Subsurface Soil Organic Carbon Research Articles

Mapping sub-surface distribution of soil organic carbon stocks in South Africa's arid and semi-arid landscapes: Implications for land management and climate change mitigation

Soil organic carbon (SOC) stocks are critical for land management strategies and climate change mitigation. However, understanding SOC distribution in South Africa's arid and semi-arid regions remains a challenge due to data limitations, and the complex spatial and sub-surface variability in SOC stocks driven by desertification and land degradation. Thus, to support soil and land-use management practices as well as advance climate change mitigation efforts, there is an urgent need to provide more precise SOC stock estimates within South Africa's arid and semi-arid regions. Hence, this study adopted remote-sensing approaches to determine the spatial sub-surface distribution of SOC stocks and the influence of environmental co-variates at four soil depths (i.e., 0-30 cm, 30-60 cm, 60-100 cm, and 100-200 cm). Using two regression-based algorithms, i.e., Extreme Gradient Boosting (XGBoost) and Random Forest (RF), the study found the former (RMSE values ranging from 7.12 t/ha to 29.55 t/ha) to be a superior predictor of SOC in comparison to the latter (RMSE values ranging from 7.36 t/ha to 31.10 t/ha). Nonetheless, both models achieved satisfactory accuracy (R2 ≥ 0.52) for regional-scale SOC predictions at the studied soil depths. Thereafter, using a variable importance analysis, the study demonstrated the influence of climatic variables like rainfall and temperature on SOC stocks at different depths. Furthermore, the study revealed significant spatial variability in SOC stocks, and an increase in SOC stocks with soil depth. Overall, these findings enhance the understanding of SOC dynamics in South Africa's arid and semi-arid landscapes and emphasizes the importance of considering site specific topo-climatic characteristics for sustainable land management and climate change mitigation. Furthermore, the study offers valuable insights into sub-surface SOC distribution, crucial for informing carbon sequestration strategies, guiding land management practices, and informing environmental policies within arid and semi-arid environments.

Remote Sensing of Surface and Subsurface Soil Organic Carbon in Tidal Wetlands: A Review and Ideas for Future Research

Tidal wetlands, widely considered the most extensive reservoir of soil organic carbon (SOC), can benefit from remote sensing studies enabling spatiotemporal estimation and mapping of SOC stock. We found that a majority of the remote-sensing-based SOC mapping efforts have been focused on upland ecosystems, not on tidal wetlands. We present a comprehensive review detailing the types of remote sensing models and methods used, standard input variables, results, and limitations for the handful of studies on tidal wetland SOC. Based on that synthesis, we pose several unexplored research questions and methods that are critical for moving tidal wetland SOC science forward. Among these, the applicability of machine learning and deep learning models for predicting surface SOC and the modeling requirements for SOC in subsurface soils (soils without a remote sensing signal, i.e., a soil depth greater than 5 cm) are the most important. We did not find any remote sensing study aimed at modeling subsurface SOC in tidal wetlands. Since tidal wetlands store a significant amount of SOC at greater depths, we hypothesized that surface SOC could be an important covariable along with other biophysical and climate variables for predicting subsurface SOC. Preliminary results using field data from tidal wetlands in the southeastern United States and machine learning model output from mangrove ecosystems in India revealed a strong nonlinear but significant relationship (r2 = 0.68 and 0.20, respectively, p < 2.2 × 10−16 for both) between surface and subsurface SOC at different depths. We investigated the applicability of the Soil Survey Geographic Database (SSURGO) for tidal wetlands by comparing the data with SOC data from the Smithsonian’s Coastal Blue Carbon Network collected during the same decade and found that the SSURGO data consistently over-reported SOC stock in tidal wetlands. We concluded that a novel machine learning framework that utilizes remote sensing data and derived products, the standard covariables reported in the limited literature, and more importantly, other new and potentially informative covariables specific to tidal wetlands such as tidal inundation frequency and height, vegetation species, and soil algal biomass could improve remote-sensing-based tidal wetland SOC studies.

