Terrestrial Soil Organic Carbon Research Articles

Changes in soil organic carbon and its response to environmental factors in the Yarlung Tsangpo River basin

The distribution of terrestrial soil organic carbon changes with climate and substrate conditions. This study investigated the spatial and temporal distribution of soil organic carbon and its influencing elements in the Yarlung Tsangpo River Basin using meteorological, subsurface, and remotely sensed data over the past 39 years, combined with random forests and geodetector. The results of the study showed that soil organic carbon has been increasing in the study area over the past 39 years, and this change has been concentrated in the central part of the watershed. Among the environmental factors, precipitation, temperature, and leaf area index were the main contributors to the soil organic carbon changes, and the effects of the three factors were more significant at higher altitudes, with a 33 % increase in the significance level. Notably, the interactions among the factors exceeded their individual effects, and the combination of climate and leaf area index showed a significant two-factor enhancement effect, with q-value enhancements generally ranging from 0.15 to 0.3. The two-factor enhancement effect was more pronounced below 3000 m above sea level, where the topography of the study area concentrates soil organic carbon in the gently sloping areas of the catchment.

Compositions and sources of sedimentary organic carbon on the tropical epicontinental sea

The deposition of fluvially derived terrestrial organic carbon (OC) together with marine OC in marine sediments has important implications for the global carbon cycle. However, dispersal extent and fates of terrestrial soil and petrogenic OC on the continental shelf have remained unclear and debated. Here, we present OC compositions and mineral properties of marine sediments from the Gulf of Thailand and the adjacent Mekong shelf, as well as river sediments from continents surrounding them in order to determine the provenance of sedimentary OC in this tropical epicontinental sea. We find that stable carbon isotope composition (δ13C) and radiocarbon activity (Δ14C) of sedimentary OC fall between those of marine OC and of river sediments, mainly composed of C3 plant-dominated pre-aged soil and petrogenic OC. Mixing model reveals that the OC in river sediments is predominantly sourced from pre-aged deep soils, accounting for 73 ± 5%, with the remaining consisting of modern surface soil (25 ± 6%) and bedrock (2 ± 1%). The sedimentary OC on the shelf is primarily derived from marine OC, accounting for 65 ± 15%, with terrestrial soil OC and petrogenic OC contributing 24 ± 14% and 10 ± 5%, respectively. The sources, degradation, and aging of OC and mineral surface area determine the spatial pattern of sedimentary OC compositions in the Gulf of Thailand and the Mekong shelf. The contents of marine, soil, and petrogenic OC decrease with increasing distance offshore, consistent with changes in spatial patterns of sedimentation rate of terrestrial materials and of marine primary production in the overlying water column. Extensive degradation of terrestrial OC and extremely low OC burial rate in this tropical epicontinental sea suggest a minor role on OC sequestration in marine sediments, but an important CO2 source to the atmosphere in the context of the global carbon cycle.

Unexpected no significant soil carbon losses in the Tibetan grasslands due to rodent bioturbation.

The Tibetan grasslands store 2.5% of the Earth's soil organic carbon. Unsound management practices and climate change have resulted in widespread grassland degradation, providing open habitats for rodent activities. Rodent bioturbation loosens topsoil, reduces productivity, changes soil nutrient conditions, and consequently influences the soil organic carbon stocks of the Tibetan grasslands. However, these effects have not been quantified. Here, using meta-analysis and upscaling approaches, we found that rodent bioturbation impacts on the Tibetan grassland soil organic carbon contents were depth-dependent, with significant (P<0.001) decreasing of 24.4% in the topsoil (0 to 10cm) but significant (P<0.05) increasing of 35.9% in the deeper soil layer (40 to 50cm), and nonsignificant changes in other soil layers. The depth-dependent responses in soil organic carbon content were closely associated with rodent tunnel burrowing, foraging, excrement deposition, and mixing of the upper and deeper soil layers. Rodent bioturbation had shown nonsignificant impacts on soil bulk density, independent of soil layer. Tibetan grasslands totally lose -35.2 Tg C yr-1 (95% CI: -48.5 to -21.1 Tg C yr-1) and -32.9 Tg C yr-1 (-54.2 to -8.6 Tg C yr-1) due to rodent bioturbation in the 0 to 10 or 0 to 30cm soil layer, while no significant net loss was found over the 0 to 90cm layer. Our findings highlight the importance of considering depth-dependent factors to robustly quantify the net changes in the terrestrial soil organic carbon stocks resulting from disturbances such as rodent bioturbation.

