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

The disturbance of reactive nitrogen (N) on ecosystems and biogeochemical cycles is now one of the most severe environmental problems worldwide. Nitrate (NO) is usually a dominant reactive N species in river ecosystems. Excessive NO concentrations in rivers have led to eutrophication and consequent ecological and environmental damages. Quantifying catchment-scale NO yield and export dynamics is crucial for effective remediation of river NO pollution. Frequently, natural abundance isotopes of NO in a river (δN/δO-NO) are applied to identify sources and potential transformations of NO at a catchment scale, while microbial molecular techniques and N pairing experiments are employed to reveal the NO 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 NO yield and fluvial export dynamics. The protocol links microscopic processes with catchment-scale geochemical characteristics to explicitly describe the NO 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 NO yield and export dynamics in the catchments.