Alfalfa Plant Age (3 to 8 Years) Affects Soil Physicochemical Properties and Rhizosphere Microbial Communities in Saline–Alkaline Soil

Increasing soil salinization can severely restrict local agricultural production. Planting alfalfa is considered an effective measure to ameliorate saline–alkali soil. However, it remains unclear how alfalfa planting years affect the sustained impact on soil and rhizosphere microecology. This study analyzed the effects of alfalfa planted 3, 6, and 8 years ago on soil physicochemical properties and key soil enzyme activities and investigated the rhizosphere microbial community structure and diversity. The results indicate that cultivating alfalfa plants for six years can improve soil physicochemical properties and enhance soil fertility to a certain extent. This is attributed to a higher abundance of plant growth-promoting bacteria, such as Bradyrhizobium and Allorhizobium, as well as degradation bacteria, such as Flavobacterium, Stenotrophomonas, Brevundimonas, and Massilia, in the rhizosphere of alfalfa plants. These microorganisms promote alfalfa growth, improve soil quality, and inhibit the accumulation of autotoxins. This not only maintains high alfalfa yields but also optimizes soil physicochemical properties and enzyme activity, facilitating more effective nutrient cycling and metabolic processes in the soil. However, extending plant growth to 8 years is not beneficial.