Straw amendment and soil tillage alter soil organic carbon chemical composition and are associated with microbial community structure

Soil organic carbon (SOC) accumulation and its components are influenced by agricultural practices, which are interfered by soil microorganisms. A field experiment was conducted to investigate how straw amendment and soil tillage influenced SOC fractions and chemical components, and their interrelations with microbial communities evaluated through phospholipid fatty acids (PLFAs) in Mollisols. Density fractionation and Fourier-transform Infrared Spectroscopy were combined to characterize OC properties, including free and occluded light fractions (fLFC, oLFC), heavy fraction (HFC), aliphatic-C and aromatic-C functional groups. Compared with control soil, straw amendment caused surface SOC increased by 6.7%–12.4% (P < 0.05). The dissolved OC, microbial biomass carbon (SMBC), fLFC, oLFC, aliphatic-C, and total PLFAs also increased after straw amendment. Whereas the aromatic-C proportions in bulk soil, fLFC, and oLFC decreased. The OC components and microbial properties didn't obviously change in soil with straw ash amendment. Straw amendment to the subsurface layer only increased subsurface SOC and OC fractions (P < 0.05). Principal component analysis revealed that soil microbial community structure changed with the amounts of straw amendment, soil tillage further underpinned the separation. The separation via PC1 was associated with labile OC components, including SMBC, fLFC, oLFC, and aliphatic-C pointing to soils with higher amounts of straw amendment, and with recalcitrant aromatic-C and HFC pointing to soils without straw or straw ash. Higher amounts of straw amendment plus soil tillage affected soil microbial community structure, and consequently influenced OC sequestration by altering labile OC fractions and components in Mollisols.

The silent carbon pool: Cryoturbic enriched organic matter in Canadian High Arctic semi-deserts

High Arctic soil organic carbon (SOC) is an important component in the global C cycle, yet there is considerable uncertainty in the estimates for the polar deserts and semi-deserts that dominate these regions. Some of this uncertainty in SOC estimates arises from the cryoturbic processes including diapirism that structure polar desert soils. Diapirism occurs when the top, viscous layer of permafrost is deformed during freezing and ejected upwards into the soil profile forming a distinct diapiric soil patch or diapir. The diapiric is often nutrient rich relative to the surrounding soil; plants seek out and forage for nutrients in these patches creating a localized mixture of old carbon ejected from the permafrost and new carbon. Here we investigate how the subsurface SOC-rich patches in frost boils resulting from diapirism contribute to overall SOC storage in these environments. We quantify the rates of diapirism and fine-scale distribution of SOC in 560 frost boils at two Canadian high Arctic polar desert sites differing in parent material (dolomite versus granitic) with strikingly different plant surface communities. Though total soil organic carbon content did not differ between the dolomite and granite polar semi-deserts, SOC was being stored differently. The dolomitic site had greater SOC content below 10 cm reflecting the more common occurrence of subsurface SOC patches (46% of frost boils) compared to the granitic site (30%). When a subsurface patch of SOCC (SOC expressed on a m−2 basis) was present in a frost boil the boil contained nearly double the SOCC compared to frost boils without subsurface patches (11 ± 6.3 kg SOC m−2 compared to 6.4 ± 3.6 kg SOC m−2). Diapirism occurs in only 35% of all frost boils, but these diapiric patches represent an important, yet heterogenous, pool of SOC in polar semi-deserts. We upscale from these data to generate an improved estimate of SOC stored in the active layer of High Arctic polar semi-deserts of 8.14 ± 0.45 Pg SOC.

