Sequestration Of Atmospheric Carbon Dioxide Research Articles

Precision and bias of carbon storage estimations in wetland and mangrove sediments.

Peaty sediments in coastal wetlands play an important role in the sequestration of atmospheric carbon dioxide and its belowground storage. Sediment cores are used to quantify organic matter (OM) density, estimated by multiplying the bulk density of a core segment by its OM fraction. This method can be imprecise, as repeated samples often differ widely. Recent studies have shown that sediment bulk density and OM fraction are not independent but tightly related by a function called the ideal-mixing model. Thus, the bulk density of the sediment can be directly estimated from its OM fraction. Statistical theory and simulations demonstrate that the high variance in the product estimation of OM density is the result of error propagation in the product of two functionally related variables with independent errors. Estimating OM density in wetland sediments using the ideal-mixing model is more precise than the traditionally used product estimate, especially in highly organic sediments.

Atmospheric Carbon Sequestration Using Microalgae

This article outlines biotechnological methods that can help reduce atmospheric and industrial carbon dioxide emissions through the use of microalgae. A general description of microalgae was provided, and the most promising species for microalgal biotechnology were identified. The metabolic process by which microalgae capture and degrade carbon dioxide was described. The microalgae-based biotechnological systems and devices available today were analyzed. The key factors that need to be considered for the effective and successful use of microalgae were highlighted. Different products obtained from microalgal biomass after atmospheric carbon dioxide sequestration were overviewed.

Spatial Database of Planted Trees (SDPT Version 2.0)

This paper describes the methods for updating the Spatial Database of Planted Trees (SDPT) in three areas: boundaries, to spatially differentiate planted forests and tree crops from natural and seminatural forests on a global scale; tree species information, to help screen for the illegal exportation of timber products; and carbon sequestration rates, or removal factors, to improve maps of atmospheric carbon dioxide sequestration by planted trees.

Characterizing the Mechanisms of Ca and Mg Carbonate Ion-Pair Formation with Multi-Level Molecular Dynamics/Quantum Mechanics Simulations.

The carbonate minerals of Ca and Mg are abundant throughout the lithosphere and have recently garnered significant research interest as possible long-term carbon sinks in the sequestration of atmospheric carbon dioxide. Nonetheless, an understanding of the atomic-level processes comprising their mineralization remains limited. Here, we characterize and contrast the mechanisms of contact ion-pair formation in aqueous Ca and Mg carbonate systems, which represents the most fundamental step leading to the formation of their mineral solids. Utilizing multilevel embedded correlated wavefunction-based ab initio molecular dynamics/quantum mechanics simulations, we characterize not only the dynamics of these processes but also factors arising from the electronic structure of the involved species, revealing further details of the fundamentally different mechanisms for the interconversion between the contact ion-pairs and solvent-shared ion-pairs of Ca versus Mg carbonate.

Estimation of Aboveground Forest Biomass and Carbon Storage of Bangladesh

Forest biomass helps mitigate climate change impacts through sequestration of atmospheric carbon dioxide and potentially storing it for long periods of time. Deforestation and timber harvesting cause the reduction of forest biomass resulting in the reduced carbon sequestration capacity and alterednatural balance of forest ecosystems. We used remote sensing and GIS tools in the four important forest cover zones within five districts of Bangladesh to compare the aboveground forest biomass (AGB) changes between 2014 and 2020. We found an increased AGB in Sundarban mangrove forest from 89.73 Mg.h-1 in 2014 to 90.76 Mg.h-1 in 2020. Similarly, the AGB was found to be increased for Ukhiya hill forest from 7.89 Mg.h-1 in 2014 to 8.89 Mg.h-1 in 2020. Contrary, the average AGB content in Nijhum Dwip mangrove forest decreased from 44.36 Mg.h-1 in 2014 to 37.46 Mg.h-1 in 2020. The average AGB of Modhupur decidious forest also found to be decreased from 110.01 Mg.h-1 in 2014 to 107.22 Mg.h-1 in 2020. The decreased biomass contents could be attributed to anthropgenic factors as indicated by the presence of human activities and this informatin will be helpful for forest restoration and management in Bangladesh.