Homoacetogenesis competes with hydrogenotrophic methanogenesis for substrates in a peatland experiencing ecosystem warming

Peatlands contain up to half of terrestrial soil organic carbon (C) while simultaneously emitting the potent greenhouse gas methane (CH4). Global change will alter C biogeochemistry in peatlands, but is hard to predict without a mechanistic understanding of the processes that control anaerobic C cycling. One of the least known anaerobic C cycling pathways in wetland systems is homoacetogenesis, the reduction of carbon dioxide (CO2) with dihydrogen (H2) to acetate. We developed a new technique to infer rates of putative homoacetogenesis by measuring the incorporation of 14C-labeled dissolved inorganic C into dissolved organic matter. We determined rates of homoacetogenesis and total and hydrogenotrophic CH4 production throughout the peat profile on two sampling dates in a whole-ecosystem warming and elevated CO2 experiment in a northern Minnesota bog, USA. Homoacetogenesis and hydrogenotrophic CH4 production were greatest in surface soils and at warmer temperatures. However, homoacetogens were stronger competitors than hydrogenotrophic methanogens for H2 in deeper soil depths and at lower temperatures. A higher ratio of acetoclastic to hydrogenotrophic methanogenesis is often associated with greater total CH4 production, indicating that homoacetogenesis potentially can indirectly increase total CH4 production. Additionally, failure to account for homoacetogenesis may lead to erroneous conclusions in stable isotope models of anaerobic C cycling and CH4 production. This work shows that homoacetogenesis responds to warming and may impact CH4 production, and thus this process should be included in future modeling of climate change effects in wetlands.

Changes in total organic carbon and organic carbon fractions of reclaimed minesoils in response to the filling of different substrates

Soil organic carbon (SOC) of reclaimed minesoil (RMS) is regarded as an important part of terrestrial SOC, and SOC losses of RMSs occur during the substantial soil compaction and the removal, storage and backfilling of the topsoil, which lead to poor soil structure. The filling of alternative soil substrates is considered to disturb the soil structure of RMSs, however, how SOC pool changes in the RMSs filled with various substrates and the mechanisms are less clearly understood. Therefore, a study on reclaimed area filled with three typical alternative soil substrates, including mining waste reclamation (MWR), river sand reclamation (RSR) and river mud reclamation (RMR), was started in mining area of Eastern China, where total SOC, labile SOC fractions, stable SOC fraction and soil physicochemical properties were measured. The results showed that (1) the total SOC, labile SOC fractions (microbial biomass carbon (MBC), readily oxidizable organic carbon (ROOC), light fraction organic carbon (LFOC) and particulate organic carbon (POC)) and stable SOC fraction (humic acids carbon (HAC)) contents of RMSs were lower than those of non-subsided cultivated land (NCL), the filling of alternative soil substrates had a significant effect on the SOC composition of RMSs, and the backfilling layer of RSR and the filling layer of MWR were more similar with NCL than other treatments in SOC composition; (2) In backfilling layer, bulk density (BD), connectivity index (τ), available phosphorus (AP), available potassium (AK) and soil urease (URE) were strong predictors for dissimilarities of SOC composition, and the difference in soil physics and soil fertility had more direct and indirect effects on the contents of SOC and SOC fractions; (3) In filling layer, water content (WC), macropore number (MN), microporosity (Φmac), available nitrogen (AN), electronic conductivity (EC), soil urease (URE) and sucrase (SUC) were strong predictors for dissimilarities of SOC composition, and the substrate texture and soil physics had more direct and indirect effects on the contents of SOC and SOC fractions. Easing the compactness and strengthening fertilizing management of backfilling layer, while improving the texture of filling substrates benefit the increasing of the SOC content of RMSs.

Warming promotes the use of organic matter as an electron acceptor in a peatland

Peatlands store approximately one-half of terrestrial soil organic carbon (C) and the future of this C in the face of ongoing global change remains a key question in global biogeochemistry. Particularly pressing is the need to understand if this C will remain in peatland soils or be returned to the atmosphere as the potent greenhouse gas methane (CH4) in response to warming. Past work has demonstrated that the microbial reduction of organic matter (OM) as an alternative terminal electron acceptor (TEA) under anaerobic conditions can suppress CH4 production and is a key mechanistic control of peatland CH4 dynamics. Here we show that warming directly enhances rates of potential OM reduction in peatland soils, enhancing acetate availability and allowing for a faster onset of CH4 production. Using soils collected from the ecosystem-scale Spruce and Peatland Responses Under Changing Environments (SPRUCE) experiment, we also show that while there were no indirect effects of 2 years of soil warming on potential OM reduction through changes in soil quality, warming-mediated lowering of the water table increased the volume of oxidized OM which will suppress CH4 production following rewetting events.

