Response of Carbon, Nitrogen and Sulfur Coupling Characteristics to Microbially‐Driven Hydro‐Environmental Fluctuations in a Typical Floodplain Lake System

ABSTRACT In lakes, complex microbially‐driven coupling of nutrient cycling processes and associated mechanisms remains unclear. Accordingly, this study systematically investigates the spatial heterogeneity of microbially regulated carbon (C), nitrogen (N) and sulfur (S) cycling processes, their associated functional genes, key functional taxa and dominant factors in a typical floodplain lake (Poyang Lake). We analysed the spatial heterogeneity of functional pathways, functional genes and the key microbial taxa involved in C, N and S cycling across areas with different hydrological conditions within a floodplain lake. Correlations between cycling of C, N and S were further examined at both the functional gene and metagenome‐assembled genomes (MAGs) levels. In addition, we identified the potential factors influencing the putative coupling of C, N and S cycling processes and how they are shaped by spatial variation in hydrological conditions. Results revealed an overall lower abundance of functional pathways and genes involved in C, N and S cycling in the sub‐lake area compared to the channel and main lake areas. The functional gene co‐occurrence network and MAGs‐based analysis revealed that the coupling of C, N and S cycling in the sub‐lake area was less complex and less restricted than expected, with higher modularity than the channel and main lake areas. The most important factors controlling potential coupled C, N and S cycles in the channel and the main lake areas were concentrations of nitrate nitrogen (NO 3 − ‐N), dissolved organic carbon (DOC) and pH levels. Conversely, dissolved total nitrogen (DTN), conductivity (COND) and NO 3 − ‐N were mainly responsible for the linkage of C, N and S cycling processes in the sub‐lake area. Seven medium‐quality MAGs were found to contain multiple functional genes responsible for C, N and S cycling processes, suggesting a potential microbial basis for the coupling. Networks of C, N and S functional genes and comparison with null models further strengthened the evidence for such coupling. Microbially mediated redox reactions and organic matter decomposition were the main processes behind the potentially coupled C, N and S cycles, which were more intense in the main lake and channel areas than sub‐lake areas. This study elucidates key processes and putative coupling mechanisms of C, N and S cycling and their associated driving factors while highlighting the importance of hydrological processes in characterising the regional distribution of microbial functions in a floodplain lake system. Our findings are intended to help accurately assess the role of lakes in global nutrient cycles.