Nitrate transformation and source tracking of Yarlung Tsangpo River using a multi-tracer approach combined with Bayesian stable isotope mixing model
Yufei Bao, Meng Sun, Yuchun Wang, Mingming Hu, Peng Hu, Leixiang Wu, Wei Huang, Shanze Li, Jie Wen, Zhongjun Wang, Qian Zhang, Nanping Wu
Abstract
Excessive levels of nitrate nitrogen (NO 3 − -N) could lead to ecological issues, particularly in the Yarlung Tsangpo River (YTR) region located on the Qinghai Tibet Plateau . Therefore, it is crucial to understand the fate and sources of nitrogen to facilitate pollution mitigation efforts. Herein, multiple isotopes and source resolution models were applied to analyze key transformation processes and quantify the sources of NO 3 − . The δ 15 N–NO 3 - and δ 18 O–NO 3 - isotopic compositions in the YTR varied between 1.23‰ and 13.64‰ and −7.88‰–11.19‰, respectively. The NO 3 − -N concentrations varied from 0.08 to 0.86 mg/L in the dry season and 0.20–1.19 mg/L during the wet season . Nitrification remained the primary process for nitrogen transformation in both seasons. However, the wet season had a widespread effect on increasing nitrate levels, while denitrification had a limited ability to reduce nitrate. The elevated nitrate concentrations during the flood season were caused by increased release of NO 3 − from manure & sewage (M&S) and chemical fertilizers (CF). Future endeavors should prioritize enhancing management strategies to improve the utilization efficiency of CF and hinder the direct entry of untreated sewage into the water system.