Nitrate sources and transformations along the Yangtze river and its changes after strict environmental regulation

Abstract

The exploration of the sources and fate of nitrate in surface waters along with the evaluation of the implementation of environmental regulations is essential for preventing and remediating the serious problem of nitrate pollution. However, the nitrate sources and transformations in the Yangtze River and its changes after strict environmental regulation remain unclear. In this pursuit, the present study investigated the spatiotemporal variations of the nitrate source and transformations in the Yangtze River by determining the nitrate isotopic compositions (δ15N-NO3− and δ18O-NO3−) and water chemistry during the wet and dry seasons. Furthermore, the current measurements of the nitrate sources and transformations after the environmental regulations were compared to the historical records in 2006. The results showed that the NO3−-N concentration gradually increased along the river, similar to that in 2006. However, the spatial pattern of δ15N and δ18O of NO3− values obviously changed after the environmental regulations. It was found that the δ15N-NO3− values increased from the headwater to the lower stream, which can be due to assimilation of phytoplankton. The δ18O-NO3− values showed a gradual decrease in the headwater area, an increase at the upper stream, and stabilized in the middle and lower stream. In addition, the δ18O-NO3− value was significantly higher during the wet season than in the dry season (P < 0.05). However, there was no significant seasonal difference in NO3−-N concentrations, δ15N-NO3− value, and nitrate source and transformations. The results of source apportionment indicated that the dominated nitrate source changed from the manure and sewage to soil N after the implementation of the environmental regulations. However, the major nitrate transformations were not affected by the environmental regulations and nitrification was still prevalent in the Yangtze River watershed. The change of the dominated nitrate source validated the effectiveness of the environmental regulation; however, the increase in the nitrate concentration reflected the heavy inputs of residual nitrate from the watershed. This study identified the nitrate sources and transformation processes and its changes after strict environmental regulations, and provided significant information for adjusting and formulating new policies to further control the nitrate pollution.