Mechanisms Driving The High Nitrate Concentrations In A Forest River On The Southeastern Qinghai-Tibet Plateau

Nitrogen is a key biogenic element in ecosystems and is an essential nutrient for the formation of all living organisms.In the context of global change,the global nitrogen cycle is undergoing dramatic changes.The Qinghai-Tibet Plateau is one of the most sensitive regions to climate change.The study of biogeochemical cycling of biogenic elements on the Qinghai-Tibet Plateau is important for understanding regional and global ecological evolution and climate change.Nitrate（NO₃^-）is the most important bioavailable species of nitrogen in aquatic ecosystems and the main driver of eutrophication.The majority of current nitrate research has focused on tracing the sources of nitrate in river waters,with the majority of the research focusing on lakes and groundwater at low elevations that have been severely disturbed by anthropogenic activities.However,few studies focused on the NO₃^-dynamics and its drivers at high elevations,such as the Qinghai-Tibet Plateau.In fact,many pristine rivers around the world or rivers that have minimal human interference may have high NO₃^-content,but the mechanism behind this is still unknown.Understanding the mechanisms driving high NO₃^-levels in rivers in a natural background is essential to the conserving of river water resources and ecosystems.This research systematically investigates the NO₃^-dynamics and its driving mechanism in this pristine catchment by integrating natural abundance isotope,¹⁵N isotope pairing techniques,and molecular biotechnology,taking Longcanggou,a typical forest river in the southeastern Qinghai-Tibet Plateau,as the research object,and revealed the driving factors of the high NO₃^-concentrations and fluxes in this catchment.The main findings of the study are as follows:1)The variations in water quality parameters and N concentrations in the Longcanggou river were clarified.The average concentration of NO₃^--N in summer was 0.98 mg L^-1 and in winter the average concentration of NO₃^--N concentration was 1.05 mg L^-1.NO₃^--N is the main nitrogen form in the catchment and its concentration is much higher than the world average.Dissolved oxygen（DO）was high in both seasons,and the overall environment was aerobic.Chlorophyll a（Chl a）was higher in winter,which may be related to the residence time of water flow in winter.The water was neutral in both seasons.2)The NO₃^-sources were identified and the contribution of each NO₃^-source was quantified.Water chemistry indicators and multiple isotopic compositions(δ¹⁸O/δ¹⁵N-NO₃^-andδD/¹⁸O-H₂O)suggest that nitrification and denitrification are stronger in summer while weaker in winter in this catchment.The results of source analysis indicated that soil organic nitrogen（SON）,atmospheric precipitation（AP）,and animal manure（AM）were potential sources of river NO₃^-.SON was the main source of river NO₃^-in summer with a contribution of 98.8%,while AM and AP contributed only 1.0%and 0.2%,respectively.In the winter,SON contributed 92.3%,while AM and AP contributed 1.6%and 6.1%,respectively.3)The rates and key driving mechanisms of NO₃^-production and removal processes in the soils of the catchment were elucidated.The ¹⁵N pairing experiments quantifies the rates of nitrification,denitrification,Anammox and DNRA processes in soils.The respective rates in summer were 10.27mg N kg^-1 d^-1,5.91 mg N kg^-1 d^-1,0.57 mg N kg^-1 d^-1,and 1.56 mg N kg^-1d^-1;in winter the respective rates were 0.16 mg N kg^-1 d^-1,2.75 mg N kg^-1d^-1,0.26 mg N kg^-1 d^-1,and 0.90 mg N kg^-1 d^-1.Nitrification is robust in the summer,and biological removal rates account for more than half of it,which is compatible with the results of the river’s natural abundance isotopes.According to structural equation modelling,summer nitrification and denitrification（the primary NO₃^-removal process）are primarily regulated by the quantity of microbial functional genes,which in turn are regulated by various abiotic variables（e.g.,altitude,water content,etc.）.In addition,there was no significant correlation between river discharge and NO₃^-concentrations in both seasons,suggesting that the release of soil NO₃^-into the river was in a“transport-limited”mode,and that soil leaching and precipitation were also important mechanisms driving the high NO₃^-concentrations in the river.This study shows that combining multi-disciplinary techniques can achieve a more comprehensive understanding of NO₃^-dynamics at the catchment scale,providing an important method for assessing the fate of NO₃^-in the catchment.The findings have implications for understanding the high NO₃^-concentrations in pristine or minimally disturbed rivers on the Tibetan Plateau as well as over the globe.