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

Abstract

The nitrate (NO3–) 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, 15N pairing techniques, and microbial functional genes to systematically elucidate the NO3− 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 NO3− in the river, while significant NO3− removal occurred in the catchment. The 15N pairing experiments support the river’s isotopes and quantitatively showed that nitrification was prevalent in soil, but more than half of the nitrification-derived NO3− 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 NO3− removal pathway) were largely regulated by microbial functional gene abundances, which in turn were regulated by varying abiotic factors. In addition, the fluvial NO3– 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 NO3– fluxes. This study shows that combining multi-disciplinary techniques can achieve a more comprehensive understanding of NO3− dynamics at the catchment scale.