Shifts in Microbial Biomass C/N/P Stoichiometry and Bacterial Community Composition in Subtropical Estuarine Tidal Marshes Along a Gradient of Freshwater–Oligohaline Water

Saltwater intrusion results in increased estuarine salinity, affecting both ecosystem nutrient dynamics and bacterial community composition in estuarine tidal marshes. However, whether soil microbial biomass (SMB) and bacterial community composition have different responses to low levels of salinity increase is largely unknown. We designed an experiment to investigate the stoichiometry of microbial biomass and bacterial community composition in both the rhizospheres and bulk soils along a low-level salinity gradient across two freshwater and three oligohaline marshes. Soil microbial biomass C (MBC) and microbial biomass nitrogen (MBN) significantly decreased in both rhizospheres and bulk soils as salinity increased, while microbial biomass phosphorus (MBP) increased only in the rhizospheres. Rhizospheric effects increased the concentrations of MBC and MBP but did not significantly affect the concentration of MBN relative to the bulk soils. MBC/MBN/MBP ratios fluctuated drastically, and no clear trends were observed for both the rhizospheres and bulk soils along the salinity gradient. The composition of the bacterial communities changed greatly with increases in salinity, affected by changes in the proportional frequencies of the bacterial taxa, while bacterial alpha diversity was not significantly affected by salinity. A redundancy analysis found that the overlying water salinity and sulfate and soil total organic carbon had a synergistic effect on SMB and bacterial community composition. Our findings demonstrate that increased salinity, even in the low-level salinity range, had a large effect on decreasing MBC and MBN pools, increasing MBP stocks, and affecting bacterial community composition. However, it had a small effect on bacterial diversity. Also, rhizospheres exerted a major control on increasing MBC and MBP but rarely affected bacterial community composition or diversity.