Biospheric Carbon Research Articles

Background conditions for an urban greenhouse gas network in the Washington, D.C. and Baltimore metropolitan region.

As city governments take steps towards establishing emissions reduction targets, the atmospheric research community is increasingly able to assist in tracking emissions reductions. Researchers have established systems for observing atmospheric greenhouse gases in urban areas with the aim of attributing greenhouse gas concentration enhancements (and thus, emissions) to the region in question. However, to attribute enhancements to a particular region, one must isolate the component of the observed concentration attributable to fluxes inside the region by removing the background, which is the component due to fluxes outside. In this study, we demonstrate methods to construct several versions of a background for our carbon dioxide and methane observing network in the Washington, DC and Baltimore, MD metropolitan region. Some of these versions rely on transport and flux models, while others are based on observations upwind of the domain. First, we evaluate the backgrounds in a synthetic data framework, then we evaluate against real observations from our urban network. We find that backgrounds based on upwind observations capture the variability better than model-based backgrounds, although care must be taken to avoid bias from biospheric carbon dioxide fluxes near background stations in summer. Model-based backgrounds also perform well when upwind fluxes can be modeled accurately. Our study evaluates different background methods and provides guidance determining background methodology that can impact the design of urban monitoring networks.

The influence of atmospheric intraseasonal oscillations on terrestrial biospheric CO2 fluxes in Southeast China Forest

With its strong carbon sequestration capacity and significantly expanding trend, forest over southeast China (SEC) is an important biospheric carbon sink. As intraseasonal oscillation (ISO) is a remarkable variability of the East Asian summer monsoon—the major climate system influencing the carbon exchange of SEC forest, the objective of this study is to examine the impact of atmospheric intraseasonal oscillation on CO2 fluxes in SEC forest based on daily CO2 fluxes and meteorological data. Significant 15‒60-day ISO of CO2 fluxes is identified in SEC forest during May‒August of 1980‒2013. The 15‒60-day ISO of net ecosystem exchange between atmosphere and biosphere mainly results from that of gross primary productivity (GPP), with the explained variances greater than 50% over most SEC forest areas. Due to the high drought-resistance of SEC forest, the 15‒60-day ISO of its GPP is controlled primarily by local solar radiation, which is in turn associated with the northward-propagating atmospheric ISO. Suppressed (active) convection anomaly and the thermally induced anomalous anticyclone (cyclone) on its northwestern side initiate from the southern South China Sea and then propagate northward to SEC coherently in the form of a Rossby wave-like pattern. Decreased (increased) cloud cover associated with the suppressed (active) convection anomaly leads to more (less) solar radiation reaching the SEC forest canopy, thus promoting (impairing) forest productivity.

Carbon Monitoring System Flux Net Biosphere Exchange 2020 (CMS-Flux NBE 2020)

Abstract. Here we present a global and regionally resolved terrestrial net biosphere exchange (NBE) dataset with corresponding uncertainties between 2010–2018: Carbon Monitoring System Flux Net Biosphere Exchange 2020 (CMS-Flux NBE 2020). It is estimated using the NASA Carbon Monitoring System Flux (CMS-Flux) top-down flux inversion system that assimilates column CO2 observations from the Greenhouse Gases Observing Satellite (GOSAT) and NASA's Observing Carbon Observatory 2 (OCO-2). The regional monthly fluxes are readily accessible as tabular files, and the gridded fluxes are available in NetCDF format. The fluxes and their uncertainties are evaluated by extensively comparing the posterior CO2 mole fractions with CO2 observations from aircraft and the NOAA marine boundary layer reference sites. We describe the characteristics of the dataset as the global total, regional climatological mean, and regional annual fluxes and seasonal cycles. We find that the global total fluxes of the dataset agree with atmospheric CO2 growth observed by the surface-observation network within uncertainty. Averaged between 2010 and 2018, the tropical regions range from close to neutral in tropical South America to a net source in Africa; these contrast with the extra-tropics, which are a net sink of 2.5±0.3 Gt C/year. The regional satellite-constrained NBE estimates provide a unique perspective for understanding the terrestrial biosphere carbon dynamics and monitoring changes in regional contributions to the changes of atmospheric CO2 growth rate. The gridded and regional aggregated dataset can be accessed at https://doi.org/10.25966/4v02-c391 (Liu et al., 2020).