Natural ocean iron fertilization and climate variability over geological periods.

Marine primary producers are largely dependent on and shape the Earth's climate, although their relationship with climate varies over space and time. The growth of phytoplankton and associated marine primary productivity in most of the modern global ocean is limited by the supply of nutrients, including the micronutrient iron. The addition of iron via episodic and frequent events drives the biological carbon pump and promotes the sequestration of atmospheric carbon dioxide (CO2 ) into the ocean. However, the dependence between iron and marine primary producers adaptively changes over different geological periods due to the variation in global climate and environment. In this review, we examined the role and importance of iron in modulating marine primary production during some specific geological periods, that is, the Great Oxidation Event (GOE) during the Huronian glaciation, the Snowball Earth Event during the Cryogenian, the glacial-interglacial cycles during the Pleistocene, and the period from the last glacial maximum to the late Holocene. Only the change trend of iron bioavailability and climate in the glacial-interglacial cycles is consistent with the Iron Hypothesis. During the GOE and the Snowball Earth periods, although the bioavailability of iron in the ocean and the climate changed dramatically, the changing trend of many factors contradicted the Iron Hypothesis. By detangling the relationship among marine primary productivity, iron availability and oceanic environments in different geological periods, this review can offer some new insights for evaluating the impact of ocean iron fertilization on removing CO2 from the atmosphere and regulating the climate.

Organic Carbon and Lignin Protection by Metal Oxides Versus Silicate Clay: Comparative Study Based on Wetland and Upland Soils

AbstractMineral binding of organic carbon (OC) is one of the key mechanisms promoting soil organic carbon (SOC) persistence, which is vital for the long‐term sequestration of atmospheric carbon dioxide into soils. However, the relative importance of two main types of soil minerals ‐ metal oxides and silicate clay—in SOC protection remains unclear, hampering our ability to predict and protect this important pool of persistent SOC. Here, we analyzed OC bound to metal oxides (OCmetal) versus silicate clay (OCclay) in soils from contrasting environments (i.e., wetlands and uplands) using sequential dissolution by dithionite and hydrofluoric acid. We found that OCmetal% was higher than OCclay% in both wetlands and uplands, indicating the dominance of metal oxides in SOC protection. The sum of OCmetal% and OCclay% constituted a higher fraction of SOC in wetlands than uplands, suggesting an underappreciated role of mineral (in particular, metal oxide) protection in wetland SOC preservation. Furthermore, using lignin phenol analysis in tandem, ∼23% of lignin phenols were additionally released after the removal of silicate clay, providing a potential means to assess “hidden” lignin in mineral matrices. Given the wide use of lignin phenols as tracers for terrestrial plant‐derived OC, our results suggest that the contribution of terrestrial or plant‐derived carbon to sedimentary carbon stocks or mineral‐protected SOC deserves reassessment. These findings highlight the differential role of various soil minerals in the protection of SOC and its components in contrasting terrestrial environments, which warrants better recognition for understanding the distribution and dynamics of mineral‐protected SOC.

Human and natural impacts on the U.S. freshwater salinization and alkalinization: A machine learning approach

