Periodic flooding alters ecological processes and carbon metabolism efficiency of riparian soil microbial communities in the three Gorges Reservoir area, China

Soil microbial communities are the most active components in the riparian biota, and are critical in driving carbon cycling. The periodic flooding in riparian zones is a primary driving force in the changes of soil microbial community structures and function. However, whether such events can induce changes in microbial carbon metabolism efficiency has not been fully revealed, especially in large reservoirs that experience counter-seasonal water level fluctuations (WLFs). In this study, high-throughput sequencing and the 18O-H2O cultivation method were applied to investigate the soil microbial community and carbon metabolism in a tributary riparian zone in China's Three Gorges Reservoir, which has experienced large WLFs. Three elevations in the riparian zone (155, 165, and 170 m) were selected as treatments for different flooding intensities. As the frequency of flooding decreased, soil enzyme activity decreased first and then increased. In contrast, soil water content, fungal alpha-diversity, microbial co-occurrence network complexity, average variation degree, beta NTI, and total cohesion decreased slowly. The assembly mechanism of microbial communities is primarily governed by homogeneous dispersion. This suggests that periodic flooding significantly alters microbial ecological processes. Additionally, we found that decreased extracellular enzyme activity increases microbial carbon use efficiency and decreases the metabolic quotient, promoting soil carbon storage. This study enhances our understanding of the response and mechanisms of soil microbial communities to periodic flooding. It provides a theoretical foundation for soil ecosystem management and conservation.