Interspecific Plant Interactions Drive Rhizosphere Microbiome Assembly to Alter Nutrient Cycling in Ilex asprella and Grona styracifolia

To address the challenges of low land use efficiency, soil degradation, and high management costs in Ilex asprella cultivation, this study established an I. asprella–Grona styracifolia intercropping system and systematically evaluated its effects on soil nutrient cycling, microbial communities, and crop growth. Field experiments were conducted in Yunfu City, Guangdong Province, with monoculture (LCK for I. asprella, DCK for G. styracifolia) and three intercropping densities (HDT, LDT, MDT). Combining 16S rRNA sequencing and metagenomics, we analyzed the functional profile of the rhizosphere microbiome. The results showed that intercropping significantly increased the biomass of G. styracifolia, with the medium-density (MDT) treatment increasing plant length and fresh weight by 41.2% and 2.4 times, respectively, compared to monoculture. However, high-density intercropping suppressed the accumulation of medicinal compounds. In terms of soil properties, intercropping significantly enhanced soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP), and available nitrogen (AN) in the rhizosphere of both plants. Specifically, AN in the I. asprella rhizosphere increased by 18.9%. Soil urease and acid phosphatase activities were also elevated, while pH decreased. Microbial analysis revealed that intercropping reshaped the rhizosphere microbial community structure, significantly increased the Shannon diversity index of bacteria in the G. styracifolia rhizosphere, and enhanced the complexity of the microbial co-occurrence network. Metagenomic analysis further confirmed that intercropping enriched functional genes related to carbon fixation, nitrogen cycling (nitrogen fixation, assimilatory nitrate reduction), and organic phosphorus mineralization (the phoD gene), thereby driving the transformation and availability of soil nutrients. These findings demonstrate that the I. asprella–G. styracifolia intercropping system, particularly at medium density, effectively improves soil fertility and land use efficiency by regulating rhizosphere microbial functions, providing a theoretical basis for the sustainable ecological cultivation of I. asprella.