Ongoing salinization and alkalinization in U.S. rivers have been attributed to inputs of road salt and effects of human-accelerated weathering in previous studies. Salinization poses a severe threat to human and ecosystem health, while human derived alkalinization implies increasing uncertainty in the dynamics of terrestrial sequestration of atmospheric carbon dioxide. A mechanistic understanding of whether and how human activities accelerate weathering and contribute to the geochemical changes in U.S. rivers is lacking. To address this uncertainty, we compiled dissolved sodium (salinity proxy) and alkalinity values along with 32 watershed properties ranging from hydrology, climate, geomorphology, geology, soil chemistry, land use, and land cover for 226 river monitoring sites across the coterminous U.S. Using these data, we built two machine-learning models to predict monthly-aggregated sodium and alkalinity fluxes at these sites. The sodium-prediction model detected human activities (represented by population density and impervious surface area) as major contributors to the salinity of U.S. rivers. In contrast, the alkalinity-prediction model identified natural processes as predominantly contributing to variation in riverine alkalinity flux, including runoff, carbonate sediment or siliciclastic sediment, soil pH and soil moisture. Unlike prior studies, our analysis suggests that the alkalinization in U.S. rivers is largely governed by local climatic and hydrogeological conditions.

Legume-based cover crop mixtures can overcome trade-offs between C inputs, soil mineral N depletion and residual yield effects

Cover crops can contribute to climate change mitigation through enhanced sequestration of atmospheric carbon dioxide into soil organic carbon. Few studies, however, have estimated the total carbon (C) input to soil, i.e. derived both from plant material (shoot and root) and phyllo- and rhizodeposition. Selection of cover crop species should account for multiple objectives, such as C inputs to soil, nitrate leaching reduction and positive residual effects on the following main crop. However, trade-offs between these objectives may occur. The aim of this study was to investigate the performance of the cover crop species winter rye, hairy vetch and oilseed radish, and to assess the ability of mixtures to overcome potential trade-offs. A randomized split-plot field trial was conducted to compare cover crop treatments and a weeded control under high and low soil nitrogen (N) availability. Multiple-pulse labeling with 14C-CO2 was carried out to trace net cover crop-derived rhizodeposition C. Soil mineral N was measured to 1.5 m depth in autumn, as well as grain and N yield in the subsequent spring barley.Cover crop species accumulated between 1250 and 2580 kg C ha−1, with significantly higher total C input (in shoot, root and phyllo- and rhizodeposits) for the mixtures compared with pure stands of either vetch or radish, while the results for rye were in between. The quantity of C lost via phyllo- and rhizodeposition (qClvPR) showed a significant positive correlation with root C and was highest for the mixtures and rye. The relative ClvPR ranged between 7% and 14% of total cover crop-derived C and tended to decrease under higher soil N availability. All cover crop treatments were able to decrease soil mineral N (0–1.5 m), with radish displaying the highest N leaching reduction potential. Despite substantial differences in cover crop total N uptake and C:N ratios, no significant differences were observed in the subsequent main crop grain or N yields. The mixtures showed the highest total C input and generally a higher or similar mineral N depletion potential than the average of the pure stands, suggesting that cover crop mixtures offer a realistic means for overcoming trade-offs among ecosystem functions.

Soil organic carbon content increase in the east and south of China is accompanied by soil acidification.

Increased soil organic carbon (OC) in China has been reported in the past two decades, suggesting the sequestration of atmospheric carbon dioxide into soil, mitigating climate change and improving soil health. On the other hand, soil pH decrease had also been reported nationwide. If the two are related, the strategy of increasing soil OC could negatively affect soil quality for food production and the environment. We investigate this thread based on large-scale soil survey data from two provinces with typical soil and cropping patterns in the east and south of China, Jiangsu (102,600 km2) and Guangdong (177,900 km2). The data include >5000 observations from soil surveys conducted over the past four decades, i.e., the 1980s, 2006-2007, and 2010-2011. Using spatiotemporal modelling, we show that across Jiangsu province, the topsoil OC on average has increased from 8.5 g kg-1 to 9.9 g kg-1 from 1980 to 2000 and a further increase to 12.6 g kg-1 in 2010. This increase was accompanied by a decrease in average pH from 7.63 to 6.90. In Guangdong, there was an overall increase in average topsoil OC content from 14.2 g kg-1, 16.5 g kg-1, and 20.2 g kg-1 with a decrease in average pH from 5.58, 4.90, and 4.98. Based on the spatiotemporal modelling results, the structural equation modelling analysis shows that OC and pH changes were significantly correlated and linked by increased soil N content. On croplands, soil N content was mainly attributed to N fertiliser application. The pH decrease was particularly significant in the east of China where the soils were neutral in pH. We recommend that more revolutionary means be taken to sequestrate atmospheric carbon into soil as the current OC increase due to increasing crop productivity via a high rate of nitrogen application may have a potential acidification effect.

