Tracing Microbial Production and Consumption Sources of NO in Rivers on the Qinghai-Tibet Plateau via Isotopocule and Functional Microbe Analyses.

Nitrous oxide (NO), a potent greenhouse gas, is produced in rivers through a series of microbial metabolic pathways. However, the microbial source of NO production and the degree of NO reduction in river systems are not well understood and quantified. This work investigated isotopic compositions (δN-NO and δO-NO) and NO site preference as well as NO-related microbial features, thereby differentiating the importance of nitrification, denitrification, and NO reduction in controlling NO emissions from five rivers on the eastern Qinghai-Tibet Plateau (EQTP). The average NO concentration in overlying water (15.2 nmol L) was close to that in porewater (17.5 nmol L), suggesting that both overlying water and sediment are potentially important sources of NO. Canonical and nitrifier denitrification dominated riverine NO production, with contribution being approximately 90%. Nitrification is a non-negligible source of NO production, and NO concentration was positively correlated with nitrification genetic potential. The degree of NO 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 ( + )/ 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 NO in rivers, and the intense microbial reduction of NO significantly accounts for the low NO emissions observed in EQTP rivers, suggesting that both the production and consumption of NO in rivers should be considered in the future.