Fire reduces riverine DOC concentration draining a watershed and alters post-fire DOC recovery patterns

The loading of dissolved organic carbon (DOC) from soils to inland waters and ultimate transport to the ocean is a critical flux pathway in the terrestrial biosphere carbon cycle. Fires can significantly affect this flux through biogeochemical processes related to oxidation and mobilization of DOC in the soil. Therefore, in order to accurately estimate and model terrestrial carbon storage and export to the marine environment, we need to better understand the effects of fire on DOC flux. In this analysis, we compiled available observational data sets from seven watersheds across the conterminous United States generally spanning the years 1999–2019. We used these data sets to examine the effects of fire on riverine DOC concentration draining a watershed to study both the immediate impacts and the post-fire recovery patterns. Our results suggest that these fires result in an immediate decrease in riverine DOC concentration draining the watershed by 26 ± 15%, and the time required for DOC concentrations to recover to pre-fire levels was estimated to be, on average, approximately 9 months. During recovery, DOC concentration was 24 ± 11% lower than the long-term average for the watershed. In addition, the larger the proportion of the watershed that burned, the greater the concentration decrease and the longer the time period for post-fire recovery.

Designing and Constructing Artificial Small RNAs for Gene Regulation and Carbon Flux Redirection in Photosynthetic Cyanobacteria.

Photosynthetic cyanobacteria are not only model organisms for studying photosynthesis and biological cycling of carbon in biosphere but also potential "green microbial factories" to produce renewable fuels and chemicals, due to their capability to utilizing solar energy and CO2. Therefore, strategies for gene regulation and carbon flux redirection are important for both fundamental research and metabolic engineering of cyanobacteria. To address the challenges, regulatory tools based on artificial small RNAs have been developed with satisfactory effects for single or multiple gene(s) regulation in various cyanobacterial species. When combined with the promoters of varying gradient strength and the inducible switches developed in recent years, it is now feasible to realize precise gene regulation in photosynthetic cyanobacteria for producing fuels and chemicals. Here in this chapter, we provide a detailed introduction of the design principles and constructing methods of the artificial sRNA tools to achieve accurate inducible regulation of cyanobacterial gene(s).

Evaluating two soil carbon models within the global land surface model JSBACH using surface and spaceborne observations of atmospheric CO<sub>2</sub>

Abstract. The trajectories of soil carbon in our changing climate are of the utmost importance as soil is a substantial carbon reservoir with a large potential to impact the atmospheric carbon dioxide (CO2) burden. Atmospheric CO2 observations integrate all processes affecting carbon exchange between the surface and the atmosphere and therefore are suitable for carbon cycle model evaluation. In this study, we present a framework for how to use atmospheric CO2 observations to evaluate two distinct soil carbon models (CBALANCE, CBA, and Yasso, YAS) that are implemented in a global land surface model (JSBACH). We transported the biospheric carbon fluxes obtained by JSBACH using the atmospheric transport model TM5 to obtain atmospheric CO2. We then compared these results with surface observations from Global Atmosphere Watch stations, as well as with column XCO2 retrievals from GOSAT (Greenhouse Gases Observing Satellite). The seasonal cycles of atmospheric CO2 estimated by the two different soil models differed. The estimates from the CBALANCE soil model were more in line with the surface observations at low latitudes (0–45∘ N) with only a 1 % bias in the seasonal cycle amplitude, whereas Yasso underestimated the seasonal cycle amplitude in this region by 32 %. Yasso, on the other hand, gave more realistic seasonal cycle amplitudes of CO2 at northern boreal sites (north of 45∘ N) with an underestimation of 15 % compared to a 30 % overestimation by CBALANCE. Generally, the estimates from CBALANCE were more successful in capturing the seasonal patterns and seasonal cycle amplitudes of atmospheric CO2 even though it overestimated soil carbon stocks by 225 % (compared to an underestimation of 36 % by Yasso), and its estimations of the global distribution of soil carbon stocks were unrealistic. The reasons for these differences in the results are related to the different environmental drivers and their functional dependencies on the two soil carbon models. In the tropics, heterotrophic respiration in the Yasso model increased earlier in the season since it is driven by precipitation instead of soil moisture, as in CBALANCE. In temperate and boreal regions, the role of temperature is more dominant. There, heterotrophic respiration from the Yasso model had a larger seasonal amplitude, which is driven by air temperature, compared to CBALANCE, which is driven by soil temperature. The results underline the importance of using sub-annual data in the development of soil carbon models when they are used at shorter than annual timescales.