Climatic controls on soil and saprock nitrogen distribution and persistence in the Sierra Nevada

AbstractBackgroundNitrogen (N) is an essential nutrient in soil that regulates plant growth, terrestrial sequestration of atmospheric carbon dioxide, and persistence of organic compounds. However, major knowledge gaps remain about how climate change may impact N accumulation and persistence, especially in deep soil and saprock (friable weakly weathered bedrock).AimsOur objective was to understand how climate impacts the accumulation of N in soil and saprock and how climate impacts soil N distribution, persistence, and mechanisms of N persistence.MethodsWe investigated N concentration in bulk soil and density fractions. We estimated N persistence along a bio‐climatic sequence—sites range from a low‐elevation oak savannah, mid‐elevation pine‐oak/mixed‐conifer forest, to a high‐elevation subalpine forest—in the southern Sierra Nevada in California. A combination of radiocarbon and elemental composition measurements along with a first‐order kinetic model was used.ResultsThe N concentration in the bulk soil and density fractions declined with depth, and there was a relatively greater mineral‐associated heavy fraction (HF) N in deeper samples. The cooler/wetter mixed conifer site held 37% of profile N in saprock, which was greater than that of the entire soil profile at the drier/hotter oak savannah. The majority of N in soil, which was in the HF, was not influenced by climate proxies tested. However, both unprotected and occluded fractions of N were strongly influenced by climate. Soil N mean residence time (MRT) showed that drier/hotter climates have a shorter MRT, compared to mid‐elevation sites with cooler/wetter climates.ConclusionsThe effect of climate on deep saprock N storage might be indirect, primarily through climatic influence on the thickness of saprock. Overall, our findings suggest the mineral‐associated HF N pool will not be vulnerable to changes in climate and will continue to contribute to the persistent soil N pool. The amount of topsoil and subsoil unprotected and occluded N can be explained by gross primary productivity and mean annual precipitation indicating that changes in climate can influence N partitioning. N stored in deep soil and saprock may be less vulnerable to climate than N stored in drier/hotter climates with less deeply stored N. It is critical to dig deeper to understand terrestrial ecosystems’ response to climate.

Evaluation of forest carbon uptake in South Korea using the national flux tower network, remote sensing, and data-driven technology

Forests provide most of the carbon sequestration of atmospheric carbon dioxide (CO2); however, accurately quantifying the uptake amount over a region remains challenging. For reginal or national estimates, the forest productivity model and forest inventories are used which provide information for national greenhouse gas inventories. However, it has some limitations, such as not considering below-ground biomass, its lack of species-specific allometric models, the restrictions it places during fieldwork, and the long period it takes to complete the survey. In contrast to inventory-based biomass estimates, the eddy covariance (EC) method can assess net CO2 exchange of a whole ecosystem continuously and automatically with a high temporal resolution. Since, these measurements only represent a site-level observation scale (∼ 1 km2), upscaling via linkages with observation data, remote sensing, and modeling methods has been used to estimate regional or national land-atmosphere carbon fluxes. In this study, we employ a data-driven method to estimate the national-scale gross primary production (GPP) and net ecosystem CO2 exchange (NEE) by combining EC flux data from 10 sites in South Korea with remote sensing data through a machine learning algorithm based on support vector regression (SVR) for the period 2000–2018. Site-level evaluation of estimated GPP and NEE from the SVR-based model shows equivalent performance compared to other continental and global upscaled models. The mean estimated annual GPP and NEE of the South Korea forests region over the period 2000–2018 were 1465 ± 37 and −243 ± 32 g C m2 year−1, respectively. The SVR-based net primary production (NPP) was consistent with the biometric-based NPP (r2 = 0.46, p < 0.05). This study shows that combining data from a national flux network and remote sensing using a data-driven approach can be used to estimate forest CO2 fluxes on a national scale.

