Sequestration Of Atmospheric Carbon Dioxide Research Articles

Microalgae at niches of bioelectrochemical systems: A new platform for sustainable energy production coupled industrial effluent treatment

Bioelectrochemical system represents a novel technology where the microbial catalytic reaction occurs at bioanode and results in bioelectricity generation from waste and renewable biomass. At the current stage, fossil-based fuel is depleting, so newly sustainable form of renewable energy resources is required, where algal-based biofuel generation provides a new source of energy along with sequestration of atmospheric carbon dioxide, which in turn decreases the global warming issues. This review emphasizes the potential applications of microalgae-based bioelectrochemical systems for renewable power generation, wastewater treatment, CO2 sequestration and value-added products. Moreover, this critical review also highlights the current developments in microalgal MFC integrated systems by discussing the current potential challenges and drawbacks.

Opinion: Soil carbon sequestration is an elusive climate mitigation tool.

The need to stabilize the greenhouse gas concentrations of the atmosphere is the great environmental challenge of this century. To control these concentrations, humanity can reduce fossil fuel emissions and/or identify mechanisms to remove greenhouse gases once they have been emitted. The scope of the problem is challenging because of the size of the fluxes involved. Presently, industry, transportation, and domestic use emits nearly 10 Gt C to the atmosphere annually, and there is no immediate hope for a drastic reversal of these rates of emission (1). Thus, sequestration of atmospheric carbon dioxide as organic carbon in the biosphere attracts attention as an alternate way to help stem the rate of greenhouse gas growth and associated changes in our climate. Some soil researchers have suggested that altered agricultural techniques can help restore much carbon to domesticated soils, thus helping mitigate climate change. But cultural and scientific challenges suggest that this proposition is overly optimistic and inherently flawed. Image courtesy of ScienceSource/Jerry Irwin. For nearly 2 decades, researchers in the soil science community have studied and estimated the potential of sequestering carbon in soil organic matter (2, 3). The premise is inherently rational: nearly 10,000 years of cultivated agriculture has reduced global soil carbon by 116 Gt (4), an amount equivalent to more than a decade of the present rates of industrial emissions. Through changed agricultural techniques, it is proposed, much of this carbon can be restored to domesticated soils and thus serve as a significant tool to mitigate climate change, providing a wider timeframe for society to decarbonize. Unfortunately, both cultural and scientific challenges suggest that this proposal is overly optimistic and inherently flawed. Nevertheless, this long and relatively well-funded (5) area of research recently gained novel international exposure because of the unveiling of the French “4 per mille” … [↵][1]1To whom correspondence should be addressed. Email: earthy{at}berkeley.edu. [1]: #xref-corresp-1-1

Sequestration of atmospheric carbon dioxide as inorganic carbon in the unsaturated zone under semi-arid forests

Inorganic carbon, as allogenic and pedogenic carbonates mixtures, can be a significant component of arid and semi-arid zone soils. It has been claimed, that the formation of pedogenic carbonates does not sequester atmospheric carbon dioxide, when formed from the preexisting limestone: Ca+2 + 2HCO3− → CaCO3 ↓ + CO2 ↑ + H2O. But if the cation is derived from a non‑carbonate source sequestration can occur. Among the unconsidered sources in nature are the contribution of Ca+2, released from adsorption sites on the clay surfaces. Analysis of DIC, δ13C and Δ14C data show in the present study that sequestration does occur within the unsaturated zones (USZ) in a semiarid region: 1) the DIC is depleted as carbonate precipitates into the USZ, 2) the released CO2, does not immediately return to the atmosphere (except for the samples nearest to the surface) but recycles to the soil-gas of high CO2 partial pressure, and 3) that radiocarbon, which is generated only in the overlying atmosphere, and exhaled into the USZ by roots and the decay of soil organic matter, is incorporated within the calcite mineral. Thus, a net sequestration of atmospheric CO2 does occur.

