The effects of clean energy production and urbanization on sources and transformation processes of nitrate in a subtropical river system: Insights from the dual isotopes of nitrate and Bayesian model

Source appointment and control of water pollution caused by nitrate in river are the key to water quality management as well as the basis for achieving aquatic environment sustainability. However, the effects of clean energy production (hydropower generation) and urbanization on the nitrate accumulation and transformation remain unclear. For that, this study quantified and identified the nitrate sources and key processes in a subtropical river, which has been impounded and urbanized in varying degree. The variations of dual isotopes and water chemistry indicated that NO3-; in agriculture and forest land mainly from the chemical fertilizer and soil N, while manure and sewage were the main riverine NO3- sources. Nitrification was the dominate process while influenced greatly by inputs of manure and sewage, especially in urban area. Progressive denitrification has been found in the upper reaches of the Beixi River where the river impounded by dams for hydropower generation, with an estimated contribution of 36% (1%-68%). Results from Bayesian model showed that fertilizer and soil N contributed more than half of the NO3-; in the wet season as a result of rainfall and surface runoff; while manure and sewage were the dominate NO3-; sources in the dry season due to the urbanization. Land use including agriculture, dam construction, and urbanization have characterized the nitrate sources and pollution in the Jiulong River, and cascade dam impoundment could enlarge nitrate retention. We concluded that the combined use of dual isotopes of NO3 , hydrochemicals and the SIAR model offers an efficient approach to understand the nitrogen transformation and quantify the contributions of nitrogen source in river waters.