Salinity effect on soil organic carbon characteristics and microbial carbon metabolism in coastal wetland of Yellow River Delta.

Soil organic carbon (SOC) stability is crucial for carbon sequestration of coastal wetlands. However, the mechanisms by which increasing salinity affects the carbon cycle remain unclear as the salinization area of coastal wetlands continues to expand. In this study, the SOC characteristics and carbon metabolism of coastal wetlands under varying salinity conditions, as well as its underlying functional mechanisms, were investigated through a field study in the Yellow River Delta of China. Results showed that SOC concentrations ranged from 1.40 to 2.53 g/kg in the upper soil layer, and salinity promoted its stabilization. Salinity reshaped the microbial community structure and favored microbial utilization of carboxylic acid carbon sources under high salinity conditions. The transformation of labile SOC into more recalcitrant forms was facilitated, increasing the proportion of alkyl carbon and enhancing the aromaticity of SOC. In this process, salinity not only suppressed microbial carbon metabolic activity but also significantly increased polyphenol oxidase activity (P < 0.01) and decreased β-1,4-glucosidase activity (P < 0.01). These changes collectively weakened carbon metabolism and significantly inhibited CO2 and CH4 emissions (P < 0.05), thereby favoring the preservation of stable SOC and enhancing the overall stability of SOC in coastal wetlands.