Estuarine wetland tidal organic carbon activates microbial carbon pump and increases long-term soil carbon stability

Capture of tidal organic carbon (TOC) in estuarine systems can increase soil respiration (SR) and OC breakdown, but it is also revealed tht TOC input intensity may influence the accumulation of microbial necromass, which affects soil carbon sink of estuarine wetland. To clarify how TOC input affects the microbial carbon pump and SOC stability, we conducted a 6-year field study in low and high flats of the Yangtze River estuary and examined C cycling processes. In low tide flats, where TOC input decreased from upstream to downstream, SR decreased with the increases of SOC, whereas the microbial necromass contribution to SOC decreased from 52.16% to 41.75%, which was higher than the contribution of plant-derived refractory C. Additionally, on a mudflat, which had the highest TOC capture but lacked plant C input, microbial necromass accounted for the largest percentage of SOC (61.45%). These indicated that TOC input could increase the contribution of microbial necromass C toward SOC, in comparation with plant lignin. And the promotion of TOC input to OC stability of wetland soil would become significant with the accumulation of TOC in soil after a long period. The analysis of bacterial and fungal community structure verified this speculation that the abundance of C-associated heterotrophic bacteria and fungi increased in soil with high TOC input. These findings further supplement our previous study about the short-term inhibition effect of TOC input to SOC sequestration.