Tracing Microbial Production and Consumption Sources of N2O in Rivers on the Qinghai-Tibet Plateau via Isotopocule and Functional Microbe Analyses

Nitrous oxide (N2O), a potent greenhouse gas, is produced in rivers through a series of microbial metabolic pathways. However, the microbial source of N2O production and the degree of N2O reduction in river systems are not well understood and quantified. This work investigated isotopic compositions (δ15N-N2O and δ18O-N2O) and N2O site preference as well as N2O-related microbial features, thereby differentiating the importance of nitrification, denitrification, and N2O reduction in controlling N2O emissions from five rivers on the eastern Qinghai-Tibet Plateau (EQTP). The average N2O concentration in overlying water (15.2 nmol L-1) was close to that in porewater (17.5 nmol L-1), suggesting that both overlying water and sediment are potentially important sources of N2O. Canonical and nitrifier denitrification dominated riverine N2O production, with contribution being approximately 90%. Nitrification is a non-negligible source of N2O production, and N2O concentration was positively correlated with nitrification genetic potential. The degree of N2O reduction ranged from 78.1 to 94.1% (averaging 90%), significantly exceeding the reported values (averaging 70%) in other freshwaters, which was attributed to the higher ratios of organic carbon to nitrogen and lower ratio of (nirS + nirK)/nosZ in EQTP rivers. This study indicates that a combination of isotopic and isotopocule values with functional microbe analysis is useful for quantifying the microbial sources of N2O in rivers, and the intense microbial reduction of N2O significantly accounts for the low N2O emissions observed in EQTP rivers, suggesting that both the production and consumption of N2O in rivers should be considered in the future.