Trade-offs in soil carbon protection mechanisms under aerobic and anaerobic conditions

Salt marsh nitrogen cycling: where land meets sea

Salt marshes sit at the terrestrial-aquatic interface of oceans around the world. Unique features of salt marshes that differentiate them from their upland or offshore counterparts include high rates of primary production from vascular plants and saturated saline soils that lead to sharp redox gradients and a diversity of electron acceptors and donors. Moreover, the dynamic nature of root oxygen loss and tidal forcing leads to unique biogeochemical conditions that promote nitrogen cycling. Here, we highlight recent advances in our understanding of key nitrogen cycling processes in salt marshes and discuss areas where additional research is needed to better predict how salt marsh N cycling will respond to future environmental change.

Drought may exacerbate dryland soil inorganic carbon loss under warming climate conditions

Low moisture conditions result in substantially more soil inorganic carbon (SIC) than soil organic carbon (SOC) in drylands. However, whether and how changes in moisture affect the temperature response of SIC in drylands are poorly understood. Here, we report that the temperature sensitivity of SIC dissolution increases but that of SOC decomposition decreases with increasing natural aridity from 30 dryland sites along a 4,500 km aridity gradient in northern China. To directly test the effects of moisture changes alone, a soil moisture control experiment also revealed opposite moisture effects on the temperature sensitivities of SIC and SOC. Moreover, we found that the temperature sensitivity of SIC was primarily regulated by pH and base cations, whereas that of SOC was mainly regulated by physicochemical protection along the aridity gradient. Given the overall increases in aridity in a warming world, our findings highlight that drought may exacerbate dryland soil carbon loss from SIC under warming.

Distinct, direct and climate-mediated environmental controls on global particulate and mineral-associated organic carbon storage

Identifying controls on soil organic carbon (SOC) storage, and where SOC is most vulnerable to loss, are essential to managing soils for both climate change mitigation and global food security. However, we currently lack a comprehensive understanding of the global drivers of SOC storage, especially with regards to particulate (POC) and mineral-associated organic carbon (MAOC). To better understand hierarchical controls on POC and MAOC, we applied path analyses to SOC fractions, climate (i.e., mean annual temperature [MAT] and mean annual precipitation minus potential evapotranspiration [MAP-PET]), carbon (C) input (i.e., net primary production [NPP]), and soil property data synthesized from 72 published studies, along with data we generated from the National Ecological Observatory Network soil pits (n = 901 total observations). To assess the utility of investigating POC and MAOC separately in understanding SOC storage controls, we then compared these results with another path analysis predicting bulk SOC storage. We found that POC storage is negatively related to MAT and soil pH, while MAOC storage is positively related to NPP and MAP-PET, but negatively related to soil % sand. Our path analysis predicting bulk SOC revealed similar trends but explained less variation in C storage than our POC and MAOC analyses. Given that temperature and pH impose constraints on microbial decomposition, this indicates that POC is primarily controlled by SOC loss processes. In contrast, strong relationships with variables related to plant productivity constraints, moisture, and mineral surface availability for sorption indicate that MAOC is primarily controlled by climate-driven variations in C inputs to the soil, as well as C stabilization mechanisms. Altogether, these results demonstrate that global POC and MAOC storage are controlled by separate environmental variables, further justifying the need to quantify and model these C fractions separately to assess and forecast the responses of SOC storage to global change.

Poorly drained depressions can be hotspots of nutrient leaching from agricultural soils

Much of the US Corn Belt has been drained with subsurface tile to improve crop production, yet poorly drained depressions often still flood intermittently, suppressing crop growth. Impacts of depressions on field-scale nutrient leaching are unclear. Poor drainage might promote denitrification and physicochemical retention of phosphorus (P), but ample availability of water and nutrients might exacerbate nutrient leaching from cropped depressions. We monitored nitrate, ammonium, and reactive P leaching across multiple depression-to-upland transects in north-central Iowa, using resin lysimeters buried and retrieved on an annual basis. Crops included conventional corn/soybean (Zea mays/Glycine max) rotations measured at fields with and without a winter rye (Secale cereale) cover crop, as well as juvenile miscanthus (Miscanthus × giganteus), a perennial grass. Leaching of nitrogen (N) and P was greater in depressions than in uplands for most transects and years. The median difference in nutrient leaching between paired depressions and uplands was 56 kg N ha^-1 year^-1 for nitrate (p = 0.0008), 0.6 kg N ha^-1 year^-1 for ammonium (p = 0.03), and 2.4 kg P ha^-1 year^-1 for reactive P (p = 0.006). Transects managed with a cover crop or miscanthus tended to have a smaller median difference in nitrate (but not ammonium or P) leaching between depressions and uplands. Cropped depressions may be disproportionate sources of N and P to downstream waters despite their generally poor drainage characteristics, and targeted management with cover crops or perennials might partially mitigate these impacts for N, but not necessarily for P.