Effects of manure on topsoil and subsoil organic carbon depend on irrigation regimes in a 9-year wheat-maize rotation

On the North China Plain, low soil organic carbon (SOC) content and a consistently dropping underground water level are seriously threatening food production security. To date, the interactive effect of fertilization types and irrigation regimes on surface and subsurface SOC contents has rarely been quantified. Here, we conducted a field study of three irrigation regimes with two fertilization types between 2009 and 2018. The three irrigation regimes included presowing irrigation (W0); presowing and jointing irrigation (W1); and presowing, jointing and anthesis irrigation (W2). The two fertilization types were mineral fertilizer only (CF) and mineral fertilizer plus composted chicken manure (CFM). Soil samples were collected from a depth of 0−100 cm in 20-cm increments to measure SOC and its composition, including labile pool I (LPI) (polysaccharides), labile pool Ⅱ (LPⅡ) (cellulose), and recalcitrant pool (RCP). Four SOC-related enzymes were determined, i.e., α-glucosidase, β-glucosidase, peroxidase and polyphenol oxidase. Stable carbon isotopic (13C) analysis was determined to distinguish the contributions of wheat- and maize-derived SOC in a wheat-maize rotation. The results showed that the 0−20-cm SOC content was highest in the W0 + CFM treatment. In the 0−20-cm layer, SOC decreased from W2 to W1 or W0 under CF, which is attributed to a reduction in the labile pool and maize-derived C and higher polyphenol oxidase. Conversely, under CFM, the 0−20-cm SOC content increased following decreased irrigation from W2 to W1 and from W1 to W0, mainly due to the increased LPⅡ and RCP. Compared with CF at W0 and W1, the 0−20-cm SOC content increased by 30.5 % and 21.8 % under CFM, respectively. Relative to the 0−20-cm soil layer, the SOC stock was larger in the 20−100-cm layer, which of the treatments had a variation manner similar to the 0−20-cm soil layer. Our results suggest that reduced irrigation regimes have significantly reduced the SOC stock and altered its composition in the 0−100-cm soil layer under CF and that CFM can increase the SOC stock in this layer when reducing the irrigation regime.

Impact of agricultural land use on soil organic carbon sequestration at sub-catchment scale

Soil organic carbon (SOC) pool determines the potential of soil to perform ecosystem services and maintain climatic stability. Land use plays a significant role in determining the carbon (C) sequestration in soil. This study aimed to determine the impact of long-term agricultural land uses on SOC and other physico-chemical properties and to determine interrelationships between SOC stock and topographic variables at sub-catchment scale in upcountry of Sri Lanka. Soil samples were collected from surface (0-30 cm) and subsurface (30-60 cm) at 109 locations from vegetable (n = 42), tea (n = 44) and forest (n = 23) land uses in a sub-catchment (360 ha) in Nuwara Eliya. Average SOC in different land uses were compared by analysis of variance followed by Tukey post-hoc mean comparison. The largest surface SOC stock was observed in forested area (71.8 Mg/ha) followed by vegetable lands (61.8 Mg/ha) and it was significantly lesser in tea lands (50.7 Mg/ha). The forest soils showed the largest subsurface SOC stock (67.1 t/ha) as well, followed by tea (64.8 Mg/ha) and vegetable (57.9 Mg/ha) land uses. No significant relationship was observed between SOC stock and topographic variables, namely elevation, slope, wetness index and stream power index. This study concludes that the land use is a key determinant of the spatial heterogeneity of SOC and forest have the greater ability of sequestering C compared to vegetable and tea land uses in the studied sub-catchment area.

