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

Roughly half of the organic carbon held in the upper metre of the world’s soils lies below the depth conventionally sampled in agricultural and inventory work, and larger amounts again occur between one and two metres. This asymmetry between where carbon is stored and where it is measured has encouraged the proposition that the subsoil represents a large, comparatively secure and underused sink for atmospheric carbon dioxide. The present review evaluates that proposition critically rather than descriptively. It examines the vertical architecture of soil organic carbon, the isotopic evidence used to infer its persistence, the role of rooting depth in delivering carbon below the plough layer, the mechanisms that govern subsoil carbon stability, the management interventions proposed to exploit them, and the measurement and accounting conventions that keep depth largely invisible in climate policy. Literature was identified through structured searching of open scholarly databases and indexes, supplemented by citation tracking and institutional sources, and appraised for design adequacy, measurement validity and consistency with independent evidence. Three conclusions are supported with reasonable confidence. First, the great age of subsoil carbon reflects slow input and low energy supply as much as intrinsic protection, and is partly an artefact of rock-derived carbon and of averaging across heterogeneous pools; it therefore provides weak assurance of permanence. Second, deep rooting increases carbon delivery to depth, but the quantities that reach and remain in the subsoil over policy-relevant horizons are small, and the strongest field evidence concerns changes in unprotected particulate fractions rather than mineral-associated carbon. Third, several interventions widely described as subsoil sequestration redistribute existing carbon rather than adding new carbon, and their net effect depends on responses in the layers from which material is removed. Deep soil carbon merits far greater attention in measurement, modelling and inventory design than it currently receives, but the evidence does not yet support treating it as a dependable, large-scale mitigation instrument.