But They Told Us It Was Safe! Carbon Dioxide Removal, Fracking, and Ripple Effects in Risk Perceptions.

Reaching net-zero for global greenhouse gas emissions by the year 2050 will require a portfolio of new technologies and approaches, potentially requiring direct removal and sequestration of atmospheric carbon dioxide using negative emissions technologies (NETs). Since energy and climate systems are fundamentally interconnected it is important that we understand the impacts of policy decisions and their associated controversies in other related technologies and sectors. Using a secondary analysis of data from a series of deliberative workshops conducted with lay publics in the United Kingdom, we suggest that perceptions of CO2 removal technologies were negatively impacted by risk perceptions and recent policy decisions surrounding shale gas and fracking. Using the social amplification of risk framework, we argue that heightened risk perceptions have extended via "ripple effects" across these technologies. Participants' attitudes were underpinned by deeper misgivings regarding the actions and motives of experts and policymakers; a pervasive discourse of "but they told us it was safe" regarding fracking negatively affected people's trust in assurances of the safety and efficacy of CO2 removal. This has the potential to undermine attempts to build societal agreement around future deployment of CO2 removal technologies.

Effect of Heat Stress and the Recovery Potential of Heterocystous Cyanobacterium, Anabaena iyengarii Bharadwaja 1935

Cyanobacteria, the major photosynthetic organisms, cover a large surface area of this planet. These organisms, being photosynthetic, have the capacity for sequestration of atmospheric carbon dioxide, a significant greenhouse gas that causes global warming. In this work, we have collected, developed pure culture, and identified 25 cyanobacterial species from semi arid agricultural rice fields of western Odisha with the high-temperature environmental setting. The purpose was to screen the cyanobacteria that can survive and grow at high temperatures with high photosynthetic efficiency. Cyanobacteria belong to genera Nostoc, Anabaena, Calothrix, and Hapalosiphon are observed to survive at 45°C. Among the cyanobacterial species, Anabaena iyengarii 17-SKD-2014 was found to exhibit higher growth, protein content, photosynthetic pigments, and photosynthetic O2 evolution at 45°C in comparison to other cyanobacterial isolates. Further, this cyanobacterium was grown at 50°C to analyze the cellular viability, and only up to ninth day incubated culture could recover from high-temperature stress after transferring to 25°C. Even though this indigenous cyanobacterial species failed to survive at 50°C in the laboratory conditions beyond a time limit, but this could be biotechnologically manipulated for effective carbon dioxide sequestration contributing to minimization of global warming.

Release of significant fraction of primary production as dissolved organic carbon in the Bay of Bengal

Despite nutrients supply through river discharge, atmospheric deposition and eddy-driven mixing, low primary production in the Bay of Bengal (BoB) is attributed to light limitation and non-availability of nutrients during summer monsoon. We report here the first ever measurements of release of significant fraction of primary production as dissolved organic carbon (DOC) in the BoB through isotopic tracer experiments. Our results suggested that about half of the primary production is released as DOC due to existence of oligotrophic conditions, warm waters and dominance of picophytankton biomass during the summer monsoon. The mean Percentage of Extracellular Release (PER) ranged between 30 and 60% (mean of 45 ± 8%) suggesting that primary production through particulate organic matter measurements were significantly underestimated total primary production in the BoB. Higher DOC release compared to particulate organic carbon (POC) production was noticed in the cyclonic eddy (CE) regions, where high nutrients were found, than anticyclonic eddy (ACE) region, where balanced POC production and DOC release observed associating with warm waters and nutrient deplete conditions. This study suggests that primary production, including the DOC release, is not low than hitherto hypothesized due to lack of measurements on DOC and this study explains the paradox between low production and high sinking carbon fluxes to the deep BoB and underestimation of sequestration of atmospheric carbon dioxide through biological pump in the BoB.

