Input characteristics and sources identification of nitrogen in the three main tributaries of the Three Gorges Reservoir, China

The Jialingjiang River, the Wujiang River and the Yangtze River mainstream were the main inputs of the Three Gorges Reservoir (TGR)’s upstream. They accounted for about 90.8 % of the total discharge of the upstream tributaries. They were also important nitrogen sources of the TGR. The nitrogen and oxygen isotope of nitrate were used to identify the nitrate source of the three rivers. The main results demonstrated as follows: (1) The analysis of nitrogen data of the three main rivers entering the TGR from 2003 to 2013 showed that the average annual total dissolved nitrogen concentrations in the Wujiang River > Jialingjiang River > Yangtze River mainstream. The TN fluxes of the Yangtze River were higher than the other two rivers from 2003 to 2013. The nitrate nitrogen was the main form of the TN, which accounted for more than 65 % of the TN. (2) The results of δ¹⁵N-NO₃ ⁻ and δ¹⁸O-NO₃ ⁻ demonstrated that during the impounding period the nitrate from the nitrification of ammonia/urea-containing fertilizer is likely the major source of the three rivers, while during the high water level stage the nitrification from the soil organic nitrogen is likely the main source. In addition, a strong positive relationship (r ² = 0.7054) was found between δ¹⁵N-NO₃ ⁻ and δ¹⁸O-NO₃ ⁻ during the impounding period, which indicated that denitrification may be responsible for the enrichment of heavy isotope of nitrate.