Impact of Regulation of Microbial Seed Coating on Alfalfa Growth and the Soil Microbial System

Seed coating technology is regarded as one of the optimal strategies to promote sustainable agricultural development. It can effectively optimize the physical and physiological characteristics of seeds, improve germplasm quality, and enhance crop resistance to abiotic and biotic stresses. Saline–alkali soils, characterized by high salinity and alkalinity, severely restrict plant growth and development. However, alfalfa, a high-quality leguminous forage, faces substantial challenges in large-scale popularization and cultivation in saline–alkali regions. At present, research on the application of microbial seed coating technology in alfalfa production under saline–alkali conditions remains insufficient, and relevant techniques and formulations still require optimization. Under field conditions, this study used a randomized complete block design with alfalfa as the research material. Different coating treatments combining plant growth-promoting rhizobacteria (PGPR), rhizobia, and extracellular polysaccharides (EPSs) were established to systematically investigate the effects of various coating formulations on alfalfa yield, nutritional quality, root system architecture, and rhizosphere soil properties. Meanwhile, high-throughput sequencing was employed to analyze shifts in rhizosphere soil microbial community structure. The results demonstrated that all microbial coating treatments exerted significant growth-promoting effects on alfalfa grown in saline–alkali soils, among which the T8 treatment (combined coating of rhizobia + PGPR + EPS) performed the best. This treatment not only significantly improved alfalfa yield and nutritional quality but also modified root system architecture and enhanced soil enzyme activities, soil nutrient contents, and soil physical structure, thereby creating a favorable growth environment for plants. Among the single microbial coating treatments, the combined coating of rhizobia and EPS outperformed other single treatments and exhibited favorable application potential. Sequencing results revealed that microbial seed coating treatments significantly increased the relative abundance of beneficial soil bacteria, decreased the abundance of harmful fungi, regulated rhizosphere microbial community structure, and consequently promoted improvements in alfalfa yield and quality by optimizing the plant growth microenvironment. The findings of this study provide important theoretical support for the popularization and application of microbial seed coating technology in crop cultivation in saline–alkali soils, offer a key reference for optimizing alfalfa-specific seed coating formulations for saline–alkali conditions, and are of great significance for promoting the efficient utilization of saline–alkali land resources and the development of ecological agriculture.