Re-examining Dust Chemical Aging and Its Impacts on Earth's Climate.

The impact of atmospheric particulate matter (i.e., aerosols) on Earth's radiative balance has been and continues to be the leading source of uncertainty with respect to predictions of future temperature increases due to climate change. Mineral dust particles transported from deserts and semiarid regions across the globe are a dominant contributor to the aerosol burden. Dust has many and diverse effects on Earth's climate: it directly scatters and/or absorbs incoming sunlight; it reacts with trace gases leading to impacts on the oxidizing capacity of the atmosphere that affect both the lifetime of the greenhouse gas methane in addition to concentrations of tropospheric ozone-a greenhouse gas and criteria air pollutant; it influences the production as well as the lifetime and radiative properties of clouds; and it deposits nutrients to aquatic and terrestrial ecosystems that can stimulate primary production and facilitate the sequestration of atmospheric carbon dioxide (CO2). This Account will focus on the last three effects. The ability of dust to affect clouds and biogeochemical cycles hinges upon the chemical nature of dust particles-in particular, whether the compounds found in dust particles are water-soluble. The solubility of nutrients found in dust is particularly critical for determining the impact of atmospheric deposition on ocean productivity. The traditional viewpoint is that dust is inherently insoluble but reactive toward trace acidic gases, a process herein referred to as chemical aging. These reactions are thought to affect the oxidizing capacity of the atmosphere while effectively transforming the chemical composition of dust by increasing its solubility. Consequently, chemical aging is hypothesized to substantially increase the impact of dust on cloud droplet formation and marine biogeochemical cycles.This Account presents recent advances in our understanding of the mechanisms that determine how efficiently dust undergoes chemical aging and what the consequences of these processes are for the different effects of dust on Earth's climate. This Account will re-examine the traditional viewpoint that dust chemical aging strongly impacts marine biogeochemical cycles as well as the ability of dust to nucleate cloud droplets. Laboratory studies on environmental samples are combined with chemical analysis of field samples collected at dust transport receptor sites to better understand chemical aging mechanisms and determine the impact of dust on tropospheric oxidants, clouds, and biogeochemical cycles. Our results highlight the important role that dust mineralogy plays in both the nucleation of clouds as well as the kinetics responsible for the chemical aging of dust. This Account will present cases where dust contains inherently soluble minerals and does not require chemical aging in order to efficiently nucleate clouds in the atmosphere. Lastly, this Account illustrates the critical role that nondust aerosols, namely, wildfire and combustion emissions, play as a supplier of soluble nutrients important for biogeochemical cycles, particularly in marine environments. This Account will discuss these findings and highlight future research directions and recommendations to better understand dust-climate interactions and the emerging role of biomass burning aerosol in marine biogeochemical cycles.

Focus on the role of forests and soils in meeting climate change mitigation goals: summary

It is clear that reducing greenhouse gas emissions alone is insufficient to avoid large global temperature increases. To avoid atmospheric concentrations of greenhouse gases that result in dangerous alterations of the climate, large reductions in carbon dioxide emissions from fossil fuel combustion and land use changes must be accompanied by an increase in atmospheric carbon dioxide sequestration. Natural Climate Solutions have become a major focus of climate policy. Land and ocean ecosystems remove and store atmospheric carbon, and forests play a major role. This focus collection includes papers that address three important aspects of the role for forests in meeting climate change mitigation goals: (i) Carbon Accounting of forest sinks and reservoirs, process emissions and carbon storage in forest products, (ii) the carbon dioxide dynamics of using Forest Bioenergy and (iii) the carbon cycle of Tropical Forests.