Interactions between biogeochemical and management factors explain soil organic carbon in Pyrenean grasslands

Abstract. Grasslands are one of the major sinks of terrestrial soil organic carbon (SOC). Understanding how environmental and management factors drive SOC is challenging because they are scale-dependent, with large-scale drivers affecting SOC both directly and through drivers working at small scales. Here we addressed how regional, landscape and grazing management, soil properties and nutrients, and herbage quality factors affect 20 cm depth SOC stocks in mountain grasslands in the Pyrenees. Taking advantage of the high variety of environmental heterogeneity in the Pyrenees, we built a dataset (n=128) that comprises a wide range of environmental and management conditions. This was used to understand the relationship between SOC stocks and their drivers considering multiple environments. We found that temperature seasonality (difference between mean summer temperature and mean annual temperature; TSIS) was the most important geophysical driver of SOC in our study, depending on topography and management. TSIS effects on SOC increased in exposed hillsides, slopy areas, and relatively intensively grazed grasslands. Increased TSIS probably favours plant biomass production, particularly at high altitudes, but landscape and grazing management factors regulate the accumulation of this biomass into SOC. Concerning biochemical SOC drivers, we found unexpected interactive effects between grazer type, soil nutrients and herbage quality. Soil N was a crucial SOC driver as expected but modulated by livestock species and neutral detergent fibre contenting plant biomass; herbage recalcitrance effects varied depending on grazer species. These results highlight the gaps in knowledge about SOC drivers in grasslands under different environmental and management conditions. They may also serve to generate testable hypotheses in later/future studies directed to climate change mitigation policies.

Soil organic carbon stock and its changes in a typical karst area from 1983 to 2015

Changes in soil organic carbon (SOC) stock have major impacts on global terrestrial carbon cycling. However, their responses to land use conversions are poorly characterized for karst areas with extremely fragile geology and intensive human disturbance. To investigate the effects of soil type and land use on SOC stock in a typical karst region of southwestern China, 0–15 cm topsoil samples were randomly collected in 2015. Furthermore, in the same locations as the sites in 1983, 0–100 cm stratified profile soil samples (0–10, 10–20, 20–30, 30–50, 50–70, and 70–100 cm) were collected to evaluate the changes in SOC stock as affected by land use conversions from 1983 to 2015. The current SOC stock in 0–15 cm differed significantly between Calcisols and Ferralsols, and was highest in the secondary forest, followed by shrubland, grassland, plantation forest, and cropland. Changes in the stratified SOC stock (both recalculated to 0–20, 20–40, 40–60, 60–80, and 80–100 cm intervals in 1983 and 2015) varied among different land use conversions. Average stratified SOC stock decreased after forest degradation and reclamation, except in 80–100 cm. After reforestation, it had decreases in 0–20 and 20–40 cm, whereas it increased in subsoil (40–60, 60–80, and 80–100 cm). However, compared with the cropland in which soil was exposed to continuous conventional tillage, average stratified SOC stock increased in all soil layers after reforestation. The increases in SOC stock after reforestation and re-cultivation (short-term reuse in abandoned croplands) indicated the positive role of agricultural abandonment in increasing terrestrial SOC stock.

Microbial dynamics and soil physicochemical properties explain large-scale variations in soil organic carbon.

First-order organic matter decomposition models are used within most Earth System Models (ESMs) to project future global carbon cycling; these models have been criticized for not accurately representing mechanisms of soil organic carbon (SOC) stabilization and SOC response to climate change. New soil biogeochemical models have been developed, but their evaluation is limited to observations from laboratory incubations or few field experiments. Given the global scope of ESMs, a comprehensive evaluation of such models is essential using in situ observations of a wide range of SOC stocks over large spatial scales before their introduction to ESMs. In this study, we collected a set of in situ observations of SOC, litterfall and soil properties from 206 sites covering different forest and soil types in Europe and China. These data were used to calibrate the model MIMICS (The MIcrobial-MIneral Carbon Stabilization model), which we compared to the widely used first-order model CENTURY. We show that, compared to CENTURY, MIMICS more accurately estimates forest SOC concentrations and the sensitivities of SOC to variation in soil temperature, clay content and litter input. The ratios of microbial biomass to total SOC predicted by MIMICS agree well with independent observations from globally distributed forest sites. By testing different hypotheses regarding (using alternative process representations) the physicochemical constraints on SOC deprotection and microbial turnover in MIMICS, the errors of simulated SOC concentrations across sites were further decreased. We show that MIMICS can resolve the dominant mechanisms of SOC decomposition and stabilization and that it can be a reliable tool for predictions of terrestrial SOC dynamics under future climate change. It also allows us to evaluate at large scale the rapidly evolving understanding of SOC formation and stabilization based on laboratory and limited filed observation.

