Soil microbial communities regulate the threshold effect of salinity stress on SOM decomposition in coastal salt marshes

Salinity stress is one of the critical environmental drivers of soil organic matter (SOM) decomposition in coastal ecosystems. Although the temperature sensitivity (Q10) of SOM decomposition has been widely applied in Earth system models to forecast carbon processes, the impact of salinity on SOM decomposition by restructuring microbial communities remains uncovered. Here, we conducted a microcosm experiment with soils collected from the coastal salt marsh in the Yellow River Estuary, which is subjected to strong dynamics of salinity due to both tidal flooding and drainage. By setting a gradient of salt solutions, soil salinity was adjusted to simulate salinity stress and soil carbon emission (CO2) rate was measured over the period. Results showed that as salinity increased, the estimated decomposition constants based on first-order kinetics gradually decreased at different temperatures. Below the 20‰ salinity treatments, which doubled the soil salinity, Q10 increased with increasing salinity; but higher salinity constrained the temperature-related response of SOM decomposition by inhibiting microbial growth and carbon metabolisms. Soil bacteria were more sensitive to salinity stress than fungi, which can be inferred from the response of microbial beta-diversity to changing salinity. Among them, the phylotypes assigned to Gammaproteobacteria and Bacilli showed higher salt tolerance, whereas taxa affiliated with Alphaproteobacteria and Bacteroidota were more easily inhibited by the salinity stress. Several fungal taxa belonging to Ascomycota had higher adaptability to the stress. As the substrate was consumed with the incubation, bacterial competition intensified, but the fungal co-occurrence pattern changed weakly during decomposition. Collectively, these findings revealed the threshold effect of salinity on SOM decomposition in coastal salt marshes and emphasized that salt stress plays a key role in carbon sequestration by regulating microbial keystone taxa, metabolisms, and interactions.