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

Soil Carbon Persistence in a Changing Climate: A Critical Appraisal of Molecular Stabilisation Mechanisms and Their Translation into Climate-Smart Land Management

2026-09-26T11:02:32+00:00

Soil holds more organic carbon than the atmosphere and the vegetation combined, and the length of time that carbon remains there governs whether land management can contribute meaningfully to climate mitigation. Over the past fifteen years the explanation for that persistence has been rebuilt. Intrinsic molecular recalcitrance has given way to an ecosystem-level account in which mineral association, spatial inaccessibility, microbial physiology and environmental constraint jointly determine residence time. This review critically evaluates that reconstruction and asks how securely it supports the land-management prescriptions now being written into carbon markets and national mitigation strategies. Peer-reviewed literature published mainly between 2005 and 8 July 2026 was identified through open scholarly indexes and citation searching, appraised for methodological adequacy, and synthesised thematically around mechanism, climate response, model representation and management translation. The evidence for mineral association as the dominant control on decadal to centennial persistence is strong and multiply replicated, yet three influential extensions of that framework remain insecure. The proposition that mineral-associated organic matter has a texture-defined saturation limit is contradicted by inventory data showing no detectable upper bound; the proposition that microbial carbon use efficiency is the principal global determinant of carbon storage is contradicted by manipulative experiments in which efficiency exerted no clear effect on mineral-associated carbon formation; and the proposition that microbial necromass dominates stabilised organic matter is not general across biomes. Warming experiments demonstrate substantial loss from whole soil profiles, including subsoil, and particulate organic carbon emerges consistently as the vulnerable fraction. A recurrent finding that carbon-rich soils contain younger rather than older carbon undermines the assumption that accrual and persistence move together. The practical consequence is that management should be appraised against the fraction in which carbon accumulates and the conditions under which it is retained, rather than against bulk stock change alone.

Beyond Total Soil Organic Carbon: Particulate and Mineral-Associated Organic Matter as Emerging Targets for Climate-Smart Soil Management

2026-09-26T11:06:59+00:00

Soil organic carbon is widely used as the principal indicator of soil carbon sequestration, yet a single bulk stock obscures organic matter pools that differ in formation pathway, persistence, nutrient stoichiometry, management responsiveness and vulnerability to environmental change. This critical narrative review evaluates particulate organic matter and mineral-associated organic matter as complementary targets for climate-smart soil management. Literature published primarily from 2000 to 8 July 2026 was selected from major open scholarly databases and indexes, with older foundational studies retained where necessary. The synthesis shows that particulate organic matter is generally more responsive to recent carbon inputs and disturbance, supports aggregation and nutrient cycling, and can persist when physically protected or environmentally inaccessible. Mineral-associated organic matter is, on average, longer lived because of organo-mineral interactions, but its formation is controlled jointly by substrate supply, microbial transformation, mineral reactivity, stoichiometry and accessibility. Evidence does not support a universal assumption that mineral-associated organic matter is either microbially derived, permanently stable or constrained by a simple clay-based saturation ceiling. Likewise, management effects cannot be reduced to a choice between rapidly accumulating particulate carbon and durable mineral-associated carbon: outcomes depend on initial soil carbon, mineralogy, texture, pH, climate, depth, plant traits, nutrient supply and timescale. Recent warming and drought studies further demonstrate that mineral association attenuates, but does not eliminate, climate vulnerability. A fraction-aware management framework is therefore most defensible when total soil organic carbon remains the accounting baseline, while particulate and mineral-associated fractions diagnose mechanisms, persistence and management leverage. Standardised fractionation, whole-profile stock accounting, age or source validation, and integration with greenhouse-gas and productivity outcomes are priorities for translating fraction science into credible climate-smart practice.

Coupling Carbon, Water and Nutrients in Climate-Smart Soils: A Critical Appraisal of the Evidence for Mitigation, Adaptation and Food Security