Large, climate-sensitive soil carbon stocks mapped with pedology-informed machine learning in the North Pacific coastal temperate rainforest

Accurate soil organic carbon (SOC) maps are needed to predict the terrestrial SOC feedback to climate change, one of the largest remaining uncertainties in Earth system modeling. Over the last decade, global scale models have produced varied predictions of the size and distribution of SOC stocks, ranging from 1000 to >3000 Pg of C within the top 1 m. Regional assessments may help validate or improve global maps because they can examine landscape controls on SOC stocks and offer a tractable means to retain regionally-specific information, such as soil taxonomy, during database creation and modeling. We compile a new transboundary SOC stock database for coastal watersheds of the North Pacific coastal temperate rainforest, using soil classification data to guide gap-filling and machine learning approaches to explore spatial controls on SOC and predict regional stocks. Precipitation and topographic attributes controlling soil wetness were found to be the dominant controls of SOC, underscoring the dependence of C accumulation on high soil moisture. The random forest model predicted stocks of 4.5 Pg C (to 1 m) for the study region, 22% of which was stored in organic soil layers. Calculated stocks of 228 ± 111 Mg C ha−1 fell within ranges of several past regional studies and indicate 11–33 Pg C may be stored across temperate rainforest soils globally. Predictions compared very favorably to regionalized estimates from two spatially-explicit global products (Pearson’s correlation: ρ = 0.73 versus 0.34). Notably, SoilGrids 250 m was an outlier for estimates of total SOC, predicting 4-fold higher stocks (18 Pg C) and indicating bias in this global product for the soils of the temperate rainforest. In sum our study demonstrates that CTR ecosystems represent a moisture-dependent hotspot for SOC storage at mid-latitudes.

Soil hydromorphy and soil carbon: A global data analysis

Wetland soils are an important component of the Global Carbon Cycle because they store about 20–25% of the terrestrial soil organic carbon (SOC). Wetlands occupy about 6% of the global land surface and any change in their use or management has potentially dramatic consequences on greenhouse gases emissions. However, the capacity of wetland soils to store carbon (C) differs from place to place due to reasons still not well understood. The objective of this review was to evaluate the global variations in wetlands SOC content (SOCC) and to relate it to key soil and environmental factors such as soil texture, intensity of soil hydromorphy, metallic element content and climate. A comprehensive data analysis was performed using 122 soil profiles from 29 studies performed under temperate, humid, sub-humid, tropical and sub-arctic conditions. The results point to average SOCC of 53.5 ± 15.8 g C kg−1 with a maximum of 540 g C kg−1. SOCC increased with increase in intensity of soil hydromorphy (r = −0.52), Al (r = 0.19) and Fe content (r = 0.21), and decreased with soil pH (r = −0.24). There was also a surprising tendency for intensity of soil hydromorphy, and thus SOCC, to decrease with increasing mean annual precipitation and soil clay content. These results contribute to a better understanding of the impact of soil hydromorphy in wetlands on organic C stabilization in the soils. However, further studies with additional information on soil bulk density to assess carbon C stocks, still need to be performed.

