Relationships among different fertilization schemes, soil microbial function, tree growth, and fruit quality in apple orchards on the semiarid Loess Plateau

The semi-arid Loess Plateau is a major apple-producing region in China; however, soil degradation and low nutrient use efficiency have constrained sustainable orchard productivity. In this study, a three-year field experiment was conducted to evaluate the effects of different fertilization regimes involving biogas fertilizer on soil physicochemical properties, microbial community structure, tree physiological performance, and fruit quality. The results revealed that fertilization regimes in which biogas was incorporated were associated with increased soil water-holding capacity, soil organic matter (SOM) content, nutrient availability, and cation exchange capacity (CEC) and the maintenance of soil pH within a suitable range (6.5–7.5). These effects were most pronounced in the 0–20 cm soil layer and became more evident with increasing application rates. In parallel, shifts in microbial community composition and diversity were observed, with an enrichment of functional groups related to organic matter decomposition and nutrient cycling. At the plant level, improved soil conditions were accompanied by enhanced leaf nutrient accumulation and photosynthetic performance, which were further associated with improvements in fruit size, colouration, sugar composition, and bioactive compounds. Among the tested regimes, treatment with 90 kg of biogas fertilizer per tree resulted in the most favourable overall performance under the experimental conditions. Overall, the findings suggest that fertilization regimes integrating biogas fertilizer may influence fruit quality formation through the coordinated regulation of soil properties, microbial communities, and plant physiological processes. However, owing to differences in nutrient inputs among treatments, the observed effects reflect integrated fertilization outcomes rather than the effects of individual fertilizer types. This study provides a basis for optimizing organic fertilization strategies in semi-arid apple orchards.