The Effects of Salinity on Microbial Metabolic Limitation and Carbon Use Efficiency in Rhizosphere and Bulk Soils of Tamarix austromongolica

Soil extracellular enzyme activity reflects microbial resource acquisition and metabolic efficiency. However, applying enzyme stoichiometry to explore microbial metabolic limitations and carbon use efficiency (CUE) in rhizosphere and bulk soils under saline conditions remains limited. In this study, rhizosphere and bulk soils of Tamarix austromongolica were sampled along a salinity gradient in the Yellow River Delta to assess microbial metabolic limitation and CUE. Results showed that increasing salinity intensified microbial metabolic limitations and markedly reduced CUE, identifying salinity as the dominant factor constraining microbial efficiency. Rhizosphere soils consistently exhibited phosphorus limitation, whereas bulk soils shifted from balanced N–P limitation to pronounced N limitation with increasing salinity. Despite stronger microbial C limitation, CUE remained significantly higher in the rhizosphere than in the bulk soils, suggesting that continuous carbon inputs and enhanced enzyme activity partially mitigated salinity-induced stress. These findings highlight the complex interplay between salinity stress and rhizosphere effects in regulating microbial nutrient acquisition and carbon metabolism. Overall, this study demonstrates the utility of enzyme stoichiometry for evaluating microbial functional adaptation in saline habitats and provides insights that may contribute to the theoretical basis for vegetation restoration in saline-alkali ecosystems.