Soil carbon-fixing bacterial communities respond to plant community change in coastal salt marsh wetlands

Autotrophic carbon-fixing bacteria affect carbon sequestration in coastal wetland systems. However, it remains uncertain how carbon-fixing bacterial communities and their carbon-fixing genes play crucial roles in soil carbon fixation and respond to plant community change. Here, we explored changes in soil properties, carbon-fixing genes, and bacterial community structure associated with 5-year-old Spartina alterniflora (SA5), 15-year-old SA (SA15), 21-year-old SA (SA21), and native plants Suaeda salsa (SS) and Phragmites australis (PA) compared to in bare flat (BF) at three soil layers (0–10, 10–30, and 30–60 cm) in coastal salt marsh wetlands. The soil organic carbon (SOC) content, absolute abundance of carbon-fixing genes (cbbM and cbbL), and relative abundance of dominant carbon-fixing genera in all soil layers significantly differed among plant communities (P < 0.05). The effect of plant community on these factors was more pronounced than that of soil depth. The SOC content peaked in SA15 soil. The absolute abundance of cbbM and cbbL was significantly higher in plant soil than in BF soil (P < 0.05). The dominant bacterial genus with carbon-fixing genes in BF, SA5, and SA15 soils was Sulfurovum, in SA21 soil was Bacillus, and in SS and PA soils was Thioalkalispira. Salinity was the key driver affecting the community structure of carbon-fixing bacteria. The soil physicochemical properties of coastal salt marsh wetlands changed with the plant community and soil depth, resulting in shifts in the carbon-fixing bacterial communities. Our findings suggest that plant community change can strongly influence carbon-fixing bacterial communities and highlight the importance of microbial carbon fixation in soil carbon accumulation in coastal salt marsh wetlands.