Unexpectedly high nitrate levels in a pristine forest river on the Southeastern Qinghai-Tibet Plateau
Wenshi Zhang, Hao Jiang, Wenjing Guo, Shen Li, Quanfa Zhang
Abstract
River nitrate (NO 3 – ) pollution is a global environmental issue . Recently, high NO 3 – levels in some pristine or minimally-disturbed rivers were reported, but their drivers remain unclear. This study integrated river isotopes (δ 18 O/δ 15 N-NO 3 – and δD/ 18 O-H 2 O), 15 N pairing experiments, and qPCR to reveal the processes driving the high NO 3 – levels in a nearly pristine forest river on the Qinghai-Tibet Plateau. The river isotopes suggested that, at the catchment scale, NO 3 – removal was prevalent in summer, but weak in winter. The pristine forest soils contributed more than 90 % of the riverine NO 3 – , indicating the high NO 3 – backgrounds. The release of soil NO 3 – to the river was “transport-limited” in both seasons, i.e., the NO 3 – production/stock in the soils exceeded the capacity of hydrological NO 3 – leaching. In summer, this regime and the NO 3 – -plentiful conditions in the soils associated with the strong NO 3 – nitrification led to the high riverine NO 3 – levels. While the in-soil nitrification was weak in winter, the leaching of legacy NO 3 – resulted in the consistently high NO 3 – levels. This study provides insights into the reasons for high NO 3 – levels in pristine or minimally-disturbed rivers worldwide and highlights the necessity to consider NO 3 – backgrounds when evaluating anthropogenic NO 3 – pollution in rivers.