Legume intercropping outperforms gramineous intercropping in improving soil multifunctionality of oil-tea agroforestry systems

Background and Aims Long‑term monoculture drives widespread soil degradation including fertility loss, nutrient imbalance, and weakened ecosystem functions in perennial woody plantations worldwide. Camellia oleifera (oil‑tea), a dominant subtropical woody oil crop, suffers similar deterioration under continuous monoculture. Current intercropping research on oil‑tea has largely focused on individual understory species, leaving systematic comparisons between legume and gramineous functional groups largely unexplored. The distinct soil‑microbial mechanisms through which these two intercrop types influence soil multifunctionality therefore remain unclear. Methods To address this gap, we conducted a two‑year field monitoring campaign (2023–2024) within a long‑term experiment established in 2019, comparing three cropping systems: oil‑tea monoculture, oil‑tea/gramineous intercropping (maize+fescue), and oil‑tea/legume intercropping (soybean+alfalfa). We measured soil physicochemical properties, microbial biomass, extracellular enzyme activities, and bacterial/fungal alpha diversity, and constructed co‑occurrence networks at both fruit‑growth and mature stages. An integrated soil multifunctionality index was developed using multi‑indicator Z‑score standardization. Results Both intercropping systems improved soil multifunctionality relative to monoculture, with legume intercropping showing a substantially stronger effect. Legume intercropping significantly increased inorganic nitrogen, available phosphorus, and carbon‑ and nitrogen‑cycling enzyme activities, accompanied by elevated microbial diversity and network complexity. In contrast, gramineous intercropping primarily alleviated soil compaction and optimized physical structure, with only limited changes in microbial community structure. Conclusion Our findings confirm that legume intercropping outperforms gramineous intercropping and monoculture in enhancing soil multifunctionality, by simultaneously increasing nutrient supply and microbial process efficiency. These results provide targeted species‑selection strategies for sustainable oil‑tea agroforestry management.