Hydrodynamic and geochemical controls on soil carbon mineralization upon entry into aquatic systems

Erosion is the most widespread form of soil degradation and an important pathway of carbon transfer from land into aquatic systems, with significant impact on water quality and carbon cycle. However, it remains debatable whether erosion induces a carbon source or sink, and the fate of eroded soil carbon in aquatic systems remains poorly constrained. Here, we collect 41 representative soils from seven erosion-influenced basins and conduct microcosm simulation experiments to examine the fate of soil carbon under three different scenarios. We showed that soil carbon mineralization was generally promoted (by up to 10 times) in water under turbulence relative to in soils, but suppressed under static conditions upon entering into aquatic systems. Moreover, the enhancement of mineralization in turbulent systems is primarily related to soil aggregate content, while suppression in static systems positively relates to macromolecule abundance, indicating that soil geochemistry affects the magnitude of hydrodynamic effects on carbon mineralization. Random forest model further predicts that erosion may induce significant carbon sources in basins dominated by turbulent waters and aggregate-rich soils. Our findings demonstrate hydrodynamic and geochemical controls on soil carbon mineralization upon delivery into aquatic systems, which is a non-negligible part of the boundless carbon cycle and must be considered when making region-specific conservation strategies to reduce CO₂ emissions from inland waters.

A field incubation approach to evaluate the depth dependence of soil biogeochemical responses to climate change

Soil biogeochemical processes may present depth-dependent responses to climate change, due to vertical environmental gradients (e.g., thermal and moisture regimes, and the quantity and quality of soil organic matter) along soil profile. However, it is a grand challenge to distinguish such depth dependence under field conditions. Here we present an innovative, cost-effective and simple approach of field incubation of intact soil cores to explore such depth dependence. The approach adopts field incubation of two sets of intact soil cores: one incubated right-side up (i.e., non-inverted), and another upside down (i.e., inverted). This inversion keeps soil intact but changes the depth of the soil layer of same depth origin. Combining reciprocal translocation experiments to generate natural climate shift, we applied this incubation approach along a 2200 m elevational mountainous transect in southeast Tibetan Plateau. We measured soil respiration (Rs) from non-inverted and inverted cores of 1 m deep, respectively, which were exchanged among and incubated at different elevations. The results indicated that Rs responds significantly (p < .05) to translocation-induced climate shifts, but this response is depth-independent. As the incubation proceeds, Rs from both non-inverted and inverted cores become more sensitive to climate shifts, indicating higher vulnerability of persistent soil organic matter (SOM) to climate change than labile components, if labile substrates are assumed to be depleted with the proceeding of incubation. These results show in situ evidence that whole-profile SOM mineralization is sensitive to climate change regardless of the depth location. Together with measurements of vertical physiochemical conditions, the inversion experiment can serve as an experimental platform to elucidate the depth dependence of the response of soil biogeochemical processes to climate change.

Interactions between organic matter and Fe oxides at soil micro-interfaces: Quantification, associations, and influencing factors

Iron (Fe) oxides are widely recognized to prevent the degradation of organic matter (OM) in environments, thereby promoting the persistence of organic carbon (OC) in soils. Thus, discerning the association mechanisms of Fe oxides and OC interactions is key to effectively influencing the dynamics and extent of organic C cycling in soils. Previous studies have focused on i) quantifying Fe oxide-bound organic carbon (Fe-OC) in individual environments, ii) investigating the distribution and adsorption capacity of Fe-OC, and iii) assessing the redox cycling and transformation of Fe-OC. Furthermore, the widespread application of high-tech instrumentation and methods has greatly contributed to a better understanding of the mechanism of organic mineral assemblages in the past few decades. However, few literature reviews have comprehensively summarized Fe-OC distributions, associations, and characteristics in soil-plant systems. Here, studies investigating the Fe-OC contents among different environments are reviewed. In addition, the mechanisms and processes related to OM transformation dynamics occurring at mineral-organic interfaces are also described. Recent studies have highlighted that diverse interactions occur between Fe oxides and OC, with organic compounds adhering to Fe oxides due to their huge specific surfaces area and active reaction sites. Moreover, we also review methods for understanding Fe-OC interactions at micro-interfaces. Lastly, developmental prospects for understanding coupled Fe-OC geochemical processes in soil environments at molecular- and nano-scales are outlined. The summary suggests that combined advanced techniques and methods should be used in future research to explore micro-interfaces and in situ descriptions of organic mineral assemblages. This review also suggests that future studies need to consider the functional and spatial complexity that is typical of soil/sediment environments where Fe-OC interactions occur.

