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.