Stand age versus tree diameter as a driver of forest carbon inventory simulations in the northeastern U.S.

Estimating the current status and future trends of carbon (C) stocks and stock changes in forests of the northeastern United States is desired by policy makers and managers as these forests can mitigate climate change through sequestration of atmospheric carbon dioxide (CO2). We developed C flux matrix models using tree and stand variables by tree diameter class and stand age class to compare size-structured models with age-structured models in their capacity to predict forest C dynamics that are central to policy decisions. The primary control variables for the C flux matrix models (diameter at breast height, stand basal area, stem density, and stand age) were all statistically significant at the α ≤ 0.05 level. Through comparing the simulation results and root mean square error of C flux matrix models by tree diameter class and stand age class, we found that tree diameter class more accurately predicted C stock change status across the broad compositional and structural conditions in the spatial and temporal domain. An uncertainty analysis revealed that predictions of aboveground C and soil C would be distinctively different whether using tree diameter class or stand age class with high certainty. Overall, this work may enable better integration of forest inventory data and remotely sensed data for the purpose of strategic-scale forest C dynamic simulations.

Atmospheric CO2 Sequestration in Iron and Steel Slag: Consett, County Durham, United Kingdom.

Carbonate formation in waste from the steel industry could constitute a nontrivial proportion of the global requirements for removing carbon dioxide from the atmosphere at a potentially low cost. To utilize this potential, we examined atmospheric carbon dioxide sequestration in a >20 million ton legacy slag deposit in northern England, United Kingdom. Carbonates formed from the drainage water of the heap had stable carbon and oxygen isotope values between -12 and -25 ‰ and -5 and -18 ‰ for δ13C and δ18O, respectively, suggesting atmospheric carbon dioxide sequestration in high-pH solutions. From the analyses of solution saturation states, we estimate that between 280 and 2900 tons of CO2 have precipitated from the drainage waters. However, by combining a 37 year long data set of the drainage water chemistry with geospatial analysis, we estimate that <1% of the maximum carbon-capture potential of the deposit may have been realized. This implies that uncontrolled deposition of slag is insufficient to maximize carbon sequestration, and there may be considerable quantities of unreacted legacy deposits available for atmospheric carbon sequestration.

Accelerated Carbonation of Coal Combustion Fly Ash for Atmospheric Carbon dioxide Sequestration and Soil Amendment: An Overview

Fly ash is one of the major solid by-products generated by coal combustion for power generation. At present most fly ash generated all over the world is dumped as waste material without any beneficial use. Only a small proportion of the total fly ash production is used in applications such as cement production, mineral wool production, recovery of metals, road sub-base construction, mine reclamation, and in agriculture. In addition, fly ash is a material that can be used to capture and store atmospheric CO2 through mineral carbonation both in situ and ex situ of the source of CO2 emissions. As a means of increasing the rate of mineral carbonation, accelerated carbonation has been researched by scientists in the recent past. In addition to achieving the benefits of carbon sequestration, accelerated carbonation can make the fly ash chemically stable, which is beneficial in overcoming problems associated with the leaching of toxic elements when using it as a soil amendment. Therefore, through accelerated carbonation followed by addition to soil this solid waste material can be successfully managed. The efficacy of the mineralization reaction depends on the mineralogy, and the physical and chemical properties of fly ash and the reaction conditions, such as CO2 partial pressure, temperature, relative humidity and the contact time of gas and the material. In order to achieve higher CO2 sequestration efficiency it is vital to select material with the required properties and to supply it with the optimum reaction conditions. This paper reviews the main issues related to the accelerated carbonation of coal combustion fly ash under different reaction conditions and its effects on CO2 sequestration efficiency. It also identifies the potential application of carbonated fly ash to soil in order to improve soil’s physical and chemical properties.