Two-decade long fertilization induced changes in subsurface soil organic carbon stock vary with indigenous site characteristics

Soil organic carbon (SOC) sequestration at subsurface layers (i.e. >20 cm) remains unclear under long-term fertilization practices. Based on long-term datasets of fertilization experiments in four typical Chinese croplands, representing soils with high fertility at Gongzhuling (GZL, black soil) and Chongqing (CQ, purple soil), and low fertility at Zhengzhou (ZZ, aquatic Chao soil) and Qiyang (QY, red soil), we calculated SOC storage, its change relative to initial condition (ΔSOC) in 0–20, 20–40 and 40–60 cm. We also obtained annual organic C inputs (OCI; stubble, roots and manure amendment) and derived soil C sequestration efficiency (CSE: the ratio of ΔSOC over OCI) in 0–20 cm and 0–60 cm. The fertilization treatments include cropping with no fertilization (CK), chemical nitrogen, phosphorus and potassium fertilizers (NPK) and combined chemical fertilizers and manure (NPKM). Results showed SOC stock significantly increased with fertilizations (i.e. initial, CK < NPK < NPKM). Relative to initial condition, surface (0–20 cm) and subsurface (20–60 cm) SOC stocks significantly decreased under CK at all sites except GZL, a site with elevated SOC stocks under all fertilizations and depths. Subsurface SOC stocks significantly increased at high fertility soils (i.e., GZL and CQ) but remained no change or significantly decreased at low fertility soils (i.e., ZZ and QY) under NPK and NPKM. Accordingly, CSE derived in 0–60 cm was consistently higher than that in 0–20 cm in high fertility soils but lower in low fertility soils. These results demonstrated that subsurface soils (20–60 cm) remained as C sinks in indigenously high fertility sites but experienced substantial C depletions in low fertility sites. This study informed the need to account for subsurface soil carbon changes for accurate estimates of soil C sequestration capacity under long-term fertilization.

Soil organic carbon stocks and their determining factors in the Dano catchment (Southwest Burkina Faso)

Although the evaluation of soil organic carbon (SOC) stocks across different types of land use and major reference soil groups is essential for mitigating climate change, there remains, to date, limited comprehensive understanding of whole tropical soil profiles. Therefore, this study aimed to explain the amount of SOC stocks in different land-use systems and across various soil groups, as well as its spatial pattern in the topsoil (0–30 cm) and subsoil (30–100 cm) within the savannah zone of Burkina Faso. Roughly 70 soil profiles were considered along with additional auger sampling to account for spatial variation in both cropland (CR) and savannah (SA). The machine learning technique random forest regression (RFR) and multiple linear regression (MLR) were used for modeling the surface and subsurface SOC stocks. For model calibration, covariates including land use, topographic, texture, and climatic data were considered as surrogate for soil forming factors. The prediction maps produced by the calibrated models were validated by an independent dataset. The results indicated that about 53% of the SOC stock over 1 m depth was held in the upper 30 cm. Only a marginal difference was recorded between the topsoil SOC stock in SA (41.4 t C ha−1) and CR (39.1 t C ha−1) soils. For the subsoil, a significant difference (p < 0.05) was observed between the SOC stock of CR soils recording about 40.2 t C ha−1 and SA soils with 26.3 t C ha−1. Among the reference soil groups, the Gleysols located at lower elevation positions revealed the highest SOC stocks over 0–30 cm (44 t C ha−1) and 100 cm depth (86.6 t C ha−1). The Stagnosols (45.2 t C ha−1) followed by the Gleysols (42.7 t C ha−1) recorded the highest SOC stocks over 30–100 cm. The variability of SOC stock in the topsoil was primarily related to site-specific elements, such as particle-size fraction and wetness index, while its distribution in the subsoil was mainly associated with the topographical orientation represented by the slope aspect. Compared to the MLR, RFR estimated mean top- and subsoil SOC stocks of the catchment fairly well, along with lower statistical error metrics, though extreme values were not covered. Nevertheless, the findings on SOC stocks reinforce the view that the semi-arid ecosystems of West Africa still offer a significant opportunity for carbon sequestration for both topsoil and subsoil, and these results represent a baseline for future modeling of SOC dynamics in the region.

Differential Effects of Conservational Management on SOC Accumulation in the Grasslands of China.

