Assessment of the sources of nitrate in the Changjiang River, China using a nitrogen and oxygen isotopic approach.

The Changjiang River is the largest freshwater river in China. Here, the sources and variability in nitrate of the Changjiang River are assessed for the first time using dual isotopic approach. Water samples were collected once in August 2006 from the main channel of the Changjiang and its major tributaries. The concentrations and isotopic composition of nitrate were then analyzed for the waters in the Changjiang River. The delta(15)N and delta(18)O of NO(3)(-) ranges from 7.3 per thousand to 12.9 per thousand and 2.4 per thousand to 11.2 per thousand in the Changjiang River waters, respectively. The ranges of isotopic compositions of nitrate suggested that nitrification (including "modified fertilizer") and urban sewage effluent are the major sources of nitrate in the Changjiang River. The high delta(18)O-NO(3)(-) values were observed in the water of the upper reaches, indicated that the current drought might be one important reason for shifting of isotopes in the special sampling period. In addition, there was a strong positive relationship between delta(15)N-NO(3)(-) and delta(18)O-NO(3)(-), which indicated that denitrification added to the enrichment of heavy isotopes of nitrate.