The European colonization of the Americas as an explanation of the Little Ice Age

The hypothesis that the European colonization of the Americas was the driving force behind the onset of the Little Ice Age, through a mechanism involving reforestation and sequestration of carbon dioxide from the atmosphere, is here tested. On the basis of a literature review, and the assessment of the present temperature sensitivity to atmospheric carbon dioxide, it is concluded that the claim is overrated. The reforestation attributed to the European colonization of the Americas is excessive. Even this excessive atmospheric carbon dioxide sequestration would not be enough to produce the Little Ice Age.

An insight into carbon balance of product streams from hydrothermal liquefaction of Scenedesmus abundans biomass

Lab-scale hybrid loop airlift photobioreactor (HLALPBR) was used to culture microalgae Scenedesmus abundans in domestic wastewater. The highest biomass productivity of 209 mg L−1 d−1 and concentration of 3.55 g L−1 were achieved. S abundans could eradicate chemical oxygen demand, dissolved inorganic nitrogen, and dissolved inorganic phosphorus by 80.19%, 90.73% and 86.31% respectively from domestic wastewater. Later, hydrothermal liquefaction of algal biomass yielded 35.5 wt% of bio-oil. Carbon mass balance between various product streams showed that per g of algae about 0.46, 0.15, 0.963 and 0.35 g of CO2 was sequestered by bio-char, bio-gas, bio-oil and post-HTL wastewater product streams respectively. Since bio-oil has low N and O content and higher heating value of 36.52 MJ/kg, those can be upgraded to transportation fuel. The conversion of S. abundans to bio-char as a carbon sink has been proposed for reducing emissions and increasing the sequestration of atmospheric carbon dioxide.

Variation in soil organic carbon stock with forest type in tropical forests of Kanyakumari Wildlife Sanctuary, Western Ghats, India.

Sequestration of atmospheric carbon-dioxide in biospheric carbon (C) pools is a key strategy towards climate change mitigation. Soil is a huge C reservoir and its storage potential varies greatly with forest types. Therefore, in the present study, the soil organic carbon (SOC) storage pattern was assessed from 70 plots laid at three selected forest types comprising seven study sites at Kanyakumari Wildlife Sanctuary, Western Ghats, India: tropical dry deciduous (TDD I and TDD II), tropical semi-evergreen (TSE I and TSE II) and tropical evergreen forest (TEF I, TEF II and TEF III) at three depths (0-10, 10.1-20 and 20.1-30 cm). Statistical analyses were performed to understand the relationships between SOC stocks with other predictor variables. The SOC stock varied markedly with forest type and site-wise. The SOC ranged from 58 (TEF III) to 123.6 (TDD I) Mg C/ha with a mean of 84.9 ± 4.4 Mg C/ha at 0-30 cm depth. SOC stock decreased, while soil bulk density increased with increase in soil depth. The TDD forest type (115.6 Mg C/ha) stocked the highest SOC compared to TEF (75.1 Mg C/ha) and TSE (68.9 Mg C/ha) forest types. Of the total SOC stock (0-30 cm), 44.2, 32.0 and 23.8% were stored in 0-10, 10.1-20 and 20.1-30 cm respectively in all the forest types. In contrast, litter C stock were high in TEF and TSE forest types and low in TDD forest type. SOC showed significant (P < 0.01) negative relationships with bulk density, litter C, and vegetation attributes. The SOC stock stored in the study sites amount to 212.9 (TEF III) to 453.6 (TDD I) Mg of CO2 equivalents. The present study reveals that forest type and site characteristics have a profound impact on SOC stock, which would, in turn, exert a great bearing on the ecosystem C cycling. These results would also enhance our ability to evaluate the role of these forest types in soil C sequestration and for developing and validating SOC models for tropical forest ecosystems.