Conservational management practices in grasslands have been considered one of the efficient options to enhance the soil organic carbon (SOC) accumulation. However, the SOC changes after the conservational management practices vary significantly under different grassland vegetation types and the environmental conditions. At present, it is not clear how the SOC accumulation changes along the soil profile if conservational management practice was adopted. In this study, we collected 663 paired observational data of SOC changes with and without conservational management practices in grasslands of China from 176 published literatures that has both the surface (0‒20 cm) and subsurface (to 40 cm depth) SOC measurements. The differences of SOC density (SOCD) between pre‒management and post‒management in the vertical soil layers were analyzed in order to establish a quantitative relationship of the SOC changes between the subsurface and the surface. The results revealed that in all grasslands, conservational management practices benefits the SOC accumulation by enhancing 0.43‒1.14 Mg C ha–1 yr–1. But the SOC increment weakened downwards along the soil profile. While the surface SOC was enhanced by 17% after conservational management, the subsurface SOC was enhanced by only 7%. The SOC accumulation was closely correlated with restoration duration, pre-management SOCD and the environmental factors and differed greatly among different grasslands and the practices adopted. The alpine and mountain grassland showed a higher annual SOC increment than the temperate grassland with the annual rate of 1.62 and 0.72 Mg C ha-1 yr-1, respectively. The SOC increment caused by the artificial plantation and the grazing exclusion conservational management was more than 2-fold that of the cropland abandonment and the extensive utilization. With the quantitative relationship of the SOC changes between soil layers, we provide a methodological option to estimate SOC changes to layers deeper than the recommendation of IPCC when only the surface layer SOC increment is available.

Sub-surface soil carbon changes affects biofuel greenhouse gas emissions

Changes in direct soil organic carbon (SOC) can have a major impact on overall greenhouse gas (GHG) emissions from biofuels when using life-cycle assessment (LCA). Estimated changes in SOC, when accounted for in an LCA, are typically derived from near-surface soil depths (<30 cm). Changes in sub-surface soil depths (>30 cm) could have a large positive or negative impact on overall GHG emissions from biofuels that are not always accounted for. Here, we evaluate how sub-surface SOC changes impact biofuel GHG emissions for corn (Zea mays L.) grain, corn stover, and switchgrass (Panicum virgatum L.) using the (Greenhouse Gases, Regulated Emissions, and Energy Use in the Transportation) GREET model. Biofuel GHG emissions showed as much as a 154% difference between using near-surface SOC stocks changes only or when accounting for both near- and sub-surface SOC stock changes. Differences in GHG emissions highlight the importance of accounting for sub-surface SOC changes especially in bioenergy cropping systems with potential for soil C storage to deeper soil depths.

Root and dissolved organic carbon controls on subsurface soil carbon dynamics: A model approach

Although subsurface horizons store more than half the total soil organic carbon (SOC) globally, the sources and dynamics of subsurface SOC are still unknown. Root litter input and dissolved organic carbon (DOC) transport are possible sources. Using a vertically extended soil C model, we explore the role of root C input and DOC transport in controlling subsurface SOC dynamics. The model involves organic matter decomposition and DOC leaching in the aboveground litter layer, the belowground input of C from roots, and SOC turnover and DOC transport along water flows throughout the soil profile for three SOC pools (active, slow, and passive — characterized by a turnover time of years, decades, and millennia, respectively). Model simulations with a range of rooting profiles demonstrate that a large proportion (36% – 78% — greater in deeper rooting profiles) of SOC is apportioned to the subsurface horizons (below the top 30 cm). A significant part (39% – 73%) of subsurface SOC was found to be associated with C pools that turn over on time scales of decades or less. DOC transport appeared to be dominant in distributing the added C to the deeper soil layers, making the SOC content profile deeper than that of the root litter (C) input. The results suggest that current soil C studies focusing on the surface alone, significantly underestimate the stock of decadally cycling C, especially for deeply rooted ecosystems. Studies ignoring subsurface C dynamics therefore underpredict the responses of soil C to changes in climate, land use, and vegetation.

