The Chronological Advancement of Soil Organic Carbon Sequestration Research: A Review

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    The Chronological Advancement of Soil Organic CarbonSequestration Research: A Review

    J. Dinakaran Mohammad Hanief

    Archana Meena K. S. Rao

    Received: 5 May 2013 / Revised: 10 December 2013 / Accepted: 29 January 2014

    The National Academy of Sciences, India 2014

    Abstract According to Kyoto protocol carbon seques-

    tration in terrestrial ecosystem is a low cost option to

    mitigate the increasing atmospheric CO2 concentrations. It

    has been understood that the worlds forests and their soils

    have a high potential to sequester the atmospheric carbon.

    For the last two decades there has been increasing interest

    among scientists in terrestrial soil carbon storage processes.

    This review made an attempt to summarize the major

    chronological advancement of soil organic carbon (SOC)

    sequestration research and also to underpin the problems

    yet to be focused on this research at global level. SOC

    sequestration research began in the early seventies, where

    most of the studies were estimating the size/stock of the

    global SOC. In the early eighties, most of the researchers

    have started to focus on the factors involved in the storage

    of organic carbon in different ecosystems. Subsequently,

    the researchers started to work on different types of SOC

    pools, their size, turnover and chemical characterization in

    different types of ecosystems. Recently the researchers

    main focus has been the temperature sensitivity of organic

    carbon in different types of soil and their mechanism of

    stabilization. There has been interest among researchers on

    the contribution of microbial derived (microbial necro-

    mass) carbon in recalcitrant pool of SOC and their

    sequestration process in different types of ecosystems.

    Arguably, the contribution in SOC sequestration research

    on the fate of sequestered carbon in different types of soil

    and their stabilization mechanisms is insufficient. India is

    the country spread over regions from temperate to dry

    desert zones with different types of fragile ecosystems and

    its vulnerable nature to the global climate change. There-

    fore, the future research must focus on SOC sensitivity to

    temperature and SOC stabilization mechanisms in different

    ecosystems for better understanding of the soil carbon


    Keywords Carbon sequestration Carbon stabilization Soil organic carbon pools Labile and recalcitrant pool


    The rising level of global surface temperature is the major

    concern for developed and developing countries around the

    world since its impact has been felt mostly on changes in

    monsoon scenario, drought, flood and sea level rise. The

    amount of CO2 present in the atmosphere plays a crucial

    role to maintain the global surface temperatures; and it has

    historically fluctuated between about 180 ppm during gla-

    cial periods and 280 ppm during interglacial periods [1, 2].

    It has increased due to the burning of fossil fuels, cement

    production, deforestation and agricultural development,

    from the pre-industrial level of about 280 ppm to

    *391 ppm now [3]. The amount of CO2 is increasing atthe rate of 0.4 % or 1.5 ppm year-1. Similarly, CH4 has

    increased from 0.80 to 1.74 ppm and increasing at the rate

    of 0.75 % or 0.015 ppm year-1 while N2O has increased

    from 288 to 311 ppb and is increasing at the rate of 0.25 %

    or 0.8 ppb year-1 [4]. Earths surface temperature has

    increased by 0.74 C since year 1850 and is expected toincrease by another 1.1 to 6.4 C by the end of this century[5]. Thus understanding the terrestrial carbon cycle is very

    important since the temperature affects almost all aspects

    of terrestrial carbon processes, it likely enhances ecosystem

    carbon flux, potentially feeding back to a buildup of

    J. Dinakaran (&) M. Hanief A. Meena K. S. RaoNatural Resource Management Laboratory, Department of

    Botany, University of Delhi, North Campus, Delhi 110007, India



    Proc. Natl. Acad. Sci., India, Sect. B Biol. Sci.

    DOI 10.1007/s40011-014-0320-0

  • atmospheric CO2 concentration and climate dynamics [6].

    The need to identify options for mitigating atmospheric

    concentrations of CO2 is widely recognized. The United

    Nations Framework Convention on Climate Change (UN-

    FCCC), adopted in 1992 in Rio de Janeiro, with the main

    aim as the stabilization of greenhouse gases in the

    atmosphere at a level that will prevent dangerous anthro-

    pogenic interference with the climate system was the first

    international treaty that set out to address the problem of

    climate change by working to prevent anthropogenic

    emissions that contribute to climate disturbances [7, 8].

    The Kyoto Protocol, ratified by several countries but not by

    the major CO2 emitters, mandates that industrialized

    nations must reduce their net emissions with reference to

    the 1990 level by an agreed amount and recognizes a wide

    range of options to minimize the risks of global warming.