2026-09-26T11:13:49+00:00

Soil management sits at the intersection of three policy ambitions that are usually pursued separately: removing carbon dioxide from the atmosphere, buffering agriculture against hydroclimatic extremes, and sustaining nutrient supply for food production. The organising premise of climate-smart soil management is that these ambitions are mutually reinforcing because soil organic carbon, soil water and soil nutrients are biophysically coupled. This review evaluates how far the published evidence supports that premise, and where it does not. Literature was identified through structured searching of open scholarly indexes and citation registries, supplemented by backward and forward citation tracking and by targeted retrieval of intergovernmental technical assessments, with a final search date of 8 July 2026. Evidence was appraised for design adequacy, sampling depth and mass conventions, measurement validity, geographical representation and the correspondence between mechanistic plausibility and demonstrated field outcome. The synthesis identifies a consistent asymmetry: coupling is strong and well demonstrated in the direction running from water and nutrient availability to carbon accrual, but comparatively weak, conditional and frequently overstated in the direction running from carbon accrual to plant-available water. Reported gains in available water capacity per unit of carbon added are small in absolute terms, strongly texture-dependent, and generally smaller than the water storage differences created by texture itself. Nutrient stoichiometry imposes a material constraint on sequestration that is rarely costed in mitigation scenarios, and several practices that raise carbon stocks simultaneously alter the moisture regime in ways that increase nitrous oxide or methane fluxes, so that partial accounting can invert the sign of the climate outcome. Adaptation benefits attributable to soil carbon appear largest under moderate rather than extreme water limitation, which weakens the drought-insurance framing common in policy discourse. Priorities for future work include factorial long-term experiments that measure carbon, water and nutrient responses on the same plots, equivalent soil mass accounting to full rooting depth, and monitoring systems capable of detecting change at the scale at which management decisions are made.

Agroforestry Below Ground: Soil Carbon Processes, Microbial Regulation and Climate-Mitigation Potential Across Agroecosystems

2026-09-26T11:17:31+00:00

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.

Carbon Saturation and Sequestration Ceilings in Agricultural Soils: A Critical Appraisal of the Realistic Potential of Soil-Based Climate Mitigation

2026-09-26T11:21:18+00:00

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.

Deep Soil Carbon as a Climate Solution: A Critical Appraisal of Rooting Depth, Subsoil Processes and Management Opportunities

2026-09-26T11:25:25+00:00

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.

Integrating Soil Organic and Inorganic Carbon Pools into Climate-smart Land Management: A Critical Narrative Review

2026-09-26T11:32:57+00:00

Soil carbon has become a central instrument of climate policy, yet the operational definition of the resource being managed remains narrow. Programmes for climate-smart land management, carbon farming and soil carbon crediting are constructed almost entirely around soil organic carbon, while soil inorganic carbon, which dominates the carbon inventory of arid and semi-arid soils and constitutes a substantial share of the global soil carbon stock, is generally treated as inert background material. This review critically examines whether that separation remains defensible, and what follows for land management if it does not. Peer-reviewed literature published between 2000 and 8 July 2026 was identified through structured searching of open bibliographic and full-text scholarly sources, supplemented by backward and forward citation tracking and by targeted retrieval of intergovernmental technical documentation. Evidence was appraised for design adequacy, measurement transparency, treatment of carbonate provenance and external validity, and synthesised thematically rather than study by study. Three findings are robust across independent lines of evidence. First, the two pools are mechanistically coupled through soil solution chemistry, so that management which raises organic carbon can simultaneously dissolve carbonate, and management which accelerates carbonate precipitation may return part of the captured carbon dioxide to the atmosphere. Second, nitrogen-driven acidification and irrigation are the dominant anthropogenic controls on inorganic carbon in managed land, and both operate at rates comparable to, or exceeding, achievable organic carbon gains in calcareous systems. Third, the climate value of any inorganic carbon change depends on the provenance of the calcium involved, a criterion that most field studies do not resolve. Confidence in current quantitative projections is limited by short observation periods, shallow sampling, inconsistent carbonate correction and severe geographical concentration of evidence in China. Integrating both pools into land management therefore requires provenance-resolved accounting, deeper and mass-based monitoring, and crediting protocols that keep organic and inorganic claims separate rather than aggregating them into a single soil carbon figure.

Soil Moisture–Carbon Feedbacks under Climate Extremes: A Critical Review of Drought, Rewetting and the Future of Terrestrial Carbon Storage

2026-09-26T11:40:24+00:00

Soil water availability governs both the assimilation of carbon by terrestrial vegetation and its return to the atmosphere through microbial decomposition, yet the two limbs of this control operate through different mechanisms, on different timescales and with different degrees of empirical support. Intensifying drought, more frequent compound dry and hot extremes, and the sharper wetting events that accompany a more intense hydrological cycle have moved soil moisture from a secondary environmental variable to a first-order determinant of the terrestrial carbon balance. This critical narrative review examines how drought and subsequent rewetting jointly shape the persistence of soil organic carbon and the strength of the land carbon sink, and evaluates whether current evidence supports the confident projections of moisture-driven carbon loss that increasingly appear in the literature. Sources were identified through structured searching of open scholarly databases and indexes, supplemented by backward and forward citation tracking, with critical appraisal directed at measurement scale, attribution strategy, temporal coverage and geographical representation. Four findings emerge. First, the suppression of heterotrophic respiration by drying and its stimulation by rewetting are asymmetric, so that the net carbon consequence of a dry–wet sequence depends on the antecedent moisture history, soil texture and event timing rather than on drought severity alone. Second, competing mechanistic explanations of the rewetting pulse remain only partially reconciled, and the microbial and physical hypotheses are not mutually exclusive. Third, ecosystem-scale attribution of carbon anomalies to soil moisture is confounded by the tight coupling between soil dryness, atmospheric demand and temperature, and different statistical treatments of this coupling yield materially different partitionings. Fourth, model projections of moisture-driven soil carbon loss remain sensitive to the choice of an empirical moisture response function that observations do not yet constrain. Confidence is highest for short-term flux responses and lowest for decadal changes in soil carbon stocks. Priorities include long-duration manipulations that impose realistic wetting sequences, coordinated measurement in tropical and dryland regions where observational coverage is sparse, and model evaluation against moisture-dependent flux behaviour rather than against mean stocks alone.