A review on lignin utilization in petroleum exploration, petroleum products formulation, bio-fuel production, and oil spill clean-up

After cellulose, lignin is considered to be the most abundant, renewable, economical natural biopolymer existing on earth. Out of the natural biosynthesized carbon in biosphere, lignin accounts for 30%. Lignin is a complex aromatic polymer and a vital cell wall structural component. This highly complex phenolic macromolecule is obtained as a low-value by-product of cellulose production and by the paper pulp industry. It is an underutilized material, so the use of lignocellulosic feedstock is increasing as a renewable and sustainable alternative to petroleum resource. Several efforts have also been made to use lignin in the petroleum field. Here, we summarize the current knowledge regarding lignin utilization for the petroleum exploration, petroleum products formulation, bio-fuel production, and oil spill clean-up. This review covers the direct use of lignin (as such) and also the chemical modifications of lignin. The future perspective and trend towards lignin-based material utility for the petroleum industry are then addressed.

Amplification of the Ocean Carbon Sink During El Niños: Role of Poleward Ekman Transport and Influence on Atmospheric CO2

AbstractAmplification of the ocean carbon sink during El Niño events partially offset terrestrial biosphere carbon losses to the atmosphere, but uncertainties in the magnitude, timing, and spatial extent of the ocean response confound our understanding of the global carbon budget and its sensitivity to climate. Here, we examine the mechanisms controlling the anomalous tropical Pacific Ocean CO2 drawdown during El Niño events harnessing multidecadal ocean pCO2 observations in conjunction with a state‐of‐the‐art ocean biogeochemical model. We show that poleward Ekman transport dramatically amplifies the near‐equatorial pCO2 anomaly identified in prior work and that this amplification varies considerably between El Niño events. During central Pacific events, the CO2 flux anomaly can vary up to fourfold between events (2002/2003 vs. 2015/2016), but it always recedes during the boreal winter to spring transition and the poleward transport of the anomaly mostly extends into the Northern Hemisphere. During eastern Pacific events characterized by an equatorially centered intertropical convergence zone (e.g., 1997/1998), the intense CO2 flux anomaly persists into boreal summer and can extend further into the Southern Hemisphere. The hemispheric asymmetry (northward vs. southward extension) and the termination (boreal winter/spring for central Pacific events vs. boreal summer for eastern Pacific events) of the ocean CO2 response are tied to the El Niño dynamics in the near‐equatorial region and also how it is amplified by poleward Ekman transport. Finally, we evaluate how differences in the ocean response influence atmospheric CO2 and discuss the potential of atmospheric CO2 satellite data to provide observational constraints.

Effects of human activities on soil organic carbon redistribution at an agricultural watershed scale on the Chinese Loess Plateau

Human activity is thought to perturb the dynamic change of organic carbon (OC), but its impact on the transportation and redistribution of erosion-induced OC is still poorly understood. Here, we use multi-source field sampling data and long-term hydrological observation data to clarify the transportation and redistribution of OC in an agricultural watershed on the Loess Plateau. The endmember mixing model based on the radiocarbon isotopes (14C) and a budget equation were used to assess the composition and budget of erosion-induced OC in a 187 km2 watershed. The results showed that compared with the period of agricultural activities (1960−1969), soil conservation activities in 1970−1999 and 2000−2019 reduced soil erosion by 31.5 % and 75.4 %, respectively. Additionally, land use changes significantly reduced the mobilized OC, from 14,370 ± 1966 (1960−1969) to 3311 ± 431 Mg C yr−1 (2000−2019). Check dam construction led to the mobilized OC was effectively buried at a rate of 2125 ± 478 and 1420 ± 282 Mg C yr-1 in periods of 1970−1999 and 2000−2019, respectively. Further radiocarbon isotopes analysis shows that the land use changes and check dam construction effectively reduced the output of ancient petrogenic organic carbon (OCpetro) and young biospheric organic carbon (OCbio) from the watershed, which has a positive impact on the regulation of atmospheric CO2 level. Our results emphasize the positive effects of soil conservation activities on terrestrial carbon sequestration, which are of great significance to clarify the effects of soil erosion on the carbon cycle.

