Source and degradation characteristics of wetland soil organic matter along a salinity gradient in Yellow River Delta

Salt marsh wetlands exhibit high carbon capture and storage capabilities, which are crucial for mitigating climate change. However, the mechanisms of soil organic carbon (SOC) sequestration in coastal deltaic salt marsh wetlands are not well understood. To bridge this gap, we present new findings on the distribution, sources, and decomposition of SOC in the Yellow River Delta wetland, focusing on four vegetation types along a salinity gradient: Phragmites australis, Tamarix chinensis, Suaeda salsa, and Spartina alterniflora. The input of litter was found to be the primary factor affecting SOC at the depth from 20 to 100 cm, while microbial degradation and clay content were the main factors in the deeper soil layers between 20 and 100 cm. The SOC in all four communities was predominantly derived from recalcitrant organic carbon (81%–99%). A Monte Carlo model revealed that terrestrial sources accounted for 61% of SOC, plant sources for 31%, and marine sources for 8%. The vertical distribution of δ13C profiles in Phragmites australis and Spartina alterniflora communities was influenced by preferential utilization of 12C and substrate, with SOC degradation rate constants of 0.28 and 1.02 per annum (a−1), respectively. The invasion of Spartina alterniflora has led to a significant increase in the easily oxidizable carbon (EOC) to SOC ratio, thus reducing SOC stability, which underscores the importance of mitigating Spartina alterniflora invasion. SOC stability was increased by evaluated salinity in the Yellow River Delta wetland, which was higher than that in Chinese coastal wetlands.