Effects of Microbial Inoculation on Osmotic Regulation, Ion Homeostasis, Soil Nutrients, and Soil Enzyme Activity in Alfalfa (Medicago sativa L.) Under Salt Stress

Microbial inoculation has been unequivocally established as an effective strategy for promoting plant growth and enhancing salinity tolerance. The objective of this work was to examine the influence of AMF (Funneliformis mosseae) inoculation, whether applied singly or together with PGPR (Bacillus mycoides and Sinorhizobium meliloti), on osmotic balance, ionic homeostasis, soil nutrient status, and enzyme activities in salt-stressed alfalfa (Medicago sativa L.). The results showed that salt stress significantly inhibited soil enzyme activities, reduced soil nutrient contents, and significantly increased the contents of osmolytes and Na+ in both leaves and roots of alfalfa. Additionally, the accumulation of Na+ in roots and leaves exhibited a significant negative correlation with soil nutrient contents (NH4+-N, NO3−-N, available phosphorus and available potassium) as well as with soil enzyme activities (sucrase and alkaline phosphatase). However, microbial inoculation significantly alleviated these adverse effects, mainly by increasing osmotic adjustment substances (e.g., proline), enhancing soil enzyme activities (e.g., urease), improving soil nutrient contents (e.g., soil organic matter), and promoting the uptake of K+ and Ca2+, thereby raising the K+/Na+ and Ca2+/Na+ ratios. Comprehensive analysis revealed that the co-inoculation treatments were superior to the single inoculations, with T7 (AMF + Rhizobium) exhibiting the best performance. This study provides a theoretical reference for the application of microbial inoculants to improve alfalfa performance under saline–alkali conditions.