Biochar-Enhanced Weathering Interactions in Agricultural Soils: Mechanisms, Carbon-Removal Accounting and Design Principles for Climate-Smart Deployment

2026-09-26T11:44:36+00:00

Biochar and enhanced rock weathering (ERW) are increasingly considered complementary soil-based carbon dioxide removal (CDR) strategies because they store carbon through different pathways while potentially sharing agronomic co-benefits. Yet co-deployment cannot be assumed to be synergistic. This critical narrative review evaluates how biochar and ERW interact through soil acidity, hydrology, cation exchange, microbial respiration, secondary mineral formation, soil organic carbon (SOC) dynamics, nutrient cycling and trace-element mobility, and asks how those interactions should shape integrated climate-smart agricultural design. Literature published from 1 January 2010 to 6 July 2026 was examined, with earlier foundational studies retained where necessary. Direct combined experiments remain a small and recent evidence base, and the latest controlled and lysimeter studies continue to show strongly context-dependent responses. Their outcomes range from near-additive CDR to soil-specific stimulation of silicate dissolution, negligible interaction under alkaline or kinetically constrained conditions, and trade-offs involving native SOC mineralisation, crop responses or greenhouse-gas fluxes. Co-pyrolysis of biomass with silicate rock can create mineral-pyrogenic interfaces that modify alkalinity generation and short-term carbon stabilisation, but field validation is still limited. The central conclusion is therefore conditional rather than universal: integrated deployment can be advantageous when the amendments correct distinct site limitations, but gross carbon gains from separate components cannot be summed without accounting for interaction effects, weathering acid sources, native SOC changes, non-CO₂ greenhouse gases and upstream emissions. A robust strategy requires amendment matching to soil pH, texture, mineralogy, water regime and cropping system, combined with carbon-pool-specific monitoring, reporting and verification. The most important research need is multi-year factorial field experimentation that couples geochemical mass balance with SOC fractionation, greenhouse-gas measurement, crop performance, contaminant surveillance and life-cycle accounting.

Regenerative Agriculture through a Soil-Process Lens: Carbon Sequestration, Greenhouse-Gas Trade-Offs and Climate Resilience

2026-09-26T11:47:44+00:00

Regenerative agriculture has become a prominent umbrella for farming strategies intended to rebuild soil function, sequester carbon and improve resilience, yet its climate claims remain difficult to evaluate because the term combines practices with different biophysical mechanisms and greenhouse-gas consequences. This critical narrative review examines regenerative agriculture through a soil-process lens, asking when management changes can generate additional and durable soil organic carbon, whether those gains survive accounting for nitrous oxide, methane and displaced emissions, and how the same processes influence resilience to climatic stress. Literature published from 1 January 2000 to 8 July 2026 was considered, prioritising peer-reviewed field evidence, quantitative syntheses and methodologically informative reviews. The synthesis shows that the strongest case for regenerative management is not a universal practice package but the deliberate manipulation of carbon inputs, disturbance, nutrient cycling, rooting depth, soil cover and water flow. Increased plant-derived carbon inputs, continuous living roots, diversified rotations, perennial phases, agroforestry and appropriately sourced organic amendments can increase soil organic carbon under many conditions. Reduced tillage can improve surface soil condition, but whole-profile carbon gains are less consistent and should not be inferred from concentration changes in shallow layers. Carbon accumulation is constrained by baseline depletion, mineral protection capacity, climate, texture, depth, management duration and eventual saturation. Climate mitigation is further conditional because nitrous oxide from nitrogen-rich amendments or wet microsites and methane from livestock can offset carbon benefits. Evidence for climate resilience is comparatively strongest for improved infiltration, aggregate stability, erosion resistance and, in some systems, yield stability under drought; direct gains in plant-available water are more variable. The review therefore argues that regenerative agriculture should be evaluated as a context-specific soil-process portfolio, with net greenhouse-gas accounting, whole-profile carbon measurement, permanence, additionality and productivity included in claims. Research priorities centre on long-term factorial field experiments, harmonised carbon-fraction and flux measurements, causal tests of resilience, and measurement–reporting–verification systems that reward demonstrable net climate benefit rather than practice adoption alone.