Efficient preservation of young terrestrial organic carbon in sandy turbidity-current deposits

Abstract Burial of terrestrial biospheric particulate organic carbon in marine sediments removes CO2 from the atmosphere, regulating climate over geologic time scales. Rivers deliver terrestrial organic carbon to the sea, while turbidity currents transport river sediment further offshore. Previous studies have suggested that most organic carbon resides in muddy marine sediment. However, turbidity currents can carry a significant component of coarser sediment, which is commonly assumed to be organic carbon poor. Here, using data from a Canadian fjord, we show that young woody debris can be rapidly buried in sandy layers of turbidity current deposits (turbidites). These layers have organic carbon contents 10× higher than the overlying mud layer, and overall, woody debris makes up >70% of the organic carbon preserved in the deposits. Burial of woody debris in sands overlain by mud caps reduces their exposure to oxygen, increasing organic carbon burial efficiency. Sandy turbidity current channels are common in fjords and the deep sea; hence we suggest that previous global organic carbon burial budgets may have been underestimated.

Millennial-scale hydroclimate control of tropical soil carbon storage.

The storage of organic carbon in the terrestrial biosphere directly affects atmospheric concentrations of carbon dioxide over a wide range of timescales. Within the terrestrial biosphere, the magnitude of carbon storage can vary in response to environmental perturbations such as changing temperature or hydroclimate1, potentially generating feedback on the atmospheric inventory of carbon dioxide. Although temperature controls the storage of soil organic carbon at mid and high latitudes2,3, hydroclimate may be the dominant driver of soil carbon persistence in the tropics4,5; however, the sensitivity of tropical soil carbon turnover to large-scale hydroclimate variability remains poorly understood. Here we show that changes in Indian Summer Monsoon rainfall have controlled the residence time of soil carbon in the Ganges-Brahmaputra basin over the past 18,000 years. Comparison of radiocarbon ages of bulk organic carbon and terrestrial higher-plant biomarkers with co-located palaeohydrological records6 reveals a negative relationship between monsoon rainfall and soil organic carbon stocks on a millennial timescale. Across the deglaciation period, a depletion of basin-wide soil carbon stocks was triggered by increasing rainfall and associated enhanced soil respiration rates. Our results suggest that future hydroclimate changes in tropical regions are likely to accelerate soil carbon destabilization, further increasing atmospheric carbon dioxide concentrations.

Soil Organic Carbon Redistribution and Delivery by Soil Erosion in a Small Catchment of the Yellow River Basin

AbstractSoil erosion modulates the atmospheric CO2 level by affecting the redistribution of young biospheric organic carbon (OCbio) and ancient petrogenic organic carbon (OCpetro) in different order streams. However, the fate of soil organic carbon (SOC) in low‐order stream systems is still uncertain due to the complex influences of terrain, land uses, and anthropogenic disturbances. Here, we used the geochemical properties fingerprinting method and a radiocarbon‐based two‐member mixing model to clarify the source, budgets, and composition of SOC in a low‐order stream catchment in the Yellow River basin. The results showed that the primary source of SOC in the catchment was agricultural activity area, which accounted for 68.5%. After vegetation restoration, the SOC delivery rate and loss rate decreased from 0.38 ± 0.06 to 0.26 ± 0.04 and 0.19 ± 0.06 to 0.14 ± 0.04 t·ha−1·yr−1, respectively. The SOC mobilized by soil erosion in the source area was 1,085.8 ± 170.5 t from 1969 to 2015, and the SOC lost during the transport process was 552.9 ± 170.1 t. The lost carbon was dominated by OCbio, whereas the loss of OCpetro was minimal. Soil erosion can mobilize the OCpetro sequestered in terrestrial ecosystems and deliver it to a river system if it is not intercepted. The oxidation of OCpetro during long‐distance fluvial transport may represent a significant carbon source for atmospheric CO2 on a geological timescale. This study contributes to our understanding of the carbon sink/source issue and quantifies the SOC budgets in stream systems.

Using SMOS soil moisture data combining CO2 flask samples to constrain carbon fluxes during 2010–2015 within a Carbon Cycle Data Assimilation System (CCDAS)

