The isotopomer ratios of N2O in the Shaying River, the upper Huai River network, Eastern China: The significances of mechanisms and productions of N2O in the heavy ammonia polluted rivers

Abstract

In order to figure out the effects of nitrogen pollution and dams on N2O production paths in the river systems, the isotopomer ratios in N2O, NH4+ and NO3−, as well as N2 concentrations and physico-chemical characteristics of water and sediments from the Shaying River system, which is the biggest tributary and major NH4+ contributor of the Huai River, were analyzed in the years 2015 and 2016. The results showed that the net productions of N2O (△N2O) in the river were pretty high, ranging from 12.9 to 440 nmol/L. N2O exhibited a narrow range in δ15Nbulk (−0.04 to 13.51‰), nevertheless a wide range in δ18O (22.54 to 59.90‰). Isotopocule diagram and Pearson correlation analysis indicated that isotopomer ratios of N2O were significantly affected by the mixing of N2O from difference production paths, not by N2O reduction. Relative contributions of nitrification and denitrification to N2O in the Shaying river system were deduced from the two end-members model. The contribution of nitrification to gross N2O was 58.5% on average, almost equal to the contribution of denitrification in summer, although denitrification was the dominant N2O source with average contribution of 75.6% in winter. No significant relationship was found either between △N2O and NH4+ or between △N2O and NO3− in the Shaying River. Heavy NO3− and COD loading reduced nitrification and increased the relative contribution of denitrification to N2O in winter. Heavy ammonia pollution caused pH values to decrease apparently, from 7.5 ± 0.3 in July to 6.3 ± 0.1 in December, resulting in denitrification being the dominant source to N2O in winter. Assimilation enhanced by the construction of dams had weakened the contribution of nitrification to N2O in summer in the Shaying River.