Effects of Grass Clipping Mulching on Alfalfa Yield and Quality Through Soil Amelioration in Soda Saline–Alkali Soils

Salinization is a major constraint to agricultural sustainability in arid and semi-arid regions worldwide. The ameliorative effects of grass clipping mulching on alfalfa (Medicago sativa L.) growth and forage quality in soda saline–alkali land in Northeast China and its regulatory mechanisms were studied. The trial used a randomized block design with four mulch treatments: no-mulch control (CK), low-density (LC, 6000 kg ha−1, or 6.0 t ha−1), medium-density (MC, 12,000 kg ha−1, or 12.0 t ha−1), and high-density (HC, 18,000 kg ha−1, or 18.0 t ha−1). Compared with CK, MC achieved the highest soil water content (18.3% vs. 14.9% in CK), the lowest bulk density (1.26 vs. 1.48 g cm−3), and the greatest porosity (52.4% vs. 44.2%); pH, electrical conductivity, total salt, and ESP decreased, while organic matter and available nutrients increased. Overwintering survival rose to 89.1% under HC (+9.3%). Net photosynthetic rate, stomatal conductance, and chlorophyll content increased, and first-cut hay yield increased by 27.3% under HC (3938 vs. 3093 kg ha−1). Crude protein reached 20.42%, and relative feed value increased from 158.4 to 167.1. Partial least squares structural equation modeling (PLS-SEM) showed that mulching acted mainly through soil nutrient enrichment, thereby enhancing plant physiology and ultimately forage yield and quality. Grass clipping mulching is a cost-effective strategy for saline–alkali soil remediation and sustainable alfalfa production.