Plant–Soil–Microbe Interactions Along a Salinity Gradient in the Songnen Plain Grasslands

The salinization of natural grasslands is a growing global concern. The Songnen Plain in northeastern China represents a typical soda–saline grassland region, yet an integrated understanding of how salinization reshapes plant, soil, and microbial components in this ecosystem remains limited. In this study, we investigated plant community characteristics, soil physicochemical properties, and soil microbial communities across a salinity gradient (from non-saline to extremely severe saline) using field surveys, laboratory analyses, and structural equation modeling (SEM). Our results showed that vegetation species diversity, the Shannon–Wiener index, and Simpson’s index all decreased from mild to severe salinization. Soil nutrient indicators, including total nitrogen (TN), total phosphorus (TP), and total potassium (TK), significantly decreased with increasing salinity. SEM revealed that plant community diversity had a significant positive effect on soil microorganisms, whereas soil properties, particularly available potassium (AK) and electrical conductivity (EC), exerted significant negative effects on microbial diversity. Together, these results provide an integrated view of how salinization restructures plant–soil–microbe interactions across the Songnen Plain grasslands. These findings improve understanding of saline–alkali grassland degradation from a plant–soil–microbe perspective and provide a theoretical basis for ecosystem restoration in this region.