Intercropping with soybean enhances walnut rhizosphere soil physicochemical conditions, microbial community composition, and ecosystem multifunctionality

Abstract

Intercropping is a sustainable agricultural practice known to enhance farmland biodiversity and productivity. However, research on rhizosphere microbial communities in walnut-soybean intercropping systems remains limited. This study investigated a 5-year-old orchard on the Loess Plateau. Rhizosphere soil characteristics and microbial communities were compared between walnut monoculture and walnut-soybean intercropping treatments, where soybean had been intercropped for three consecutive years. Intercropping significantly increased soil organic matter (SOM), microbial biomass carbon (MBC) and nitrogen (MBN), and the activities of carbon- and phosphorus-cycling enzymes (e.g., β-glucosidase), whereas it markedly reduced total nitrogen (TN) and total phosphorus (TP). Bacterial and fungal community composition and abundance differed significantly between intercropping and monoculture treatments, with Mortierellomycota and Proteobacteria enriched under intercropping. Mantel tests revealed that bacterial community composition was correlated solely with SOM, while fungal community composition was correlated with both TN and SOM. Bacterial co-occurrence networks exhibited higher connectivity than fungal networks. The mean soil multifunctionality (SMF) index was significantly higher under intercropping (0.54) than under monoculture (0.43, P  < 0.01). Bacterial PC1 emerged as the dominant positive predictor of SMF (parameter estimate = +0.39, P  < 0.05), and bacterial variables collectively explained 70.51% of the variation in SMF, further indicating that bacteria were the primary microbial drivers of soil multifunctionality in this intercropping system. This study suggests walnut-soybean intercropping may improve soil properties via regulating soil properties and microbial dynamics, providing preliminary guidance for sustainable walnut orchard management.