Soil Processes and Climate Solutions: From Carbon Cycling to Climate-Smart Land Management

Agroforestry is widely promoted as a land-based climate solution, yet its mitigation value is often inferred from increases in bulk soil organic carbon (SOC) without resolving the below-ground processes that determine whether new carbon is rapidly cycled, physically protected or persistently associated with minerals. This critical narrative review integrates evidence on carbon inputs, particulate and mineral-associated organic matter, aggregation, rhizosphere processes, microbial transformation, subsoil storage and non-carbon-dioxide greenhouse-gas fluxes across temperate, tropical, humid, semi-arid and arid agroforestry systems. Literature published principally from 2000 to 8 July 2026 was selected through iterative searches of open scholarly databases and citation networks, with older foundational studies retained where essential to interpret soil-carbon mechanisms. The evidence supports three conclusions. First, agroforestry commonly raises SOC relative to simplified cropland, but the direction and magnitude of change depend strongly on antecedent land use, system age, climate, soil mineralogy, depth and the spatial weighting of tree and crop zones. Second, greater plant inputs and microbial activity do not automatically imply durable sequestration. Root-derived carbon and microbial products can feed mineral-associated organic matter, but particulate organic matter often responds faster, mineral-associated pools are constrained by mineral capacity, and rhizosphere stimulation can simultaneously accelerate decomposition of pre-existing SOC. Third, climate-mitigation claims remain weaker than SOC-stock claims because relatively few studies integrate equivalent-soil-mass accounting, deep sampling, carbon fractions, repeated temporal measurements and nitrous-oxide or methane fluxes. The strongest near-term research priority is therefore not merely to demonstrate higher SOC beneath trees, but to quantify whole-field, whole-profile and whole-greenhouse-gas carbon balance while distinguishing responsive particulate pools from more persistent mineral-associated pools. Agroforestry can be a credible climate-mitigation strategy when designed for additionality, persistence and productivity, but its below-ground benefit is conditional rather than universal.