Effects of land use and mineral characteristics on the organic carbon content, and the amount and composition of Na-pyrophosphate-soluble organic matter, in subsurface soils

Land use and mineral characteristics affect the ability of surface as well as subsurface soils to sequester organic carbon and their contribution to mitigation of the greenhouse effect. There is less information about the effects of land use and soil properties on the amount and composition of organic matter (OM) for subsurface soils as compared with surface soils. Here we aimed to analyse the long-term (≥ 100 years) impact of arable and forest land use and soil mineral characteristics on subsurface soil organic carbon (SOC) contents, as well as on amount and composition of OM sequentially separated by Na pyrophosphate solution (OM(PY)) from subsurface soil samples. Seven soils with different mineral characteristics (Albic and Haplic Luvisol, Colluvic and Haplic Regosol, Haplic and Vertic Cambisol, Haplic Stagnosol) were selected from within Germany. Soil samples were taken from subsurface horizons of forest and adjacent arable sites continuously used for >100 years. The OM(PY) fractions were analysed for their OC content (OCPY) and characterized by Fourier transform infrared spectroscopy. Multiple regression analyses for the arable subsurface soils indicated significant positive relationships between the SOC contents and combined effects of the (i) exchangeable Ca (Caex) and oxalate-soluble Fe (Feox) and (ii) the Caex and Alox contents. For these soils the increase in OC (OCPY multiplied by the relative C=O content of OM(PY)) and increasing contents of Caex indicated that OM(PY) mainly interacts with Ca2+. For the forest subsurface soils (pH < 5), the OCPY contents were related to the contents of Na-pyrophosphate-soluble Fe and Al. The long-term arable and forest land use seems to result in different OM(PY)-mineral interactions in subsurface soils. On the basis of this, we hypothesize that a long-term land-use change from arable to forest may lead to a shift from mainly OM(PY)-Ca2+ to mainly OM(PY)-Fe3+ and -Al3+ interactions if the pH of subsurface soils significantly decreases to <5.

Vegetation controls on soil organic carbon dynamics in an arid, hyperthermic ecosystem

The large land area occupied by arid lands, roughly 36% to 40% globally, underscores the importance for understanding how these ecosystems function in the global carbon cycle. Few studies have directly examined soil organic carbon (SOC) dynamics and the effect of vegetation on SOC and microbial community structure in arid ecosystems. The objective of this study was to determine the effect of vegetation type on SOC dynamics in an arid, hyperthermic Sonoran Desert ecosystem. We specifically examined the influence of Prosopis velutina (mesquite), Larrea tridentata (creosote), and a combination of Bouteloua barbata, Bouteloua aristidoides, Aristida adscensionis, and Cynodon dactylon (mixed grass) vegetation types on SOC dynamics by quantifying: (i) local scale SOC stocks; (ii) soil aggregate stability; (iii) SOC turnover; and (iv) soil microbial community composition. There was significantly greater SOC in mesquite A-horizons relative to creosote and grass sites with values of 46.7, 30.4, and 24.4 g m − 2 , respectively. Subsurface SOC content did not vary significantly between vegetation types. Aggregate stability determined using an ultrasonic dispersion technique was found to be similar among vegetation types. The only significant difference noted was greater energy required to disperse stable aggregates in mesquite relative to grass soils, 1500 and 735 J g − 1 soil, respectively. Laboratory incubations were performed to determine SOC dynamics, pool sizes, and active pool mean residence times (MRT) for each vegetation type. Incubation results indicated significant variation in the cumulative respired CO 2 under mesquite, creosote, and grasses with 151, 186, and 207 mg C g − 1 soil C respired from each respective vegetation type. The incubation data indicated that 7–11% of total SOC was highly labile across all vegetation types with modeled active pool SOC MRT averaging 17 days. Bacterial community analysis by Terminal Restriction Fragment Length Polymorphism (TRFLP) indicated significant differences in microbial community structure among vegetation types. Microbial composition was highly correlated with soil pH and electrical conductivity. Furthermore, community composition was correlated with cumulative respired CO 2, suggesting an interaction among vegetation type, soil properties and microbial community control SOC dynamics in this ecosystem. The combined results indicated significant variation in SOC dynamics within a specific ecosystem by vegetation type. Understanding local-scale vegetation controls of soil carbon cycling may improve efforts to model regional carbon dynamics in arid environments.

