Small tree patches enhance soil carbon stabilization in pastoral landscapes

Small tree patches (<1 ha) embedded within pastoral landscapes can enhance soil carbon (C) storage, yet these gains remain poorly represented in soil C decomposition models that assume spatially uniform plant inputs and microbial processes. Sharp vegetation boundaries at tree-pasture edges generate strong gradients in rooting, litter quality, and microbial activity over short distances, with important implications for soil C stabilization that uniform models fail to capture. Using Aotearoa New Zealand as a model system, where pastoral landscapes are widespread and small tree patches are common features, we quantified plant inputs, microbial community composition and function, and soil processes across established tree-pasture boundaries to inform a microbially explicit soil C decomposition model. Tree-pasture edges exhibited elevated root biomass and lower leaf litter C:N ratios, indicating greater belowground C allocation and higher-quality organic inputs, while rhizosphere extracellular enzyme activity tended to be lower. Shotgun metagenomic analyses revealed a shift from microbial communities in forest soils characterized by greater investment in resource acquisition to microbial communities in grassland soils prioritizing faster growth and resource use, with edge communities displaying intermediate functional profiles. Together, these patterns suggest that tree-pasture edges function as biogeochemical ecotones, where increased root inputs and reduced enzyme investment are consistent with a shift toward microbial utilization of root-derived substrates rather than enzyme-mediated decomposition of more complex organic matter. To examine whether these rhizosphere-associated mechanisms could account for observed soil C gains beneath small tree patches, we refined a microbially explicit soil C decomposition model (FUN-CORPSE) to represent edge-driven root-microbe interactions. Simulations of small tree patch establishment in long-term pasture systems produced higher soil C stocks than pasture alone, with spatially explicit root inputs associated with increased soil C protection beneath tree-pasture edges. These findings highlight small tree patches as important landscape features with the potential to enhance soil C stabilization and contribute meaningfully to climate mitigation in pastoral systems.