Niche differentiation of anaerobic ammonium-oxidizing bacteria and denitrifying bacteria in artificial and natural water-level fluctuation zones of the Three Gorges Reservoir area

AIMS: The water-level fluctuation zone (WLFZ) serves as a critical ecotone for nitrogen cycling in reservoirs, but microbial differentiation between artificial and natural WLFZs remains unclear. This study aims to explore how WLFZ type (artificial vs. natural) and hydrological gradients influence the niche differentiation of anaerobic ammonium-oxidizing (anammox) and denitrifying bacteria. METHODS AND RESULTS: Using soil physicochemical analysis, qPCR, and high-throughput sequencing, we investigated the communities of anammox and denitrifying bacteria in artificial and natural WLFZs at four elevation levels (150-180 m) in the Three Gorges Reservoir area. The results revealed that artificial WLFZ had significantly greater abundance and diversity of anammox bacteria and nirS/nirK-type denitrifiers than natural WLFZ. The abundance of anammox increased with increasing water level. Moreover, the diversity and abundance of nirS-type denitrifiers were generally higher than those of nirK-type denitrifiers, indicating better adaptation of nirS-type denitrifiers to the WLFZ environment in the Three Gorges Reservoir area. Anammox and denitrifiers predominantly exhibited competitive ecological interactions driven by NO₂- dynamics. In contrast, nirS/nirK-type denitrifiers were influenced by the C/N ratio and moisture content and could coexist in the WLFZ. CONCLUSIONS: WLFZ type and water level jointly shape the structure and ecological relationships of nitrogen-transforming bacteria. Artificial WLFZ provides more favorable conditions for nitrogen cycling. These findings offer insights into microbial nitrogen dynamics and inform reservoir management strategies.