Divergent Assembly Mechanisms and Functional Dominance of Ammonia Oxidizers in Lancang River Cascade Reservoir Sediments

AIMS: Cascade hydropower development profoundly alters riverine ecosystems, yet how it influences ammonia-oxidizing microorganisms in reservoir sediments remains unclear. This study aimed to elucidate the relative functional contributions, community assembly mechanisms, and environmental drivers of ammonia-oxidizing archaea (AOA) versus bacteria (AOB) across seven cascade reservoirs in the Lancang River. METHODS AND RESULTS: We collected surface sediments seasonally and employed quantitative PCR, high-throughput sequencing, and multivariate analyses. AOA amoA gene abundance consistently exceeded that of AOB, and potential nitrification rate correlated significantly only with AOA abundance, indicating AOA functional dominance. AOA communities exhibited higher diversity, stronger spatial structuring, and deterministic assembly governed by environmental selection, while AOB assembly was more stochastic. Dominant AOA (Nitrososphaera) and AOB (Nitrosospira) shifted spatially, with marine-related Nitrosopumilus appearing only in mid- and downstream reservoirs. Water depth and hydraulic retention time primarily influenced AOB, whereas AOA were regulated by C/N ratio, pH, and reservoir age. CONCLUSIONS: This study demonstrates that AOA, rather than AOB, dominate ammonia oxidation among canonical ammonia oxidizers in cascade reservoir sediments and that damming-induced environmental gradients drive divergent assembly mechanisms between these functional groups. These findings challenge conventional nitrification paradigms in river systems and provide a predictive framework for managing nitrogen cycling in the rapidly expanding global network of dammed rivers.