Sources and carbon sequestration mechanism of soil inorganic carbon in coastal wetland of Jiaozhou Bay, China

As an important area for the exchange of matter between land and sea, the storage, source and fixation of soil inorganic carbon in coastal wetlands are of great significance for the sequestration of CO2. To clarify the source and carbon sequestration mechanism of soil insoluble inorganic carbon in coastal wetlands, this study selected four representative wetlands in Jiaozhou Bay, including mudflat wetland, aquaculture pond wetland, Suaeda salsa wetland and Phragmites australis wetland, and used stable carbon isotope technology to measure the δ13C values of soil insoluble inorganic carbon (SIC), soil organic carbon (SOC) and soil CO2. The results showed that the contents of SIC, dissolved inorganic carbon (DIC) and pedogenic carbonate (PIC) were 3.17–12.27 g∙kg−1, 0.05–0.09 g∙kg−1, 0.19–5.22 g∙kg−1, respectively, and there were significant differences in the content of different forms of soil inorganic carbon in soil of different wetland types. The main component of SIC in coastal wetland soils was PIC (59.7 %–86.5 %) (except for the aquaculture pond wetland). The CO2 fixed in the process of PIC formation was mainly from biogenic CO2, with atmospheric CO2 as a supplement, and the restoration of coastal wetland vegetation can promote the production of biogenic CO2 and the formation of PIC. High soil pH and SOC were conducive to the fixation of CO2 during PIC formation. Ca2+ and SWC had a positive effect on the inorganic sequestration of atmospheric CO2, but a negative effect on the inorganic sequestration of biogenic CO2 in coastal wetlands. The content of PIC was equivalent to or even higher than the SOC, and the conversion process of SOC to PIC promoted the conversion of short-term to long-term geological carbon cycling. Therefore, the role of PIC should not be ignored in estimating the carbon budget of the 'blue carbon' habitat in coastal wetlands in the future.