    The efforts in the recent deliberations on climate change/

    global warming in Copenhagen (2010) and Durban (2011)

    among developed and developing countries became almost

    futile to adapt the noteworthy regulations to combat cli-

    mate change. India is also one of the more susceptible

    nations to the influence of climate change than its devel-

    oped counterparts. According to the Kyoto protocol (KP),

    carbon sequestration in terrestrial sinks can be used to

    offset greenhouse gas emissions [9].

    Current growth rate in the economy and commitment to

    environmental protection of India makes it necessary to

    protect the forests and other important ecosystems. How-

    ever, still, there is a lack of study on SOC sequestration in

    fragile ecosystems such as forests, croplands, wetlands and

    grass/pasturelands of India. An attempt has been made in

    this review to summarize the major chronological

    advancement of SOC sequestration research in terrestrial

    ecosystems and also to underpin the problems yet to be

    focused on this research at global and regional level. This

    review has abridged the following major SOC research

    areas viz., (i) estimation of carbon stock in different eco-

    system (ii) factors affecting the SOC storage (iii) Soil

    organic matter (SOM) models (iv) separation of discrete

    SOC pools (v) to understand the temperature sensitivity of

    SOC in different ecosystems (vi) chemical characterisation

    of SOC (vii) measurement of carbon isotopes in SOC

    dynamics and (viii) understanding the mechanisms/pro-

    cesses involved in SOC stabilization in different


    Carbon Sequestration

    Carbon sequestration implies capture and storage of

    atmospheric CO2 into stable pools in the ocean, geologic

    basalts, vegetation and soil by abiotic (engineered) and

    biotic strategies

    Abiotic Strategies

    They involve separation, capture, compression, transport,

    and injection of CO2 from industrial flue gases and efflu-

    ents deep into geological basalts and the ocean [4]. Abiotic

    strategies of sequestration of carbon are still in infancy, and

    the implementation of abiotic strategies to sequester carbon

    requires lot of money, chemicals and equipments, because

    these strategies are totally based on new engineering


    Consequences of Abiotic Carbon Sequestration

    The oceans are absorbing CO2 from the atmosphere and

    this is causing chemical changes by making them more

    acidic. In the past 200 years the oceans have absorbed

    approximately half of the CO2 produced by fossil fuel

    burning and cement production. The pH of the ocean sur-

    face waters has already fallen by about 0.1 units from about

    8.16 to 8.05 since the beginning of the industrial revolution

    around 200 years ago. If global emission of CO2 from

    human activities continues to rise on current trend then the

    average pH of the oceans could fall by 0.5 units (equivalent

    to a threefold increase in the concentration of hydrogen

    ions) by the year 2100 [10]. Ocean acidification, as the

    phenomenon is called, over time will create major negative

    impacts on corals and other marine life, with anticipated

    adverse consequences for fishing, tourism, and related

    economies [10]. However, many of the ecological, chem-

    ical, and geological elements of the deep sea are little

    understood, and therefore, the effects of injecting carbon

    dioxide into the ocean, are widely unknown. In addition, if

    the CO2 leaks out of a storage formation, hazards may exist

    for humans, ecosystems and groundwater.

    Biotic Strategies

    In contrast to oceanic and geologic carbon storage tech-

    niques, the terrestrial carbon sequestration in biotic strate-

    gies is based on the natural process of photosynthesis and

    transfer of fixed atmospheric CO2 into vegetative biomass

    and SOM pools [11, 12]. SOM is a complex mixture of

    plants, animals and microbial materials in different stages of

    decay pooled with a variety of decomposition products of

    different turnover rates. SOM is a major storehouse of car-

    bon on a global scale. The accumulation of SOM, of which

    about 58 % is carbon, occurs during ecosystem development

    as a result of interactions between biota (e.g., autotrophs and

    heterotrophs) and environmental controls (e.g., temperature,

    moisture) [13]. The rate of SOM accumulation in different

    types of natural ecosystems depends upon the litter inputs

    (quantity and quality) and the rate of decomposition.

    J. Dinakaran et al.


  • Obviously the rate and accumulation of SOM is strongly

    related to the productivity of the recent/past vegetation,

    physical and biological conditions in the soil, and the past

    history of SOM inputs and land management activities. Thus

    the SOM accumulation differs from natural ecosystems to

    man-made ecosystems. Schlesinger [14] documented the

    rate of SOC accumulation from polar desert

    (0.2 g C m2 year-1) to temperate forest (12 g C m2 year-1)

    ecosystem across the world. SOC accumulation in Indian

    forest soils is about 20.31 Tg year-1 [15]. However, for the

    last two decades there have been considerable amount of

    work based on simulation models to calculate the SOM

    accumulation rate/turnover in different types of ecosystems

    (see in detail in SOM model section). The naturally occur-

    ring organic carbon (OC) forms are derived from the

    decomposition of plants and animals. In addition to that, the

    anthropogenic additions such as pesticides and municipal

    wastes are also the part of OC in soils [12]. In soil a wide

    variety of OC forms are present, ranging from freshly

    deposited litter such as leaves, twigs, and branches to highly

    decomposed forms such as humus. Plant litter and microbial

    biomass are the major parent materials for SOM formation.

