dadu and Jiazela catchments on the Qinghai-Tibet Plateau, and demonstrated the effectiveness of the protocol in determining NO₃^- yield and export dynamics in the catchments.