Oxygen availability regulates the quality of soil dissolved organic matter by mediating microbial metabolism and iron oxidation

Dissolved organic matter (DOM) plays a vital role in biogeochemical processes and in determining the responses of soil organic matter (SOM) to global change. Although the quantity of soil DOM has been inventoried across diverse spatio-temporal scales, the underlying mechanisms accounting for variability in DOM dynamics remain unclear especially in upland ecosystems. Here, a gradient of SOM storage across 12 croplands in northeast China was used to understand links between DOM dynamics, microbial metabolism, and abiotic conditions. We assessed the composition, biodegradability, and key biodegradable components of DOM. In addition, SOM and mineral-associated organic matter (MAOM) composition, soil enzyme activities, oxygen availability, soil texture, and iron (Fe), Fe-bound organic matter, and nutrient concentrations were quantified to clarify the drivers of DOM quality (composition and biodegradability). The proportion of biodegradable DOM increased exponentially with decreasing initial DOM concentration due to larger fractions of depolymerized DOM that was rich in small-molecular phenols and proteinaceous components. Unexpectedly, the composition of DOM was decoupled from that of SOM or MAOM, but significantly related to enzymatic properties. These results indicate that microbial metabolism exhibited a dominant role in DOM generation. As DOM concentration declined, increased soil oxygen availability regulated DOM composition and enhanced its biodegradability mainly through mediating microbial metabolism and Fe oxidation. The oxygen-induced oxidation of Fe(II) to Fe(III) removed complex DOM compounds with large molecular weight. Moreover, increased oxygen availability stimulated oxidase-catalyzed depolymerization of aromatic substances, and promoted production of protein-like DOM components due to lower enzymatic C/N acquisition ratio. As global changes in temperature and moisture will have large impacts on soil oxygen availability, the role of oxygen in regulating DOM dynamics highlights the importance of integrating soil oxygen supply with microbial metabolism and Fe redox status to improve model predictions of soil carbon under climate change.

Rape straw application facilitates Se and Cd mobilization in Cd-contaminated seleniferous soils by enhancing microbial iron reduction

Many naturally seleniferous soils are faced with Cd contamination problem, which severely limits crop cultivation in these areas. Straw returning has been widely applied in agricultural production due to its various benefits to soil physicochemical properties, soil fertility, and crops yield. However, effects of straw application on the fates of Se and Cd in Cd-contaminated seleniferous soils remain largely unclear. Therefore, the effects of straw application on the fates of Se and Cd in Cd-contaminated seleniferous soils were investigated in this study. The results showed that iron reduction driven by Clostridium and Anaeromyxbacter was responsible for the variations in Se and Cd fates in soil. Straw application respectively increased the gene copy numbers of Clostridium and Anaeromyxbacter by 19.5-56.3% and 33.6-39.8%, thus promoting iron reductive dissolution, eventually resulting in a high release amount of Se and Cd from Fe(III) (oxyhydr) oxides. Under reducing conditions, the released Cd was adsorbed by the newly formed metal sulfides or reacted with sulfides to generate CdS precipitates. Straw application decreased the soil exchangeable Se and soil exchangeable Cd concentration during flooding phase. However, straw application significantly increased Se/Cd in soil solution which had the highest bioavailability during flooding. In addition, straw application increased soil exchangeable Se concentration, but it had no significant effects on soil exchangeable Cd concentration after soil drainage. Taken together, straw application increased Se bioavailability and Cd mobility. Therefore, straw application is an effective method for improving Se bioavailability, but it is not suitable for the application to Cd-contaminated paddy soils. In the actual agricultural production, straw could be applied in seleniferous soils to improve Se bioavailability. At the same time, straw application should be cautious to avoid the release of Cd from Cd-contaminated soil.

