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

Agricultural soils are widely promoted as a large, low-cost reservoir for atmospheric carbon dioxide removal, yet the physical and biological limits to that reservoir remain contested. The central point of contention is whether soils possess a finite capacity to stabilise organic carbon, and, if so, whether that capacity constrains what agricultural management can realistically deliver within policy-relevant timescales. This critical narrative review examines the concept of soil carbon saturation and its relationship to the wider question of sequestration ceilings in cropland and grassland soils. Literature was identified through open scholarly indexes and citation searching, appraised for methodological adequacy, and synthesised thematically rather than catalogued. Three findings emerge. First, disagreement about whether saturation exists is substantially a disagreement about measurement: estimates of maximum mineral-associated organic carbon depend on the statistical estimator, the dispersion energy applied during fractionation, the soil types retained in the analysis, and whether mineralogy is characterised beyond particle-size distribution. Second, even where a mineralogical limit cannot be detected empirically, carbon accrual in most managed systems is bounded earlier by the supply of organic inputs, by nutrient stoichiometry, and by the approach of stocks towards a new steady state, so that the operative ceiling is rarely the mineral surface itself. Third, the mitigation ceiling is lower still once nitrous oxide responses, reversibility, verification thresholds, and competition for biomass are accounted for. Estimates of global potential that omit saturation and these coupled constraints are therefore not directly comparable with those that include them, which explains much of the apparent conflict in the policy literature. Confidence is highest for the claim that sequestration rates decline as stocks rise, and lowest for any single numerical value of maximum capacity. Priorities include standardised fractionation, deliberate sampling of high-carbon soils, and long-term experiments designed to test saturation rather than assume it.