Nitrate dynamics in a forested catchment on the southeastern Tibetan Plateau: A study integrating natural-abundance isotopic, 15N pairing, and microbial molecular techniques

Abstract

• Isotopic and molecular techniques were combined to study NO 3 – dynamics in a catchment. • In-soil nitrification was the dominant source of NO 3 – in the river. • Denitrification, the major NO 3 – removal pathway in the catchment, was prevalent. • Nitrification and denitrification rates were regulated by functional gene abundances. • River runoff significantly regulated the fluvial NO 3 – flux. The nitrate (NO 3 – ) dynamics on the Tibetan Plateau, one of the most climatic sensitive regions on Earth, has not yet been well understood. This study synthesized natural-abundance isotopes, 15 N pairing techniques, and microbial functional genes to systematically elucidate the NO 3 − cycling dynamics as well as their biotic and abiotic driving mechanisms in a pristine forested catchment on the Tibetan Plateau. The river’s natural-abundance isotopes showed that mineralization-nitrification of soil organic nitrogen was the major source of NO 3 − in the river, while significant NO 3 − removal occurred in the catchment. The 15 N pairing experiments support the river’s isotopes and quantitatively showed that nitrification was prevalent in soil, but more than half of the nitrification-derived NO 3 − was removed. In turn, the river isotopes verified the representativeness of the in-soil processes in the catchment. Structural equation models suggested nitrification and denitrification (the major NO 3 − removal pathway) were largely regulated by microbial functional gene abundances, which in turn were regulated by varying abiotic factors. In addition, the fluvial NO 3 – export rates in the sub-catchments were significantly correlated with the river runoff, highlighting the role of hydrological conditions and associated soil leaching in regulating fluvial NO 3 – fluxes. This study shows that combining multi-disciplinary techniques can achieve a more comprehensive understanding of NO 3 − dynamics at the catchment scale.