Shared Microbial Taxa Respond Predictably to Cyclic Time-Varying Oxygen Limitation in Two Disparate Soils

Periodic oxygen (O₂) limitation in humid terrestrial soils likely influences microbial composition, but whether communities share similar responses in disparate environments remains unclear. To test if specific microbial taxa share consistent responses to anoxia in radically different soils, we incubated a rainforest Oxisol and cropland Mollisol under cyclic, time-varying anoxic/oxic cycles in the laboratory. Both soils are known to experience anoxic periods of days to weeks under field conditions; our incubation treatments consisted of anoxic periods of 0, 2, 4, 8, or 12 d followed by 4 d of oxic conditions, repeated for a total of 384 d. Taxa measured by 16S rRNA gene sequences after 48 d and 384 d of experimental treatments varied strongly with increasing anoxic period duration, and responses to anoxia often differed between soils at multiple taxonomic levels. Only 19% of the 30,356 operational taxonomic units (OTUs) occurred in both soils, and most OTUs did not respond consistently to O₂ treatments. However, the OTUs present in both soils were disproportionally abundant, comprising 50% of sequences, and they often had a similar response to anoxic period duration in both soils (p < 0.0001). Overall, 67 OTUs, 36 families, 15 orders, 10 classes, and two phyla had significant and directionally consistent (positive or negative) responses to anoxic period duration in both soils. Prominent OTUs and taxonomic groups increasing with anoxic period duration in both soils included actinomycetes (Micromonosporaceae), numerous Ruminococcaceae, possible metal reducers (Anaeromyxobacter) or oxidizers (Candidatus Koribacter), methanogens (Methanomicrobia), and methanotrophs (Methylocystaceae). OTUs decreasing with anoxic duration in both soils included nitrifiers (Nitrospira) and ubiquitous unidentified Bradyrhizobiaceae and Micromonosporaceae. Even within the same genus, different OTUs occasionally showed strong positive or negative responses to anoxic duration (e.g., Dactylosporangium in the Actinobacteria), highlighting a potential for adaptation or niche partitioning in variable-O₂ environments. Overall, brief anoxic periods impacted the abundance of certain microbial taxa in predictable ways, suggesting that microbial community data may partially reflect and integrate spatiotemporal differences in O₂ availability within and among soils.

Modeling Soil Organic Carbon Changes under Alternative Climatic Scenarios and Soil Properties Using DNDC Model at a Semi-Arid Mediterranean Environment

Soil organic carbon (SOC) is one of the central issues in dealing with soil fertility as well as environmental and food safety. Due to the lack of relevant data sources and methodologies, analyzing SOC dynamics has been a challenge in Morocco. During the last two decades, process-based models have been adopted as alternative and powerful tools for modeling SOC dynamics; whereas, information and knowledge on the most sensitive model inputs under different climate, and soil conditions are still very limited. For this purpose, a sensitivity analysis was conducted in the present work, using the DeNitrification-DeComposition (DNDC) model based on the data collected at a semi-arid region (Merchouch station, Morocco). The objective is to identify the most influential factors affecting the DNDC-modeled SOC dynamics in a semi-arid region across different climatic and soil conditions. The results of sensitivity analysis highlighted air temperature as the main determinant of SOC. A decrease in air temperature of 4 °C results in an almost 161 kg C ha−1 yr−1 increase in C sequestration rate. Initial SOC was also confirmed to be one of the most sensitive parameters for SOC. There was a 96 kg C ha−1 yr−1 increase in C sequestration rate under low initial SOC (0.005 kg C ha−1). In the DNDC, air temperature in climatic factors and initial SOC in variable soil properties had the largest impacts on SOC accumulation in Merchouch station. We can conclude that the sensitivity analysis conducted in this study within the DNDC can contribute to provide a scientific evidence of uncertainties of the selected inputs variables who can lead to uncertainties on the SOC in the study site. The information in this paper can be helpful for scientists and policy makers, who are dealing with regions of similar environmental conditions as Merchouch Station, by identifying alternative scenarios of soil carbon sequestration.