The terrestrial carbon cycle is an important component of the global carbon budget due to its large gross exchange fluxes with the atmosphere and their sensitivity to climate change. Terrestrial biosphere models show large uncertainties in simulating carbon fluxes, which impact global carbon budget assessments. The land surface carbon cycle is tightly controlled by soil moisture through plant physiological processes. Accurate soil moisture observations thereby have the potential to improve the modeling of carbon fluxes in a model-data fusion framework. We employ the Carbon Cycle Data Assimilation System (CCDAS) to assimilate six years of surface soil moisture provided by the SMOS satellite in combination with global-scale observations of atmospheric CO2 concentrations. We find that assimilation of SMOS soil moisture exhibits better performance on soil hydrology modeling at both global and site-level than only assimilating atmospheric CO2 concentrations, and it improves the soil moisture simulation particularly in mid- to high-latitude regions where the plants suffer from water stress frequently. The optimized model also shows good agreements with inter-annual variability in simulated Net Primary Productivity (NEP) and Gross Primary Productivity (GPP) from an atmospheric inversion (Jena CarboScope) and the up-scaled eddy covariance flux product (FLUXNET-MTE), respectively. Correlation between SIF (Solar Induced Fluorescence) and optimized GPP also shows to be the highest when soil moisture and atmospheric CO2 are simultaneously assimilated. In general, CCDAS obtains smaller annual mean NEP values (1.8 PgC/yr) than the atmospheric inversion and an ensemble of Dynamic Global Vegetation Models (DGVMs), but larger GPP values (167.8 PgC/yr) than the up-scaled eddy covariance dataset (FLUXNET-MTE) and the MODIS based GPP product for the years 2010 to 2015. This study demonstrates the high potential of constraining simulations of the terrestrial biosphere carbon cycle on inter-annual time scales using long-term microwave observations of soil moisture.

Increasing soil carbon stocks in eight permanent forest plots in China

Abstract. Forest soils represent a major stock of organic carbon (C) in the terrestrial biosphere, but the dynamics of soil organic C (SOC) stock are poorly quantified, largely due to lack of direct field measurements. In this study, we investigated the 20-year changes in SOC stocks in eight permanent forest plots, which represent boreal (1998–2014), temperate (1992–2012), subtropical (1987–2008), and tropical forest biomes (1992–2012) across China. SOC contents increased significantly from the 1990s to the 2010s, mostly in the upper 0–20 cm soil depth, and soil bulk densities do not change significantly during the same period. As a result, the averaged SOC stocks increased significantly from 125.2±85.2 Mg C ha−1 in the 1990s to 133.6±83.1 Mg C ha−1 in the 2010s across the forest plots, with a mean increase of 127.2–907.5 kg C ha−1 yr−1. This SOC accumulation resulted primarily from increasing leaf litter and fallen logs, which accounts 3.6 %–16.3 % of above-ground net primary production. Our findings provided direct evidence that China's forest soils have been acting as significant C sinks, although their strength varies in forests with different climates.

The impact of improved satellite retrievals on estimates of biospheric carbon balance

Abstract. The Orbiting Carbon Observatory 2 (OCO-2) is NASA's first satellite dedicated to monitoring CO2 from space and could provide novel insight into CO2 fluxes across the globe. However, one continuing challenge is the development of a robust retrieval algorithm: an estimate of atmospheric CO2 from satellite observations of near-infrared radiation. The OCO-2 retrievals have undergone multiple updates since the satellite's launch, and the retrieval algorithm is now on its ninth version. Some of these retrieval updates, particularly version 8, led to marked changes in the CO2 observations, changes of 0.5 ppm or more. In this study, we evaluate the extent to which current OCO-2 observations can constrain monthly CO2 sources and sinks from the biosphere, and we particularly focus on how this constraint has evolved with improvements to the OCO-2 retrieval algorithm. We find that improvements in the CO2 retrieval are having a potentially transformative effect on satellite-based estimates of the global biospheric carbon balance. The version 7 OCO-2 retrievals formed the basis of early inverse modeling studies using OCO-2 data; these observations are best equipped to constrain the biospheric carbon balance across only continental or hemispheric regions. By contrast, newer versions of the retrieval algorithm yield a far more detailed constraint, and we are able to constrain CO2 budgets for seven global biome-based regions, particularly during the Northern Hemisphere summer when biospheric CO2 uptake is greatest. Improvements to the OCO-2 observations have had the largest impact on glint-mode observations, and we also find the largest improvements in the terrestrial CO2 flux constraint when we include both nadir and glint data.

