Deep carbon in agricultural soils: considerations for its treatment in soil carbon studies
Carbon reaches deep soil via roots, dissolved and colloidal transport, preferential flow, bioturbation, and soil mixing. Droughts may promote deep rooting but also suppress microbial activity. At depth, carbon persistence and vulnerability are regulated by interacting constraints that differ fundamentally from those operating near the surface. Hence, its study must acknowledge the unique carbon inputs and environmental constraints. This review offers a perspective on the utility and potential shortcomings of current approaches to studying deep soil organic carbon (dSOC) in the context of sources, transport pathways, stabilization and vulnerability across depth and their interactions with pedogenic horizonation, climate and environmental change. Evidence suggests that dSOC functions as a distinct component of the soil carbon cycle rather than a slow-cycling reflection of surface SOC. Persistence of dSOC cannot be solely explained by the chemical recalcitrance of SOC. Instead, dSOC stability is a composite of the hydrological, gaseous, mineralogical and biological constraints controlling microbial functions and access to SOC. Importantly, these are more accurately described as horizon-dependent rather than strictly depth-dependent. Although the particulate/mineral-associated organic matter framework remains useful, its interpretation may become ambiguous in deep soils. Hence, a mechanistic understanding of dSOC persistence requires coupling changes to C inputs, transfer pathways, mineral interactions, hydrological connectivity and biological constraints across pedogenic horizons. As soil horizon-dependent heterogeneity governs the persistence of chemically labile C compounds, horizon-aware sampling and testing would avoid misrepresenting mechanisms controlling dSOC. Incubation and warming studies imposing ecologically implausible conditions may measure short-term microbial physiological stress responses rather than climate-relevant SOC vulnerability.