    As the plants grow, they absorb CO2 through photosynthesis

    and incorporate a portion of this carbon into their body

    structure. Thus, carbon sequestration by growing perennial

    vegetation has been shown to be a cost-effective option for

    mitigation of global climatic change.

    The biological management of carbon in tackling climate

    change has therefore essentially two components: (i) the

    reduction in emissions from biological systems and (ii) the

    increase in their storage of carbon. These can be achieved in

    three ways: existing stores could be protected and the current

    high rate of loss reduced; historically depleted stores could be

    replenished by restoring ecosystems and soil; and, potentially,

    new stores could be created by encouraging greater carbon

    storage in areas that currently have little, through afforestation

    [16]. At the same moment, carbon sequestration (by growing

    forests) can be appropriate from ecological, environmental

    and a socioeconomic point of view. The environmental per-

    spective includes the removal of CO2 from the atmosphere,

    the improvement of soil quality, and the increase in biodi-

    versity. Socioeconomic benefits can be reflected through

    increased yields [17] and monetary income from potential

    carbon trading schemes [18]. Carbon sequestration projects

    could also enhance local participation and understanding of

    sustainable forest management practices.

    Chronological Progress of SOC Research Across

    the World

    Table 1 shows the major progress and development in SOC

    research. The major hitch in SOC research across the world

    is that most of the studies have focused only on carbon

    stock estimation and/or the process involved in the storage

    of organic carbon in different ecosystems. This is due to

    some error and/or lack of understanding of the estimation

    of SOC stock in different ecosystems.

    SOC stocks

    In the early seventies to the nineties, most of the

    researchers around the world showed their interest in

    estimating the size/stock of the global SOC [19]. They

    followed two different approaches to estimate the global

    SOC stock. They are (a) soil taxonomy and (b) ecosystem-

    based estimates. In the first approach (taxonomic

    approach), they tabulated the SOC stocks based on the

    determination of the area/extent and the average carbon

    content in each of major soil taxonomic groups of the

    world [20]. Two major soil classification/mapping schemes

    were used in global carbon tabulation, i.e., US department

    of agriculture soil taxonomy (USDA) and the food and

    agricultural organizations (FAO) world soil map. Eswaran

    et al. [20] calculated the SOC stocks (1576 Pg C) of the

    worlds different soil orders. Among the different soil

    orders, SOC stock was low in vertisols and high in histo-

    sols. In ecosystem based approach, SOC stock was esti-

    mated on the basis of generalized ecological life zones

    which are distributed in relation to specific combinations of

    mean annual precipitation and mean annual temperature

    [21]. This approach estimated the global soil storage about

    to be 2542.3 Pg of carbon. A recent study [22] estimated

    the global SOC storage in the top 3 m depth of soil as

    2,344 Pg C, with the first, second and third meter mea-

    suring 1,502, 491 and 351 Pg C, respectively.

    In India, for the first time, Jenny and Raychaudhuri [23]

    studied extensively on estimating organic carbon status in

    forests and cultivated lands with respect to different rainfall

    and temperature. After a long span of time Gupta and Rao

    [24] estimated the organic carbon stock of about 24.33 Pg of

    carbon in soil ranging from surface to an average subsurface

    depth of 0.44 to 1.86 m with the database of 48 soil series.

    Subsequently Bhattacharya et al. [25] estimated the OC stock

    as 63 Pg in five major physiographic regions of India

    (northern mountains, the great plains, peninsular India, pen-

    insular plateau and coastal plains and the islands) up to

    150 cm depth. Among different orders of soils in India,

    aridisols and inceptisols stored more amount of carbon which

    is about 20.3 and 15.07 Pg C respectively followed by alfi-

    sols, vertisols, entisols, mollisols, oxisols and ultisols [26].

    Bhadwal and Singh [27] used three Land Use and Carbon

    Sequestration (LUCS) models to estimate the sequestration of

    carbon in different land use scenarios in India. The amount of

    carbon sequestered in scenario LUCS-III is estimated to be

    The Chronological Advancement of Soil Organic Carbon


  • Table 1 Major progress and development in soil organic carbon sequestration research

    S.No Types of SOC research Major findings Research problem yet to be addressed References

    1. OC stock estimation in different

    types of soils/ecosystems

    Total OC stock in different soil taxonomic

    orders: 1,576 Pg (up to 1 m)

    Total carbon stock of different ecosystems:

    biomass contains 486.4 Pg C. Soil

    contains 2,056 Pg C (up to 1 m)

