Rhizobial Inoculation Improves Soil Properties and Microbial Network Stability to Support Medicago sativa L. Production in Cold Arid Regions

The Qinghai–Tibet Plateau, a globally significant ecological barrier and a core pastoral region, is persistently constrained by cold and arid climatic conditions, nutrient poor soils, and progressive grassland degradation. These challenges necessitate maintaining forage productivity while enhancing ecological stability. Medicago sativa L., valued for its high nutritional quality and capacity for biological nitrogen fixation, has been widely incorporated into regional grassland systems. Rhizobial inoculation, as an environmentally sustainable agronomic practice, is regarded as an effective approach to improving nutrient use efficiency and promoting ecological restoration; however, its underlying mechanisms in cold and arid environments remain insufficiently understood. This study established a field experiment in Delingha, Qaidam Basin, using the cultivar ‘Beilin 201’. Treatments included an uninoculated control (CK) and four rhizobial seed coating rates: E1 (0.75 g·m−2), E2 (1.50 g·m−2), E3 (2.24 g·m−2), and E4 (3.00 g·m−2). The effects on yield, rhizosphere soil physicochemical properties, bacterial community structure, and molecular ecological networks were systematically evaluated. The composite microbial inoculant maintained Medicago sativa L. yield, with only modest and non-significant increases in some treatments. In contrast, soil organic matter increased significantly with application rate (p < 0.001), suggesting a stronger short-term effect on soil properties than on yield. Although network vulnerability was lowest in E4, the differences among treatments were not statistically significant. Mixed effects modeling showed that soil factors (74.79%) and microbial factors (25.12%) jointly influenced yield variation. Structural equation modeling further revealed that microbial factors exerted a positive direct effect on yield (0.3), whereas soil factors exhibited a stronger direct effect (0.57), with inoculation rate primarily influencing yield indirectly through soil mediated pathways. This study elucidates the ecological functions and regulatory mechanisms of rhizobial formulations in high elevation dryland ecosystems and provides both theoretical support and practical guidance for the rational application of microbial fertilizers and the sustainable management of forage systems on the Qinghai–Tibet Plateau.