Characteristics of Soil Nutrients and Microorganisms at the Grassland–Farmland Interface in the Songnen Agro-Pastoral Ecotone of Northeast China

The ecological interface between grasslands and farmlands forms a critical landscape component, significantly contributing to the stability and functioning of ecosystems within the agro-pastoral transition zone of northern China. Nevertheless, the variation patterns and interactions between soil physicochemical attributes and microbial community diversity at this interface remain poorly understood. In this study, we investigated nine sites located within 50 m of the grassland–farmland boundary in the Songnen Plain, northeastern China. We assessed the soil’s physicochemical properties and the composition of bacterial and fungal communities across these sites. Results indicated a declining gradient in soil physicochemical characteristics from grassland to farmland, except for pH and total phosphorus (TP). The composition of bacterial and fungal communities differed notably in response to contrasting land-use types across the ecological interface. Soil environmental variables were closely aligned with shifts observed in bacterial and fungal assemblages. Concentrations of total nitrogen (TN), available phosphorus (AP), alkali-hydrolyzable nitrogen (AN), and available potassium (AK) exhibited inverse correlations with both bacterial and fungal populations. Alterations in microbial community composition were significantly linked to TN, TP, total potassium (TK), AN, AP, AK, and soil pH levels. Variability in soil properties, as well as microbial biomass and diversity, was evident across the grassland–cropland boundary. Long-term utilization and conversion of grassland into cultivated land altered the soil’s physicochemical environment, thereby indirectly shaping the structure of microbial communities, including both bacteria and fungi. These findings provide a valuable basis for understanding the ecological implications of land-use transitions and inform microbial-based indicators for assessing soil health in agro-pastoral ecotones.