Erosion-induced exposure of SOC to mineralization in aggregated sediment

During slope-scale erosion events, re-distribution of eroded soil and the associated soil organic carbon (SOC) is not always uniform, but very often affected by preferential transport and deposition. Under given flow conditions, the site of SOC deposition depends on the transport distances of sediment particles containing the SOC. Very often, soil and SOC erosion risk are assessed by applying mineral particle specific SOC distributions to erosion models. However, soil is not always eroded as individual mineral particles, but mostly in a form of aggregates. Aggregates are likely to increase settling velocities of individual mineral particles, which may considerably reduce the transport distance of sediment fractions and the associated SOC, skewing SOC redistribution and its subsequent fate. Yet, little is known about the potential effects of aggregation on the movement and fate of eroded SOC.To assess the effect of preferential deposition, a simulated rainfall was applied to two soils in this study, with the Movelier silty clay having greater SOC content and aggregate stability than the Möhlin silty loam. The eroded sediments of the two soils were fractionated by a settling tube apparatus according to their potential transport distances. The CO2 emissions of the fractionated and incubated sediments were then measured for 50days enabling the assessment of the bioactivity of eroded SOC for weeks after deposition. Our results show that: 1) the re-deposition of eroded SOC into terrestrial systems increased by 64% if considering the actual aggregate specific rather than the mineral particle specific SOC distribution. 2) The CO2 emission rates differed across settling fractions, with the most pronounced rates in the finest fractions from the Möhlin silty loam sediment and in the medium-size fractions from the Movelier silty clay sediment. 3) Over 50-day incubation, the CO2 emissions from the Möhlin silty loam sediment was 114% greater than that from the non-eroded Möhlin soil, whereas CO2 emissions were roughly equivalent for the Movelier sediment and non-eroded Movelier silty clay. These data demonstrate that erosion and preferential deposition of SOC-enriched aggregates can enhance terrestrial SOC deposition. This can further result in greater CO2 efflux than commonly applied mineral particle size specific SOC distribution would suggest. The different performances of deposition and CO2 emissions between the two soils also suggest that these effects can vary with soil type. Our observations certainly illustrate that sediment aggregation requires further investigation to assess the redistribution and subsequent fate of eroded SOC appropriately.

Large amounts of labile organic carbon in permafrost soils of northern Alaska.

Permafrost-affected soils of the northern circumpolar region represent 50% of the terrestrial soil organic carbon (SOC) reservoir and are most strongly affected by climatic change. There is growing concern that this vast SOC pool could transition from a net C sink to a source. But so far little is known on how the organic matter (OM) in permafrost soils will respond in a warming future, which is governed by OM composition and possible stabilization mechanisms. To investigate if and how SOC in the active layer and adjacent permafrost is protected against degradation, we employed density fractionation to separate differently stabilized SOM fractions. We studied the quantity and quality of OM in different compartments using elemental analysis, 13 C solid-phase nuclear magnetic resonance (13 C-NMR) spectroscopy, and 14 C analyses. The soil samples were derived from 16 cores from drained thaw lake basins, ranging from 0 to 5500years of age, representing a unique series of developing Arctic soils over time. The normalized SOC stocks ranged between 35.5 and 86.2kgSOCm-3 , with the major amount of SOC located in the active layers. The SOC stock is dominated by large amounts of particulate organic matter (POM), whereas mineral-associated OM especially in older soils is of minor importance on a mass basis. We show that tremendous amounts of over 25kgOC per square meter are stored as presumably easily degradable OM rich in carbohydrates. Only about 10kgOC per square meter is present as presumably more stable, mineral-associated OC. Significant amounts of the easily degradable, carbohydrate-rich OM are preserved in the yet permanently frozen soil below the permafrost table. Forced by global warming, this vast labile OM pool could soon become available for microbial degradation due to the continuous deepening of the annually thawing active layer.

Historical advances in the study of global terrestrial soil organic carbon sequestration

This paper serves two purposes: it provides a summarized scientific history of carbon sequestration in relation to the soil-plant system and gives a commentary on organic wastes and SOC sequestration. The concept of soil organic carbon (SOC) sequestration has its roots in: (i) the experimental work of Lundegårdh, particularly his in situ measurements of CO2 fluxes at the soil-plant interface (1924, 1927, 1930); (ii) the first estimates of SOC stocks at the global level made by Waksman [Waksman, S.A., 1938. Humus. Origin, Chemical Composition and Importance in Nature, second ed. revised. Williams and Wilkins, Baltimore, p. 526] and Rubey [Rubey, W.W., 1951. Geologic history of sea water. Bulletin of the Geological Society of America 62, 1111-1148]; (iii) the need for models dealing with soil organic matter (SOM) or SOC dynamics beginning with a conceptual SOM model by De Saussure (1780-1796) followed by the mathematical models of Jenny [Jenny, H., 1941. Factors of Soil Formation: a System of Quantitative Pedology. Dover Publications, New York, p. 288], Hénin and Dupuis [Hénin, S., Dupuis, M., 1945. Essai de bilan de la matière organique. Annales d'Agronomie 15, 17-29] and more recently the RothC [Jenkinson, D.S., Rayner, J.H., 1977. The turnover of soil organic matter in some of the Rothamsted classical experiments. Soil Science 123 (5), 298-305] and Century [Parton, W.J., Schimel, D.S., Cole, C.V., Ojima, D.S., 1987. Analysis of factors controlling soil organic matter levels in great plains grasslands. Soil Science Society of America Journal 51 (5), 1173-1179] models. The establishment of a soil C sequestration balance is not straightforward and depends greatly on the origin and the composition of organic matter that is to be returned to the system. Wastes, which are important sources of organic carbon for soils, are taken as an example. For these organic materials the following factors have to be considered: the presence or absence of fossil C, the potential of direct and indirect emissions of non-CO2 greenhouse gases (CH4 and N2O) following application and the agro-system which is being used as a comparative reference.