High carbon losses from oxygen-limited soils challenge biogeochemical theory and model assumptions

Oxygen (O₂ ) limitation contributes to persistence of large carbon (C) stocks in saturated soils. However, many soils experience spatiotemporal O₂  fluctuations impacted by climate and land-use change, and O₂ -mediated climate feedbacks from soil greenhouse gas emissions remain poorly constrained. Current theory and models posit that anoxia uniformly suppresses carbon (C) decomposition. Here we show that periodic anoxia may sustain or even stimulate decomposition over weeks to months in two disparate soils by increasing turnover and/or size of fast-cycling C pools relative to static oxic conditions, and by sustaining decomposition of reduced organic molecules. Cumulative C losses did not decrease consistently as cumulative O₂ exposure decreased. After >1 year, soils anoxic for 75% of the time had similar C losses as the oxic control but nearly threefold greater climate impact on a CO₂ -equivalent basis (20-year timescale) due to high methane (CH₄ ) emission. A mechanistic model incorporating current theory closely reproduced oxic control results but systematically underestimated C losses under O₂  fluctuations. Using a model-experiment integration (ModEx) approach, we found that models were improved by varying microbial maintenance respiration and the fraction of CH₄ production in total C mineralization as a function of O₂ availability. Consistent with thermodynamic expectations, the calibrated models predicted lower microbial C-use efficiency with increasing anoxic duration in one soil; in the other soil, dynamic organo-mineral interactions implied by our empirical data but not represented in the model may have obscured this relationship. In both soils, the updated model was better able to capture transient spikes in C mineralization that occurred following anoxic-oxic transitions, where decomposition from the fluctuating-O₂ treatments greatly exceeded the control. Overall, our data-model comparison indicates that incorporating emergent biogeochemical properties of soil O₂ variability will be critical for effectively modeling C-climate feedbacks in humid ecosystems.

Drivers of Organic Carbon Stocks in Different LULC History and Along Soil Depth for a 30 Years Image Time Series

Soil organic carbon (SOC) stocks are a remarkable property for soil and environmental monitoring. The understanding of their dynamics in crop soils must go forward. The objective of this study was to determine the impact of temporal environmental controlling factors obtained by satellite images over the SOC stocks along soil depth, using machine learning algorithms. The work was carried out in São Paulo state (Brazil) in an area of 2577 km2. We obtained a dataset of boreholes with soil analyses from topsoil to subsoil (0–100 cm). Additionally, remote sensing covariates (30 years of land use history, vegetation indexes), soil properties (i.e., clay, sand, mineralogy), soil types (classification), geology, climate and relief information were used. All covariates were confronted with SOC stocks contents, to identify their impact. Afterwards, the abilities of the predictive models were tested by splitting soil samples into two random groups (70 for training and 30% for model testing). We observed that the mean values of SOC stocks decreased by increasing the depth in all land use and land cover (LULC) historical classes. The results indicated that the random forest with recursive features elimination (RFE) was an accurate technique for predicting SOC stocks and finding controlling factors. We also found that the soil properties (especially clay and CEC), terrain attributes, geology, bioclimatic parameters and land use history were the most critical factors in controlling the SOC stocks in all LULC history and soil depths. We concluded that random forest coupled with RFE could be a functional approach to detect, map and monitor SOC stocks using environmental and remote sensing data.

Temporal and Agricultural Factors Influence Escherichia coli Survival in Soil and Transfer to Cucumbers

Biological soil amendments of animal origin (BSAAO) increase nutrient levels in soils to support the production of fruits and vegetables. BSAAOs may introduce or extend the survival of bacterial pathogens which can be transferred to fruits and vegetables to cause foodborne illness. Escherichia coli survival over 120 days in soil plots (3 m²) covered with (mulched) or without plastic mulch (not mulched), amended with either poultry litter, composted poultry litter, heat-treated poultry pellets, or chemical fertilizer, and transfer to cucumbers in 2 years (2018 and 2019) were evaluated. Plots were inoculated with E. coli (8.5 log CFU/m²) and planted with cucumber seedlings (Supremo). The number of days needed to reduce E. coli levels by 4 log CFU (dpi4log) was determined using a sigmoidal decline model. Random forest regression and one-way analysis of variance (ANOVA; P < 0.05) identified predictors (soil properties, nutrients, and weather factors) of dpi4log of E. coli and transfer to cucumbers. The combination of year, amendment, and mulch (25.0% increase in the mean square error [IncMSE]) and year (9.75% IncMSE) were the most prominent predictors of dpi4log and transfer to cucumbers, respectively. Nitrate levels at 30 days and soil moisture at 40 days were also impactful predictors of dpi4log. Differing rainfall amounts in 2018 (24.9 in.) and 2019 (12.6 in.) affected E. coli survival in soils and transfer to cucumbers. Salmonella spp. were recovered sporadically from various plots but were not recovered from cucumbers in either year. Greater transfer of E. coli to cucumbers was also shown to be partially dependent on dpi4log of E. coli in plots containing BSAAO.IMPORTANCE Poultry litter and other biological soil amendments are commonly used fertilizers in fruit and vegetable production and can introduce enteric pathogens such as Escherichia coli O157:H7 or Salmonella previously associated with outbreaks of illness linked to contaminated produce. E. coli survival duration in soils covered with plastic mulch or uncovered and containing poultry litter or heat-treated poultry litter pellets were evaluated. Nitrate levels on day 30 and moisture content in soils on day 40 on specific days were good predictors of E. coli survival in soils; however, the combination of year, amendment, and mulch type was a better predictor. Different cumulative rainfall totals from year to year most likely affected the transfer of E. coli from soils to cucumbers and survival durations in soil. E. coli survival in soils can be extended by the addition of several poultry litter-based soil amendments commonly used in organic production of fruits and vegetables and is highly dependent on temporal variation in rainfall.