Soil organic carbon changes and distribution in cultivated and restored grassland soils in Saskatchewan

The impacts of grassland restoration on amounts, forms and distribution of soil organic carbon (SOC) were examined in paired cultivated and restored grassland catenae of the Missouri Coteau region in south-central Saskatchewan, Canada. Total SOC (0–15 cm depth) and light fraction organic carbon (LFOC) (0–7.5 cm) contents were determined in paired catenae in upland areas, and in the surface (0–15 cm) and at depth (>15 cm) in the wetland fringe areas. Mass of SOC was higher in the restored grassland catenae than in the cultivated equivalents. In both the cultivated and restored grassland catenae at the three sites, footslope positions consistently had a higher mass of SOC. However, the shoulder positions showed the greatest response in soil C sequestration to grass seed-down, with a 1.4–2.9 Mg ha−1 year−1 SOC increase apparent over an approximately eight-year period. The mass of LFOC and the proportion of SOC comprised of LFOC was also higher in the restored grassland, reflective of higher recent C inputs. Rates of C sequestration in the Missouri Coteau based on SOC differences in the paired comparisons were estimated to be 0.3–2.9 Mg C ha−1 year−1, depending upon site and slope position. In the wetland fringe region of the landscape, the three sites also had higher surface or subsurface SOC in the grassland restoration. In general, SOC changes at depth (below 15 cm) in the restored grasslands appeared to be less consistent than changes in SOC in the surface 0–15 cm soil. In conclusion, the findings suggest that a switch to permanent cover on these soils will significantly increase C sequestered in the soil.

Aquatic humic substances; influence on fate and treatment of pollutants

High Arctic soil organic carbon (SOC) is an important component in the global C cycle, yet there is considerable uncertainty in the estimates for the polar deserts and semi-deserts that dominate these regions. Some of this uncertainty in SOC estimates arises from the cryoturbic processes including diapirism that structure polar desert soils. Diapirism occurs when the top, viscous layer of permafrost is deformed during freezing and ejected upwards into the soil profile forming a distinct diapiric soil patch or diapir. The diapiric is often nutrient rich relative to the surrounding soil; plants seek out and forage for nutrients in these patches creating a localized mixture of old carbon ejected from the permafrost and new carbon. Here we investigate how the subsurface SOC-rich patches in frost boils resulting from diapirism contribute to overall SOC storage in these environments. We quantify the rates of diapirism and fine-scale distribution of SOC in 560 frost boils at two Canadian high Arctic polar desert sites differing in parent material (dolomite versus granitic) with strikingly different plant surface communities. Though total soil organic carbon content did not differ between the dolomite and granite polar semi-deserts, SOC was being stored differently. The dolomitic site had greater SOC content below 10 cm reflecting the more common occurrence of subsurface SOC patches (46% of frost boils) compared to the granitic site (30%). When a subsurface patch of SOCC (SOC expressed on a m−2 basis) was present in a frost boil the boil contained nearly double the SOCC compared to frost boils without subsurface patches (11 ± 6.3 kg SOC m−2 compared to 6.4 ± 3.6 kg SOC m−2). Diapirism occurs in only 35% of all frost boils, but these diapiric patches represent an important, yet heterogenous, pool of SOC in polar semi-deserts. We upscale from these data to generate an improved estimate of SOC stored in the active layer of High Arctic polar semi-deserts of 8.14 ± 0.45 Pg SOC.