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

doi:10.1016/j.jhazmat.2023.132047

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

J Hazard Mater. 2023 Sep 15:458:132047. doi: 10.1016/j.jhazmat.2023.132047. Epub 2023 Jul 13.

Authors

Wenshi Zhang¹, Hao Jiang², Wenjing Guo¹, Shen Li¹, Quanfa Zhang³

Affiliations

¹ Key Laboratory of Aquatic Botany and Watershed Ecology, Wuhan Botanical Garden, Chinese Academy of Sciences, Wuhan 430074, China.
² Key Laboratory of Aquatic Botany and Watershed Ecology, Wuhan Botanical Garden, Chinese Academy of Sciences, Wuhan 430074, China; Danjiangkou Wetland Ecosystem Field Scientific Observation and Research Station, Chinese Academy of Sciences & Hubei Province, Wuhan 430074, China. Electronic address: jianghao@wbgcas.cn.
³ Key Laboratory of Aquatic Botany and Watershed Ecology, Wuhan Botanical Garden, Chinese Academy of Sciences, Wuhan 430074, China; Danjiangkou Wetland Ecosystem Field Scientific Observation and Research Station, Chinese Academy of Sciences & Hubei Province, Wuhan 430074, China.

PMID: 37453353
DOI: 10.1016/j.jhazmat.2023.132047

Abstract

River nitrate (NO₃^-) pollution is a global environmental issue. Recently, high NO₃^- levels in some pristine or minimally-disturbed rivers were reported, but their drivers remain unclear. This study integrated river isotopes (δ¹⁸O/δ¹⁵N-NO₃^- and δD/¹⁸O-H₂O), ¹⁵N pairing experiments, and qPCR to reveal the processes driving the high NO₃^- levels in a nearly pristine forest river on the Qinghai-Tibet Plateau. The river isotopes suggested that, at the catchment scale, NO₃^- removal was prevalent in summer, but weak in winter. The pristine forest soils contributed more than 90 % of the riverine NO₃^-, indicating the high NO₃^- backgrounds. The release of soil NO₃^- to the river was "transport-limited" in both seasons, i.e., the NO₃^- production/stock in the soils exceeded the capacity of hydrological NO₃^- leaching. In summer, this regime and the NO₃^--plentiful conditions in the soils associated with the strong NO₃^- nitrification led to the high riverine NO₃^- levels. While the in-soil nitrification was weak in winter, the leaching of legacy NO₃^- resulted in the consistently high NO₃^- levels. This study provides insights into the reasons for high NO₃^- levels in pristine or minimally-disturbed rivers worldwide and highlights the necessity to consider NO₃^- backgrounds when evaluating anthropogenic NO₃^- pollution in rivers.

Keywords: Background; Isotope; Nitrate; Qinghai-Tibet Plateau; Source.