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
2019
Ma, Pei | Li, Xinyan | Chen, Feng | Liu, Shuaixia | Hou, Cuicui
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.
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