Salt-tolerant plant moderates the effect of salinity on soil organic carbon mineralization in a subtropical tidal wetland

Although salinization is widely known to affect cycling of soil carbon (C) in tidal freshwater wetlands, the role of the presence or absence of plants in mediating the responses of soil organic carbon (SOC) mineralization to salinization is poorly understood. In this study, we translocated soils collected from a tidal freshwater wetland to sites with varying salinities along a subtropical estuarine gradient and established unplanted and planted (with the salt-tolerant plant Cyperus malaccensis Lam.) mesocosms at each site. We simultaneously investigated cumulative soil CO2 emissions, C-acquiring enzyme activities, availability of labile organic C (LOC), and structures of bacterial and fungal communities. Overall, in the planted mesocosm, the soil LOC content and the activities of β-1,4-glucosidase, cellobiohydrolase, phenol oxidase, and peroxidase increased with salinization. However, in the unplanted mesocosm, soil LOC content decreased with increasing salinity, whereas all the C-acquiring enzyme activities did not change. In addition, salinization favored the dominance of bacterial and fungal copiotrophs (e.g., γ-Proteobacteria, Bacteroidetes, Firmicutes, and Ascomycota) in the planted mesocosms. Contrarily, in the unplanted mesocosms salinization favored bacterial and fungal oligotrophs (e.g., α-Proteobacteria, Chloroflexi, Acidobacteria, and Basidiomycota). In both planted and unplanted mesocosms, cumulative soil CO2 emissions were affected by soil LOC content, activities of C-acquiring enzymes, and microbial C-use trophic strategies. Overall, cumulative soil CO2 emissions increased by 35% with increasing salinity in the planted mesocosm but decreased by 37% as salinity increased in the unplanted mesocosm. Our results demonstrate that the presence or absence of salt-tolerant plants can moderate the effect of salinity on SOC mineralization in tidal wetland soils. Future C prediction models should embed both planted and unplanted modules to accurately simulate cycling of soil C in tidal wetlands under sea level rise.