Comparative Na+ and K+ Profiling Reveals Microbial Community Assembly of Alfalfa Silage in Different Saline-Alkali Soils

Alfalfa cultivated in saline-alkaline soils exhibits a high buffering capacity but low carbohydrate content, posing challenges in the production of high-quality silage feed. This study investigated alfalfa plants grown under varying conditions: mild (QE, salt content 1–2%), moderate (ZE, salt content 2–3%), severe (HE, salt content 3–4%), and non-saline-alkaline (CON, salt content < 1%). Employing a two-factor experimental design, we analyzed the fluctuations in nutritional quality, microbial abundance, and community composition of different salt–alkaline alfalfa materials and silage feeds. Furthermore, we elucidated the fermentation mechanism involved in salt–alkaline alfalfa ensiling. Following a 60-day ensiling period, the ZE and HE treatments led to substantial reductions in pH, acetic acids (AA), branched-chain butyric acids (BA) content, facultative anaerobic bacteria, and Escherichia coli populations (p < 0.05). Conversely, the ZE and HE treatments increased lactic acid (LA) content and the population of lactic acid bacteria (LAB) (p < 0.05). Additionally, these treatments significantly mitigated protein losses in both raw alfalfa and silage feeds (p < 0.05), while remarkably augmenting the water-soluble carbohydrates (WSC), Na+, and K+ content of alfalfa materials. Sodium ions were found to exert a considerable influence on bacterial community composition during salt–alkaline alfalfa ensiling, with Enterococcus, Lactococcus, and Lactobacillus identified as the predominant fermentative microorganisms. Overall, moderately salt-alkaline alfalfa materials displayed optimal nutritional and fermentation quality, ensuring favorable nutritional attributes and fermentation outcomes under such conditions.