The Influence of Planting Different Green Manures in Apple Orchards on Soil Fertilizer and Apple Quality in the Loess Plateau

Long-term clean tillage in apple orchards on the Loess Plateau has degraded soil properties and diminished fruit quality, highlighting the urgent need for sustainable cover cropping strategies. In a two-year field experiment (two consecutive growing seasons), eight green manure species (annual ryegrass, perennial ryegrass, white clover, smooth vetch, common vetch, rapeseed, hairy vetch, and cocksfoot) were compared with a clean-tillage control. Each treatment was replicated three times, and each plot was 100 m2. Soil physicochemical properties, microbial communities (16S rRNA and ITS amplicon sequencing), and fruit quality parameters were systematically assessed. All grass treatments significantly lowered soil pH and increased soil organic matter and total nitrogen, phosphorus, and potassium contents. Hairy vetch (T7) produced the highest soil organic matter (25.86 g kg−1) and total nitrogen (1.43 g kg−1), while rapeseed (T6) reduced pH most markedly (4.28%). White clover (T3) enhanced bacterial and fungal alpha-diversity and enriched beneficial genera (Sphingomonas, Lysobacter, Mortierella, Chaetomium). However, functional predictions based on 16S/ITS data using FAPROTAX and Tax4Fun (PICRUSt-type predictive tools) suggested a potential increase in chemoorganoheterotrophy and saprotrophy; these predicted functions should be interpreted as hypothesis-generating rather than as direct evidence of microbial function. Common vetch (T5) increased soluble sugar content by 28.1%, and cocksfoot (T8) decreased titratable acidity by 30.8%. The comprehensive analysis indicates that hairy vetch and white clover as the most effective species for improving soil fertility, microbial diversity, and fruit quality. We therefore recommend these two legumes as optimal cover crops for sustainable orchard floor management in rain-fed semi-arid regions of the Loess Plateau.