Distinctive Microbial Processes and Controlling Factors of Nitrous Oxide Emission in an Agricultural River Network: Perspective in Riparian Zone Type and Season

The emission of nitrogen oxides (N2O) in rivers is an important source of potent greenhouse gases. However, the mechanism at the interface between rivers and riverbanks remains unclear. This study quantified N2O emissions from natural and artificial riparian zones across seasons and explored the microbial mechanisms affecting N2O production and consumption in an intensive agricultural river network in China. Significant seasonal variability in N2O emission rates was observed (p < 0.05), with mean values of 0.56 ± 0.09 mmol·m−2·h−1 in autumn and 1.13 ± 0.32 mmol·m−2·h−1 in spring. In spring, emissions from natural riparian zones (1.38 ± 0.28 mmol·m−2·h−1) were significantly higher than those from artificial riparian zones (0.89 ± 0.05 mmol·m−2·h−1). All wind-based models significantly overestimated N2O emissions (p < 0.05) due to inflated IPCC emission factors (EF5r), exceeding measured values by 1.76–3.09 times. Dissolved organic carbon and nitrite nitrogen were identified as key environmental drivers of N2O emissions. Nitrogen fixation and ammonification accounted for 82.3% of N2O production. Network analysis revealed a dominant microbial niche containing nitrifiers, sulfate-reducing bacteria, and carbohydrate-degrading taxa. Partial least squares path modeling indicated that riparian zone type altered DOC and NO2− availability, regulated nifH and ureC gene abundances, and enhanced N2O production. These findings underscore the importance of riparian-zone-specific microbial regulation of riverine N2O emissions and demonstrate the necessity of refining EF5r estimates for agricultural river networks.