Study on the Population and Activity of Nitrogen-Fixing Microorganisms in Sediments of Wujiang Cascade Reservoirs

Microbial nitrogen fixation is a key process in the nitrogen cycle of ecosystems. The molybdenum-iron nitrogenase encoded by the nif gene plays a dominant role in the nitrogen fixation process. However, research on nitrogen-fixing microorganisms and functional genes in reservoir ecosystems is currently limited. In this study, seven cascade reservoirs on the main stream of the Wujiang River were taken as research objects. Using metagenomic and metatranscriptomic techniques, the relative abundances and activities of nitrogen-fixing microorganisms and functional genes in the surface layer (0-3 cm) and lower layer (5-15 cm) sediments were systematically analyzed. The results showed that the alpha diversity of nitrogen-fixing microorganisms and the expression activity of nitrogen fixation genes in the surface sediments were significantly higher than those in the lower layer. The phylum Pseudomonadota is the dominant nitrogen-fixing phylum in sediments (21.89%-51.36%). Beta diversity analysis showed that there were significant differences in the population structures of nitrogen-fixing microorganisms in the surface and lower layers. Reservoir age and total sulfur (TS) are key factors affecting the population of nitrogen-fixing microorganisms and nitrogen fixation pathways. The relative abundance of nitrogen fixation genes is significantly positively correlated with reservoir age and TS contents, but significantly negatively correlated with sulfate (SO42–) content. In addition, the relative abundance of sulfur reduction functional genes (dsrAB) in sediments is positively correlated with nitrogen fixation genes. This study reveals the distribution characteristics and environmental driving mechanisms of nitrogen-fixing microorganisms in cascade reservoirs, providing a theoretical basis for the study of nitrogen cycles in artificial reservoirs.