Equilibrium Responses of Soil Carbon to Climate Change: Empirical and Process-Based Estimates

We use a new version of the Terrestrial Ecosystem Model (TEM), which has been parameterized to control for reactive soil organic carbon (SOC) across climatic gradients, to evaluate the sensitivity of SOC to a 1?C warming in both empirical and process-based analyses. In the empirical analyses we use the steady state SOC estimates of TEM to derive SOC-response equations that depend on temperature and volumetric soil moisture, and extrapolate them across the terrestrial biosphere at 0.50 spatial resolution. For contemporary climate and atmospheric C02, mean annual temperature explains 34.8% of the variance in the natural logarithm of TEM-estimated SOC. Because the inclusion of mean annual volumetric soil moisture in the regression explains an additional 19.6%, a soil moisture term in an equation of SOC response should improve estimates. For a 1?C warming, the globally derived empirical model estimates a terrestrial SOC loss of 22.6 1015 g (Pg), with 77.9% of the loss in extra-tropical ecosystems. To explore whether loss estimates of SOC are affected by the spatial scale at which the response equations are derived, we derive equations for each of the eighteen ecosystems considered in this study. The sensitivity of terrestrial SOC estimated by summing the losses predicted by each of the ecosystem empirical models is greater (27.9 Pg per ?C) than that estimated by the global empirical model; the 12.2 Pg loss (43.7%) in tropical ecosystems suggests that they may be more sensitive to warming. The global process-based loss of SOC estimated by TEM in response to a 1?C warming (26.3 Pg) is similar to the sum of the ecosystem empirical losses, but the 13.6 Pg loss (51.7%) in extra-tropical ecosystems suggests that they may be slightly less sensitive to warming. For the modelling of SOC responses, these results suggest that soil moisture is useful to incorporate in empirical models of SOC response and that globally derived empirical models may conceal regional sensitivity of SOC to warming. The analyses in this study suggest that the maximum loss of SOC to the atmosphere per?C warming is less than 2% of the terrestrial soil carbon inventory. Because the NPP response to elevated CO2 has the potential to compensate for this loss, the scenario of warming enhancing soil carbon loss to further enhance warming is unlikely in the absence of land use or changes in vegetation distribution.

Use of soil survey data to select measurement techniques for hydraulic conductivity

Wetland soils are an important component of the Global Carbon Cycle because they store about 20–25% of the terrestrial soil organic carbon (SOC). Wetlands occupy about 6% of the global land surface and any change in their use or management has potentially dramatic consequences on greenhouse gases emissions. However, the capacity of wetland soils to store carbon (C) differs from place to place due to reasons still not well understood. The objective of this review was to evaluate the global variations in wetlands SOC content (SOCC) and to relate it to key soil and environmental factors such as soil texture, intensity of soil hydromorphy, metallic element content and climate. A comprehensive data analysis was performed using 122 soil profiles from 29 studies performed under temperate, humid, sub-humid, tropical and sub-arctic conditions. The results point to average SOCC of 53.5 ± 15.8 g C kg−1 with a maximum of 540 g C kg−1. SOCC increased with increase in intensity of soil hydromorphy (r = −0.52), Al (r = 0.19) and Fe content (r = 0.21), and decreased with soil pH (r = −0.24). There was also a surprising tendency for intensity of soil hydromorphy, and thus SOCC, to decrease with increasing mean annual precipitation and soil clay content. These results contribute to a better understanding of the impact of soil hydromorphy in wetlands on organic C stabilization in the soils. However, further studies with additional information on soil bulk density to assess carbon C stocks, still need to be performed.