Contrasting the Microbial Communities in Rhizosphere and Bulk Soils Across Different Eucommia ulmoides Planting Sites and Their Soil Chemical Driving Mechanisms

Soil multifunctionality (SMF) is a core indicator of plantation soil ecological function, and microbial diversity plays a vital role in sustaining it. However, cross-site rhizosphere and bulk SMF disparities and their microbial driving mechanisms remain unclear in Eucommia ulmoides plantations. Here, we collected rhizosphere and bulk soils from three typical sites (Mengzhou, MZ; Liangyuan, LY; and Yuanyang, YY). Soil chemical properties, extracellular enzymes, microbial alpha diversity, community composition and cross-kingdom network topology were measured. Correlation heatmaps, random forest, Redundancy analysis (RDA) and Partial least path modeling (PLS-PM) were adopted to quantify SMF predictors and regulatory pathways. Rhizosphere soils possessed significantly higher alkaline hydrolyzable nitrogen (AN), available phosphorus (AP) and available potassium (AK) than bulk soils at all sites. LY rhizosphere showed the greatest soil organic carbon (SOC), total potassium (TK), available nutrients and enzyme activities, while YY had higher total nitrogen (TN) and AP, yet the lowest enzyme levels. Rhizosphere bacterial and fungal alpha diversity was consistently higher across locations. SMF varied distinctly by site and compartment: LY had substantially higher SMF than MZ and YY in both rhizosphere and bulk soils, with rhizosphere SMF being consistently greater than bulk values across all sites. The PLS-PM (GOF = 0.70) indicated that soil chemical properties regulated SMF via dual pathways: they directly promoted microbial co-occurrence networks and indirectly modified network structure by altering fungal diversity, while suppressing bacterial diversity. Total effect analysis identified soil chemical properties and microbial co-occurrence networks as the core drivers of SMF variation. This work clarifies that rhizosphere effects and soil chemical properties jointly drive SMF by regulating microbial diversity and co-occurrence networks, offering theoretical guidance for sustainable soil management in E. ulmoides plantations.