Land use change and soil carbon pools: evidence from a long-term silvopastoral experiment

Multi-functional silvopastoral systems provide a wide range of services to human society including the regulation of nutrients and water in soils and the sequestration of atmospheric carbon dioxide (CO2). Although silvopastoral systems significantly contribute to enhance aboveground carbon (C) sequestration (e.g. C accumulation in woody plant biomass), their long-term effects on soil C pools are less clear. In this study we performed soil physical fractionation analyses to quantify the C pool of different aggregate fractions across three land use types including (1) silvopastoral system with ash trees (Fraxinus excelsior L.), (2) planted woodland with ash trees, and (3) permanent grassland, which were established in 1989 at Loughgall, Northern Ireland, UK. Our results show that 26 years after the conversion of permanent grassland to either silvopastoral or woodland systems, soil C (and N) stocks (0–20 cm depth) did not significantly change between the three land use types. We found, however, that permanent grassland soils were associated with significantly higher C pools (g C kg−1 soil; P < 0.03) of the large macro-aggregate fraction (> 2 mm) whereas soil C pools of the micro-aggregate (53–250 μm) and silt and clay (< 53 μm) fractions were significantly higher in the silvopastoral and woodland systems (P < 0.05). A key finding of this study is that while tree planting on permanent grassland may not contribute to greater soil C stocks it may, in the long-term, increase the C pool of more stable (recalcitrant) soil micro-aggregate and silt and clay fractions, which could be more resilient to environmental change.

Calcium carbonate (CaCO3) sediment dissolution under elevated concentrations of carbon dioxide (CO2) and nitrate (NO3−)

Ocean acidification (OA), attributed to the sequestration of atmospheric carbon dioxide (CO2) into the surface ocean, and coastal eutrophication, attributed in part to land-use change and terrestrial runoff of fertilizers, have received recent attention in an experimental framework examining the effects of each on coral reef net ecosystem calcification (Gnet). However, OA and eutrophication in conjunction have yet to receive attention from the perspective of coral reef sediment dissolution. To address this omission, CO2 and nitrate (NO3−) addition experiments were performed in Mo'orea, French Polynesia. Incubation chambers were used to measure sediment Gnet during the day and night under three different [NO3−] (0, 9.8, and 19.7μM) that were nested within four separate constructed coral reef communities maintained at different PCO2 levels (417, 721, 1030, and 1333μatm, respectively). PCO2 negatively affected sediment Gnet during the day and night, resulting in a shift to diel net dissolution at a PCO2 of 1030μatm. Elevated NO3− alone, and the combination of NO3− and PCO2, both negatively affected sediment Gnet at night. However, the response of Gnet to NO3− was less clear during the day, where diurnal sediment Gnet was enhanced under the combined treatment of elevated NO3− and PCO2, resulting in no net effect of NO3− on sediment Gnet on diel timescales. Overall, these results show that ocean acidification represents a greater threat to the balance of calcification and dissolution in Mo'orea's back reef sediment communities than the potential impact of NO3− enrichment on relatively short timescales.

Characterization and predictive reaction model for cement-sand-kaolin composite for CO2 sequestration

The sustainability and sturdiness of cement based composite is increasingly coming to the forefront of infrastructural design, maintenance and as a green material that is environmentally benign. This paper seeks to develop a predictive mathematical global reaction rate model for atmospheric carbon dioxide sequestration in sandcrete composite of cement, mineral kaolin as supplementary aggregate material (SAM), sand and water. Standard experimental techniques and procedures were used. MatCad Professional (R14) software was used to simulate the developed model equation. Results obtained indicate that the overall global reaction rates for CO2 sequestration was higher in both experimental and simulated for the developed sandcrete-kaolin (S-K) composite (9.064×10−6 and 9.532×10−6mol/kgs) at 30°C compared to the reference kaolin-free sandcrete (S-S) composite (4.058×10−7 and 4.056×10−7mol/kgs) at an early stage of setting (9h). The quantity of carbon dioxide sequestered after 9h of carbonation ranges between 0.0185–0.0214kg/m3 for sandcrete-kaolin composite, while for the reference specimen it ranges from 0.0128 to 0.011kg/m3 at 30–45°C. The simulated results based on the developed overall global reaction rates compared strongly with the experimental in most of the parameters investigated. The mineralogical compositions and microstructure changes of the carbonated samples characterized using X-ray Diffractometer (XRD), X-ray Fluorescence (XRF), Scanning Electron Microscopy (SEM) and Diffraction Spectrometer (EDS) showed that addition of mineral kaolin to sandcrete enhanced the rate of CO2 sequestration.

