Soil depth‐dependent relationships between biodiversity and function in subtropical forest conversion

Forest management strategies fundamentally alter above‐ground biodiversity; however, their cascading effects on below‐ground ecological processes, particularly the vertical stratification of biodiversity and its linkage to soil ecosystem multifunctionality (SMF), remain poorly understood. To bridge this gap, we employed a multi‐method framework integrating regional‐scale standardized field experiments, amplicon sequencing and soil profiling to dissect biodiversity–function relationships across multitrophic groups during the conversion of 10 subtropical forests from Cunninghamia lanceolata monocultures to mixed plantations ( Phoebe bournei + C. lanceolata ). Monoculture conversion to mixed plantations significantly increased soil multidiversity (biodiversity across multitrophic levels) and SMF ( p < 0.05). Subsoils demonstrated tighter coupling between multiple biodiversities, especially fungi, and function, particularly for organic matter decomposition, nutrient cycling functional potential and antibiotic resistance gene regulation. Furthermore, we revealed the context dependency of such relationships, which varies markedly with organismal size and rarity, and the importance of biodiversity in maintaining multiple dimensions of functioning in soil ecological networks. More importantly, while plant diversity enhancement in mixed plantations supported SMF compared to monocultures, this effect operated through potential trophic‐level‐specific interactions rather than via multidiversity. Synthesis and applications . Our results indicate that the effect of forest conversion on biodiversity–ecosystem function relationships depends on soil depth. These findings expand our understanding of the conventional topsoil‐centric perspective on biodiversity and ecosystem functioning relationships and highlight the need to integrate soil biota into biodiversity conservation frameworks, particularly in subsoils, to achieve the sustainability of subtropical forest management practices.