Salinity-associated shifts in r-strategist microorganisms correlate with contrasting accumulation of fungal versus bacterial necromass in coastal soils

Soil salinization is a significant environmental issue worldwide, severely affecting the carbon cycling process of ecosystems. However, the mechanism by which salinity influences the accumulation of soil organic carbon (SOC), especially the contribution of microbial-derived carbon, is still not fully understood. In this study, by setting up three salinity gradients of high salinity, medium salinity, and low salinity, and using amino sugar biomarker methods and molecular biology techniques, the microbial-driven mechanism of SOC accumulation was systematically investigated. The results showed that compared with high salinity conditions (2.82 g kg −1 ), SOC content increased by 40.07% and 162.76%, respectively, under medium and low salinity conditions. The microbial necromass carbon showed a specific response to salinity changes, with the fungal necromass carbon increasing from 1.03 g kg −1 to 2.52 g kg −1 , while the bacterial necromass carbon decreased from 1.14 g kg −1 to 0.63 g kg −1 . The analysis of microbial community structure revealed that r-strategy fungi such as Ascomycota and Mortierellomycota had the highest abundance under low salinity conditions, and were significantly positively correlated with the content of fungal necromass carbon; r-strategy bacteria such as Proteobacteria and Bacteroidota had the highest abundance under high salinity conditions, and were significantly positively correlated with the content of bacterial necromass carbon. This study elucidates how salinity-driven shifts in microbial strategies regulate carbon dynamics, providing a basis for assessing the carbon sequestration potential of salinity soils.