The role of poorly crystalline iron oxides in the stability of soil aggregate-associated organic carbon in a rice–wheat cropping system

The stability of the soil organic carbon (SOC) in a rice–wheat rotation paddy soil has been of interest because of the global emission of greenhouse gases from the soil and the sequestration of atmospheric carbon dioxide in the soil, as well as in terms of soil fertility in subtropical Asia. The purpose of this study was to explore the relationship between poorly crystalline iron oxides (Feo) and SOC in bulk soil and soil aggregates. Soil samples were collected after a wheat harvest in June 2014 and after a rice harvest in October 2014 and were separated into large macroaggregates, small macroaggregates, microaggregates, and silt and clay by wet-sieving. The long-term (5-year) application of pig manure compost plus chemical fertilizer (NPKM) increased the SOC content more efficiently than other treatments. The specific carbon mineralization rate (SCMR, rate per unit SOC) increased in the following order: microaggregate<macroaggregate<silt and clay, suggesting that SOC in the microaggregates is more stable than in the silt and clay fraction. The Feo concentration was significantly positively correlated with the SOC content in the bulk soil (P<0.001) and the soil aggregates (P<0.001), but negatively correlated with the SCMR (P<0.001). Compared with chemical fertilization alone (NPK), NPKM not only significantly improved soil aggregation but also efficiently activated the iron oxides as indicated by an increase in the aggregate mean-weight diameter (MWD) and the Feo content. Therefore, we suggested that the seasonal variation of wetting and drying drives the redox transformation of iron oxides and the mobility of Feo, and then affects the distribution of Feo in soil aggregates, which may endow paddy soil with a physico-chemical protection of SOC while application of organic fertilizers enhances this protection.

Detailed life cycle assessment of Bounty® paper towel operations in the United States

Life Cycle Assessment (LCA) is a well-established and informative method of understanding the environmental impacts of consumer products across the entire value chain. However, companies committed to sustainability are interested in more methods that examine their products and activities' impacts. Methods that build on LCA strengths and illuminate other connected but less understood facets, related to social and economic impacts, would provide greater value to decision-makers. This study is a LCA that calculates the potential impacts associated with Bounty® paper towels from two facilities with different production lines, an older one (Albany, Georgia) representing established technology and the other (Box Elder, Utah), a newer state-of-the-art platform. This is unique in that it includes use of Industrial Process Systems Assessment (IPSA), new electricity and pulp data, modeled in open source software, and is the basis for the development of new integrated sustainability metrics (published separately). The new metrics can guide supply chain and manufacturing enhancements, and product design related to environmental protection and resource sustainability. Results of the LCA indicate Box Elder had improvements on environmental impact scores related to air emission indicators, except for particulate matter. Albany had lower water use impacts. After normalization of the results, fossil fuel depletion is the most critical environmental indicator. Pulp production, electricity, and fuels for product production drive fossil fuel depletion. Climate change, land occupation, and particulate matter are also relevant. Greenhouse gas (GHG) emissions by pulp, electricity, papermaking, and landfill methane from the disposed product, drive climate change impacts. Pulp provides significant offsets to balance climate change impacts due to sequestration of atmospheric carbon dioxide. Ninety-nine percent of land occupation is for the growth of the trees for pulp production. Papermaking, electricity, and pulp production cause the most potential particular matter formation.