Survival of graphitized petrogenic organic carbon through multiple erosional cycles

Graphite forms the endpoint for organic carbon metamorphism; it is extremely resilient to physical, biological and chemical degradation. Carbonaceous materials (CM) contained within sediments, collected across Taiwan and from the Gaoping submarine canyon, were analyzed using Raman spectroscopy to determine the crystallinity. This allowed the erosional and orogenic movements of petrogenic organic carbon (OCpetro) during the Taiwanese orogeny to be deduced. After automatically fitting and classifying spectra, the distribution of four groups of CM within the sediments provides evidence that many forms of OCpetro have survived at least one previous cycle of erosion, transport and burial before forming rocks in the Western Foothills of the island. There is extensive detrital graphite present in rocks that have not experienced high-grade metamorphism, and graphite flakes are also found in recently deposited marine sediments off Taiwan. The tectonic and geological history of the island shows that these graphite flakes must have survived at least three episodes of recycling. Therefore, transformation to graphite during burial and orogeny is a mechanism for stabilizing organic carbon over geological time, removing biospheric carbon from the active carbon cycle and protecting it from oxidation during future erosion events.

Comment on carbon dioxide emissions by rock organic carbon oxidation

In a holistic study of the modern-day Mackenzie River Basin, [Horan and others (2019)][1] budget the uptake and release of atmospheric CO2 by chemical weathering of minerals and biospheric organic carbon burial and, in particular, quantify the important contribution that kerogen oxidation plays in

A new model of the coupled carbon, nitrogen, and phosphorus cycles in the terrestrial biosphere (QUINCY v1.0; revision 1996)

Abstract. The dynamics of terrestrial ecosystems are shaped by the coupled cycles of carbon, nitrogen, and phosphorus, and these cycles are strongly dependent on the availability of water and energy. These interactions shape future terrestrial biosphere responses to global change. Here, we present a new terrestrial ecosystem model, QUINCY (QUantifying Interactions between terrestrial Nutrient CYcles and the climate system), which has been designed from scratch to allow for a seamless integration of the fully coupled carbon, nitrogen, and phosphorus cycles with each other and also with processes affecting the energy and water balances in terrestrial ecosystems. This new model includes (i) a representation of plant growth which separates source (e.g. photosynthesis) and sink (growth rate of individual tissues, constrained by temperature and the availability of water and nutrients) processes; (ii) the acclimation of many ecophysiological processes to meteorological conditions and/or nutrient availability; (iii) an explicit representation of vertical soil processes to separate litter and soil organic matter dynamics; (iv) a range of new diagnostics (leaf chlorophyll content; 13C, 14C, and 15N isotope tracers) to allow for a more in-depth model evaluation. In this paper, we present the model structure and provide an assessment of its performance against a range of observations from global-scale ecosystem monitoring networks. We demonstrate that QUINCY v1.0 is capable of simulating ecosystem dynamics across a wide climate gradient, as well as across different plant functional types. We further provide an assessment of the sensitivity of key model predictions to the model's parameterisation. This work lays the ground for future studies to test individual process hypotheses using the QUINCY v1.0 framework in the light of ecosystem manipulation observations, as well as global applications to investigate the large-scale consequences of nutrient-cycle interactions for projections of terrestrial biosphere dynamics.

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.

Soil organic carbon storage varies with stand ages and soil depths following afforestation

Soil organic carbon (SOC) is the largest component of the terrestrial biosphere carbon pool. Afforestation is an effective solution to mitigate Carbon (C) emission and sequester C into soils. However, how and to which extent afforestation influences SOC stock changes are not well understood. This study conducts a quantitative review that synthesizes 544 data points from 261 sites from 90 papers, to examine the impact of afforestation on SOC changes in three soil layers (0-20 cm, 20-40 cm and 40-60 cm). 212 data points are obtained by standardization and/or extrapolationwith high reliability. The results indicate that stand age has significant effects on the SOC stock dynamics under different conditions of previous land use types, plant functional types, temperature or precipitation. The effect is greatest at the topsoil layer of 0-20 cm. Previous land use types significantly influence SOC accumulations, but these effects are not significant in the first 10 years or after 30 years of afforestation. Besides, afforestation on grassland seems to sequester more SOC than that of cropland in the long term. Plant functional types also significantly affect SOC dynamics, with deciduous hardwood reporting a continuous increase of SOC contents at soil depth of 0-60 cm during the whole afforestation period. On the other hand, the accumulation of SOC in evergreen hardwood and evergreen softwood start from the third decades. Higher SOC accumulation rates are observed under evergreen hardwood but no significant differences were found between deciduous hardwood and evergreen softwood for the longer period after afforestation (>20 years). Mean annual temperature and precipitation negatively affect SOC accumulation in the first two decades of afforestation, however, the effects become positive in the later years.We also found that initial SOC stocks did not play a major role in SOC sequestration. In other words, lower SOC soils could also sequestrate a significant amount of SOC after reforestation.