Salinity constrains rhizosphere-driven soil carbon mineralization through shifts in microbial communities and enzyme activities

Functional assessment of how the rhizosphere regulates soil carbon (C) mineralization is critical for predicting ecosystem C dynamics under global climate change. However, the mechanisms by which rhizosphere-mediated microbial traits and extracellular enzyme activities influence soil C mineralization under saline conditions remain poorly understood. Here, we compared C mineralization and associated biotic and abiotic drivers in rhizosphere and bulk soils across three salinity levels in a saline–alkali ecosystem. We found that elevated salinity significantly inhibited C mineralization in both soil compartments. Under medium- and high-salinity levels, cumulative C mineralization in the rhizosphere significiantly decreased by 11.67% and 19.41%, respectively, relative to low-salinity, corresponding to declines in hydrolytic enzyme activities and bacterial oligotroph-to-copiotroph (Oli/Cop) ratios, along with an increase in salt-tolerant copiotrophic taxa. Moreover, network analysis showed that 58.88% of bacterial and 45.70% of fungal ASVs were positively correlated with SOC and C mineralization but negatively correlated with salinity. Under low-salinity, rhizosphere soils exhibited significantly higher C mineralization than bulk soils, whereas this rhizosphere advantage was no longer statistically detectable under medium- and high-salinity levels. As salinity increased, fungal communities in the rhizosphere shifted toward oligotrophic taxa, whereas copiotrophic fungi dominated in bulk soil, utilizing sporadic labile C sources to sustain metabolic activity and potentially offsetting salinity-induced suppression of C mineralization. Overall, salinity constrained soil C cycling by limiting C input, suppressing hydrolytic enzyme activity, and altering microbial functional groups. Although the rhizosphere effect enhanced local enzymatic and microbial activities, salinity-induced physiological constraints dominated, ultimately leading to an overall reduction in soil C mineralization. These findings provide a functional assessment of the rhizosphere-soil-microbe continuum under salinity stress, revealing how shifts in microbial life-history strategies and enzyme activities constrain soil carbon mineralization. Overall, our findings provide new insights into the mechanisms by which salinity regulates rhizosphere-driven soil C mineralization and support the development of rhizosphere-oriented management strategies to sustain vegetation restoration and soil carbon sequestration in saline ecosystems.