Coupled ocean‐atmosphere loss of marine refractory dissolved organic carbon

AbstractThe oceans hold a massive quantity of organic carbon, nearly all of which is dissolved and more than 95% is refractory, cycling through the oceans several times before complete removal. The vast reservoir of refractory dissolved organic carbon (RDOC) is a critical component of the global carbon cycle that is relevant to our understanding of fundamental marine biogeochemical processes and the role of the oceans in climate change with respect to long‐term storage and sequestration of atmospheric carbon dioxide. Here we show that RDOC includes surface‐active organic matter that can be incorporated into primary marine aerosol produced by bursting bubbles at the sea surface. We propose that this process will deliver RDOC from the sea surface to the atmosphere wherein its photochemical oxidation corresponds to a potentially important and hitherto unknown removal mechanism for marine RDOC.

Vertical decoupling of nitrate assimilation and nitrification in the Sargasso Sea

The fraction of phytoplankton growth that leads to the rain of organic carbon out of the sunlit surface ocean (“export production”) is central to the ocean׳s sequestration of atmospheric carbon dioxide. Nitrate assimilation has long been taken as a measure of export production; however, this has recently been questioned by suggestions that much of the nitrate in the sunlit layer (the “euphotic zone”) originates from biological nitrogen (N) already in surface waters. This view has been supported by recent observations of euphotic zone nitrate elevated in δ18O relative to its δ15N, taken as indicative of nitrification (the oxidation of recycled ammonium to nitrite and then nitrate). To evaluate the potential importance of nitrification in the Sargasso Sea euphotic zone, we measured the δ15N and δ18O of seawater nitrate for samples with ≥0.2µM nitrate collected on 18 cruises in the Sargasso Sea, and here we present the first large data set to correct for the low but often measurable concentrations of nitrite. Regardless of season, nitrate (i.e., nitrate-only rather than nitrate+nitrite as is commonly reported) δ15N and δ18O increase in unison from below the base of the euphotic zone toward the surface. This pattern derives from nitrate assimilation by phytoplankton, implying that nitrification is much slower than the upward transport of nitrate into the lower Sargasso Sea euphotic zone. In the twilight zone below the euphotic zone, we observe a rise in nitrate δ18O (relative to deeper waters) that is not accompanied by a rise in δ15N, suggesting nitrification co-occurring with nitrate assimilation. Nitrification, therefore, does not appear to occur in euphotic zone waters overlapping with nitrate assimilation only in the ~150m-thick twilight zone layer below it. In net, the data argue for a simpler N cycle for the Sargasso Sea euphotic zone than has recently been suggested, with the rate of euphotic zone nitrate assimilation approximating that of organic carbon export to the deep ocean.

Threshold dynamics in soil carbon storage for bioenergy crops.

Because of increasing demands for bioenergy, a considerable amount of land in the midwestern United States could be devoted to the cultivation of second-generation bioenergy crops, such as switchgrass and miscanthus. The foliar carbon/nitrogen ratio (C/N) in these bioenergy crops at harvest is significantly higher than the ratios in replaced crops, such as corn or soybean. We show that there is a critical soil organic matter C/N ratio, where microbial biomass can be impaired as microorganisms become dependent upon net immobilization. The simulation results show that there is a threshold effect in the amount of aboveground litter input in the soil after harvest that will reach a critical organic matter C/N ratio in the soil, triggering a reduction of the soil microbial population, with significant consequences in other microbe-related processes, such as decomposition and mineralization. These thresholds are approximately 25 and 15% of aboveground biomass for switchgrass and miscanthus, respectively. These results suggest that values above these thresholds could result in a significant reduction of decomposition and mineralization, which, in turn, would enhance the sequestration of atmospheric carbon dioxide in the topsoil and reduce inorganic nitrogen losses when compared to a corn-corn-soybean rotation.

