Life-form composition of spontaneous vegetation drives root biomass patterns in southeastern Amazonian agroforestry systems

Agroforestry systems (AFS) enhance biodiversity and contribute to carbon storage in plant biomass and soil organic matter. Unraveling the ecological role of spontaneous vegetation and its relationship with belowground carbon pools helps clarify how this component influences ecosystem services, which remain poorly understood in early-stage systems. This study evaluated five AFS modules in southeastern Pará, Brazil, to assess how vegetation richness, density, and life-form composition influence fine- and coarse-root biomass. The modules combined cocoa, banana, cassava, pineapple, and maize, with an adjacent secondary forest serving as a control. Established on uniform edaphic conditions to isolate biotic effects, root biomass was sampled at 0–10 cm depth and spontaneous individuals ≤ 1.5 m were identified. Forest plots exhibited substantially higher root biomass (fine: 284 g·m⁻²; coarse: 58 g·m⁻²) compared to AFS modules (fine: 56–116 g·m⁻²; coarse: 1.5–7.2 g·m⁻²). Richness and density of spontaneous vegetation were negatively correlated with root biomass (p < 0.01), supporting a balanced-growth strategy where plants prioritize aboveground allocation in managed soils. Redundancy analysis revealed that vegetation life forms explained 65.2% of root variation; herbaceous species were strongly associated with fine roots, whereas woody life forms (trees, shrubs, and palms) correlated with coarse-root biomass. These findings highlight that life-form composition, rather than species numbers alone, is the primary biotic signature driving early belowground dynamics in AFS. Managing the balance between herbaceous and woody species can enhance soil carbon storage and align productive land use with restoration and climate mitigation goals.