Mineralogical associations with soil carbon in managed wetland soils

Carbon (C)-rich wetland soils are often drained for agriculture due to their capacity to support high net primary productivity. Increased drainage is expected this century to meet the agricultural demands of a growing population. Wetland drainage can result in large soil C losses and the concentration of residual soil minerals such as iron (Fe) and aluminum (Al). In upland soils, reactive Fe and Al minerals can contribute to soil C accumulation through sorption to poorly crystalline minerals and coprecipitation of organo-metal complexes, as well as C loss via anaerobic respiration by Fe-reducing bacteria. The role of these minerals in soil C dynamics is often overlooked in managed wetland soils and may be particularly important in both drained and reflooded systems with elevated mineral concentrations. Reflooding drained soils have been proposed as a means to sequester C for climate change mitigation, yet little is known about how reactive Fe and Al minerals affect C cycling in restored wetlands. We explored the interactions among soil C and reactive Fe and Al minerals in drained and reflooded wetland soils. In reflooded soils, soil C was negatively associated with reactive Fe and reduced Fe(II), a proxy for anaerobic conditions (reactive Fe: R²  = .54-.79; Fe(II): R²  = .59-.89). In drained soils, organo-Al complexes were positively associated with soil C and Fe(II) (Al R²  = .91; Fe(II): R²  = .54-.60). Soil moisture, organo-Al, and reactive Fe explained most of the variation observed in soil C concentrations across all sites (p < .01). Reactive Fe was negatively correlated to soil C concentrations across sites, suggesting these Fe pools may drive additional C losses in drained soils and limit C sequestration with reflooding. In contrast, reactive organo-Al in drained soils facilitates C storage via aggregation and/or formation of anaerobic (micro)sites that protect residual soil C from oxidation and may at least partially offset C losses.

Organic Carbon Stabilization Mechanisms in Mangrove Soils: A Review

Despite the recognized organic carbon (OC) sequestration potential of mangrove forests, the ongoing climate change and anthropogenic disturbances pose a great threat to these ecosystems. However, we currently lack the ability to mechanically understand and predict the consequences of such impacts, primarily because mechanisms underlying OC stabilization in these ecosystems remain elusive. Research into OC stabilization has focused on terrestrial soils and marine sediments for decades, overlooking the vegetated coastal ecosystems including mangroves. In terrestrial soils and marine sediments, it is widely accepted that OC stabilization is the integrated consequence of OM’s inherent recalcitrance, physical protection, and interactions with minerals and metals. However, related discussion is rarely done in mangrove soils, and recalcitrance of roots and high net ecosystem production (high primary production and low heterotrophic respiration) have been considered as a primary OC sequestration mechanism in mangrove peat and mineral soils, respectively. This review presents the available information on the mechanisms underlying OC stabilization in mangrove soils and highlights research questions that warrant further investigation. Primary OC stabilization mechanisms differ between mangrove peat and mineral soils. In mangrove mineral soils, physico-chemical stabilization processes are important, yet grossly understudied OC stabilization mechanisms. In mangrove peat, recalcitrance of mangrove roots and the inhibition of phenoloxidase under the anoxic condition may be the primary OC stabilization mechanisms. Salinity-induced OC immobilization likely plays a role in both type of soils. Finally, this review argues that belowground production and allochthonous inputs in mangrove forests are likely underestimated. More studies are needed to constrain C budgets to explain the enigma that mangrove OC keeps accumulating despite much higher decomposition (especially by large lateral exports) than previously considered.