Preservation of terrestrial organic carbon in marine sediments offshore Taiwan: mountain building and atmospheric carbon dioxide sequestration

Abstract. Geological sequestration of atmospheric carbon dioxide (CO2) can be achieved by the erosion of organic carbon (OC) from the terrestrial biosphere and its burial in long-lived marine sediments. Rivers on mountain islands of Oceania in the western Pacific have very high rates of OC export to the ocean, yet its preservation offshore remains poorly constrained. Here we use the OC content (Corg, %), radiocarbon (Δ 14Corg) and stable isotope (δ13Corg) composition of sediments offshore Taiwan to assess the fate of terrestrial OC, using surface, sub-surface and Holocene sediments. We account for rock-derived OC to assess the preservation of OC eroded from the terrestrial biosphere and the associated CO2 sink during flood discharges (hyperpycnal river plumes) and when river inputs are dispersed more widely (hypopycnal). The Corg, Δ14Corg and δ 13Corg of marine sediment traps and cores indicate that during flood discharges, terrestrial OC can be transferred efficiently down submarine canyons to the deep ocean and accumulates offshore with little evidence for terrestrial OC loss. In marine sediments fed by dispersive river inputs, the Corg, Δ14Corg and δ 13Corg are consistent with mixing of terrestrial OC with marine OC and suggest that efficient preservation of terrestrial OC (&gt;70%) is also associated with hypopycnal delivery. Sub-surface and Holocene sediments indicate that this preservation is long-lived on millennial timescales. Re-burial of rock-derived OC is pervasive. Our findings from Taiwan suggest that erosion and offshore burial of OC from the terrestrial biosphere may sequester &gt;8 TgC yr−1 across Oceania, a significant geological CO2 sink which requires better constraint. We postulate that mountain islands of Oceania provide a strong link between tectonic uplift and the carbon cycle, one moderated by the climatic variability which controls terrestrial OC delivery to the ocean.

Sequestration of atmospheric Carbon dioxide by microbial carbonic anhydrase

The increase in atmospheric concentrations of carbon dioxide contributes to global warming to a large extent. In this context biological sequestration of carbon dioxide hold promise as a means for addressing this problem. In the present study atmospheric carbon dioxide sequestration is achieved with the help of carbonic anhydrase produced by Bacillus schlegelii. A maximum activity of 0.0453 µ mol/ml/min was observed at pH 9 and 37 0 C. The enzyme was active up to 70 0 C.

Capturing atmospheric carbon: biological and nonbiological methods

Atmospheric carbon dioxide is one of the primary greenhouse gases on earth and its continuous emission by manmade activities is leading to a rise in atmospheric temperature. On the other hand, various natural phenomena exist that contribute to the sequestration of atmospheric carbon dioxide, i.e. its capture and long-term storage. These phenomena include oceanic, geological and chemical processes happening on earth. In addition to the above-mentioned nonbiological methods, various biological methods viz. soil carbon sequestration and phytosequestration have also been contributing to fixation of atmospheric carbon. Phytosequestration is mainly performed by several photosynthetic mechanisms such as C3, C4 and crassulacean acid metabolism (CAM) pathways of plants, carboxysomes of cyanobacteria and pyrenoids of microalgae. For an effective mitigation of global climate change, it is required to stabilize the CO2 concentration to viable levels. It requires various permutations and combinations of naturally existing and engineering strategies. Although numerous strategies are in commodious use in the present times, the issues of sustainability and long-term stability still exist. We present an overview of the natural and manmade biological and nonbiological processes used today to reduce atmospheric CO2 levels and discuss the scope and limitations of each of them.

