Metarhizium anisopliae reshapes the citrus rhizosphere microbiome to enhance fruit quality via nutrient cycling

The rhizosphere microbiome is a critical regulator of nutrient acquisition and plant growth in citrus. Here, we evaluated the effects of the entomopathogenic fungus <i>Metarhizium anisopliae</i> CQMa421 on soil nutrient status, rhizosphere bacterial community structure, and fruit quality in citrus using soil physicochemical assays, plant physiological measurements, and 16S rRNA amplicon high-throughput sequencing. CQMa421 application markedly reshaped soil properties, increasing available potassium by 128.50% and organic matter by 75.05%. In addition, total nitrogen, alkali-hydrolyzable nitrogen, and available phosphorus increased by 112.68%, 155.30%, 305.74% respectively, while soil pH decreased by 0.4 units. CQMa421 treatment significantly increased leaf total nitrogen content and elevated fruit vitamin C by 12.00%. Microbial community profiling showed an enrichment of putatively beneficial taxa, including Proteobacteria and Firmicutes, in treated soils. Functional prediction suggested enhanced nutrient cycling potential, with increased representation of genes associated with carbohydrate metabolism and inorganic ion transport. Collectively, these results indicate that <i>M. anisopliae</i> CQMa421 acts as a plant growth-promoting fungus by enhancing soil nutrient availability and restructuring the rhizosphere microbiome, thereby improving the overall nutrient status of the soil and enhancing citrus fruit quality.