Unexpectedly high nitrate levels in a pristine forest river on the Southeastern Qinghai-Tibet Plateau.

River nitrate (NO) pollution is a global environmental issue. Recently, high NO levels in some pristine or minimally-disturbed rivers were reported, but their drivers remain unclear. This study integrated river isotopes (δO/δN-NO and δD/O-HO), N pairing experiments, and qPCR to reveal the processes driving the high NO levels in a nearly pristine forest river on the Qinghai-Tibet Plateau. The river isotopes suggested that, at the catchment scale, NO removal was prevalent in summer, but weak in winter. The pristine forest soils contributed more than 90 % of the riverine NO, indicating the high NO backgrounds. The release of soil NO to the river was "transport-limited" in both seasons, i.e., the NO production/stock in the soils exceeded the capacity of hydrological NO leaching. In summer, this regime and the NO-plentiful conditions in the soils associated with the strong NO nitrification led to the high riverine NO levels. While the in-soil nitrification was weak in winter, the leaching of legacy NO resulted in the consistently high NO levels. This study provides insights into the reasons for high NO levels in pristine or minimally-disturbed rivers worldwide and highlights the necessity to consider NO backgrounds when evaluating anthropogenic NO pollution in rivers.