Hydrodynamic processes influence nitrous oxide production and emission potential in a mountainous river estuary: Insights from microbial community patterns and model predictions

Abstract Estuaries are important emission hot spots of nitrous oxide (N 2 O) to the atmosphere, which are typically highly dynamic due to complex hydrodynamic processes. However, N 2 O production processes remain poorly understood in estuarine profiles, especially in mountainous river estuaries with complex hydrodynamic conditions. Therefore, we explored the vertical distribution and structural characteristics of the microbial community and related mechanisms of N 2 O production in estuarine hydrodynamic profiles of the mountainous river by combining ecological theory, hydrodynamic modeling, and machine learning. Our results revealed pronounced shifts in microbial community states along the estuarine continuum across vertical profiles. Upstream regions were enriched with nitrogen‐cycling functional microorganisms and exhibited complex microbial networks, whereas downstream areas showed a decline in functional taxa and a less complex microbial interaction. These structural changes were closely associated with N 2 O concentrations: the loss of key functional groups and changed microbial interactions reduced the N 2 O production potential. Environmental gradients shaped by hydrodynamic processes—particularly salinity and substrate availability—emerged as the primary drivers of these microbial states shift. We further quantified the contributions of hydrodynamic processes to N 2 O production and emissions by modeling. The strong mixing process caused by the high flow discharge resulted in a 1.9% increase in dissolved N 2 O concentration and a 3.2% increase in emissions. The N 2 O production and emission hot spot also shifted downstream with increased flow discharge. Our study clarifies that microbiota‐mediated N 2 O production processes in a mountainous river estuary emphasize the crucial role of hydrodynamic processes in regulating and potentially mitigating estuarine N 2 O emissions.