A wide view of no-tillage practices and soil organic carbon sequestration

Many believe that conservation tillage practices could increase the sequestration of atmospheric carbon dioxide into agricultural soils and this sequestered carbon may partially offset the greenhouse gas effect and thus reduce the impact of global warming. Recent advances in soil carbon (C) and greenhouse gas analysis have made it possible to evaluate the impacts of conservation tillage on C sequestration from various perspectives. Although conservation tillage favors soil and water conservation, there are biased estimates of C sequestration associated with conservation tillage, and it is particularly an issue for a “pure” no-tillage (NT) system. Accordingly, this paper presents an overview of the progress achieved in evaluating C sequestration in no-till (the extreme type of conservation tillage) and conventional tillage production systems. In addition to extended discussion of how soil sampling and calculations could influence the estimates of C gains or losses in no-till versus conventional tilled soil, this review will also focus on following aspects, including (1) the impact of NT on crop yields which governs organic C inputs to soil from crop residue, (2) the impact of NT on soil organic C mineralization which is a major pathway of soil C output, and (3) the roles of the initial levels of C stocks and soil erosion rates which are crucial for estimating soil C sequestration under different tillage systems. Many soil C studies have indicated that the impacts of NT on soil C sequestration are compounded by many factors and should not be generalized.

Iron speciation in aerosol dust influences iron bioavailability over glacial‐interglacial timescales

Iron deposition influences primary production and oceanic sequestration of atmospheric carbon dioxide (CO2). Iron has two oxidation states, Fe(II) and Fe(III), with Fe(II) being more soluble and available for oceanic phytoplankton uptake. The past proportions of soluble iron in aerosol dust remain unknown. Here we present iron speciation (Fe2+ and Fe3+) in the Antarctic Talos Dome ice core over millennial time scales. We demonstrate that iron speciation over the last 55 kyr is linked to increasing quantities of fine dust (FD) (0.7–5 µm) and intensified long‐range dust transport. We propose that Fe(II) and Fe2+ production is principally enhanced in FD by photoreduction, although pH and organic complexation may also contribute to the speciation dynamics. During the Last Glacial Maximum, Fe2+ concentrations in dust increased by up to seven times more than interglacial levels, while Fe3+ only doubled. Cold and dusty climatic periods may increase the percentage of biologically available Fe(II) and Fe2+ deposited in the nutrient‐limited Southern Ocean, allowing greater phytoplankton uptake and perhaps increased CO2 drawdown.

Carbon storage capacity of monoculture and mixed-species plantations in subtropical China

Mixed-tree plantations can be a good silvicultural alternative to large-scale monoculture coniferous plantations for climate change mitigation, which is facilitated by the sequestration of atmospheric carbon dioxide. To evaluate the impacts of tree species compositions on the carbon storage capacity of plantation ecosystems, we measured the above and belowground biomass, as well as the carbon content, in three 27-year old forest plantations of monoculture Castanopsis hystrix (CH stand), monoculture Pinus massoniana (PM stand), and mixed C. hystrix and P. massoniana (mixed CH/PM stand) stands. We developed an allometric equation to estimate tree carbon storage. The carbon storage levels of understory, litter, and soil components were also estimated. Results show that biomass is positively correlated with carbon storage. The ecosystem carbon storage of the mixed CH/PM stand (327.03Mgha−1) was higher than those of the CH (314.59Mgha−1) and PM (293.60Mgha−1) stands. The majority of carbon storage was found in the soil pool (84.73%, 63.54%, and 75.80% in the CH, PM and mixed CH/PM stands, respectively). Almost 40% of soil carbon at a depth of 0–60cm was stored in the upper 20cm of the soil pool. Except for the vegetation layer, each layer of the CH and mixed CH/PM stands consisted of a higher amount of carbon than did the layers of the PM stand. These findings suggest that mixed CH/PM plantation stands or valuable indigenous CH plantation stands more substantially improve carbon storage in litter, soil, and ecosystems than do monoculture PM plantation stands. The results also imply that developing valuable indigenous tree species is a good silvicultural option for enhancing carbon sequestration and valuable timber cultivation in subtropical China.