    Carbon stock of major forest biomes is

    1,240 Pg C

    Carbon sequestered in crop lands in first,

    second and third meter is 157, 210 and

    248 Gt C

    Still there is a debate over the calculation

    of carbon stock from regional level to

    national level

    Also there is a lack of data on SOC stock of

    some fragile ecosystems like tropical

    wetlands and watersheds (including river



    2 Factors involved in the storage of


    Effects of land use changes on

    storage of SOC

    Salinity, sodicity and soil erosion

    Litter decomposition and

    Microbial population

    Temperature, precipitation, types of soil

    and vegetation cover of the region has the

    influence on storage of carbon

    Vegetation cover has the major influence

    on the storage of carbon through their

    different types (chemically) of litter


    Globally agricultural activity reduces the

    original carbon content by 3050 %

    Salinity, sodicity and soil erosion have an

    impact on SOC storage

    Litter quality and microbes play a pivotal

    role in the storage of SOC in any


    Still there is a lack of understating on

    carbon storage with respect to

    temperature and precipitation

    Very limited data on other factors like

    altitude, topography, soil parent

    materials, soil clay minerals and heavy

    metals affecting the storage of carbon in

    different ecosystems

    Still there is an error in estimating the

    global SOC stocks of agricultural soils

    There is a scarcity of knowledge on

    inorganic carbon impact on storage of

    organic carbon in arid environment soils

    Still there is a paucity of information on the

    microbial contribution to the long lived

    carbon pool in different ecosystems

    [19, 22,


    3. SOC pools separation and SOM


    SOC is heterogeneous in nature and

    divided into three to four distinct pools by

    physical, particle and chemical

    fractionation techniques

    SOM divided into active/labile,

    intermediate, slow and passive/

    recalcitrant pools

    SOC models like CENTURY/ROTH C

    developed to discrete the pools with

    different turnover rates

    No proper/standardised technique to

    separate organic matter into different


    No clarity of determining the active/labile

    and stable/recalcitrant pool of carbon in


    Yet to quantify the labile and stable pool of

    carbon in major soil ecosystems of the


    There is no single widely accepted model

    to tell the fate of sequestered carbon in

    different types of soils under different

    vegetation cover


    4. Temperature sensitivity of OC in

    different types of ecosystems

    SOC is temperature sensitive Rising level

    of global surface temperature would

    affect the stored carbon (deep layer) in


    Which pool (labile or recalcitrant) of OC is

    more sensitive to rising global surface


    [19, 109,



    5. Chemical characterisation of SOC Proton (1H1) and C-13 NMR was used to

    characterise the OC in soil and OC pools

    Spectra is divided into four distinct

    chemical shift regions: 050 alkyl carbon

    (C) (for 1H1 NMR spectra it is

    0.52 ppm), 6093 ppm O-alkyl C (for1H1 NMR spectra it is 35 ppm),

    112140 ppm aromatic C (for 1H1 NMR

    spectra it is 78 ppm) and 165190 ppm

    carboxyl C (for 1H1 NMR spectra it is

    1012 ppm)

    There is a lack of data on chemical

    characterisation of OC in different

    taxonomic orders of soil at different



    J. Dinakaran et al.


  • 6.937 billion tonnes, which is the highest among those

    sequestered in all the three scenarios. According to this sce-

    nario, the carbon sequestered in above ground vegetation of

    India will be more than double by the year 2050. Lal [28]

    estimated SOC pool at 21 Pg up to 30 cm depth and 63 Pg up

    to 150 cm depth and also estimated the total potential of

    carbon sequestration being 0.0390.049 Pg C year-1. Ra-

    machandran et al. [29] estimated both above and below

    ground carbon stocks of the natural forest area of southern

    India. They estimated about 0.00274 and 0.00348 Pg of car-

    bon in above ground biomass and soils respectively.

    According to the Forest Survey of India, FSI [30] the national

    carbon stock is about 6,621 million tonnes. Kumar et al. [31]

    suggests that carbon sequestration through cultivation of cold

    resistant agroforestry species like willow and poplar can also

    restore degraded lands of trans-Himalayan region and

    improve SOC along with global CO2 mitigation. A dynamic

    growth model (CO2FIX) was used by Kaul et al. [32] to

    estimate the carbon sequestration potential of Sal (Shorea

    robusta Gaertn. f.), eucalyptus (Eucalyptus tereticornis Sm.),

    poplar (Populus deltoides Marsh), and teak (Tectona grandis

    Linn. f.) forests in India. Their results indicated that poplar

    (8 Mg C ha-1 year-1) (1 Mg = 1 9 10-9 Pg) and euca-

    lyptus (6 Mg C ha-1 year-1) plantations had a highest car-

    bon sequestration rate followed by moderate growing teak

    forests (2 Mg C ha-1 year-1) and slow growing long rota-

    tion sal forests (1 Mg C ha-1 year-1). Based on different

    forest types in India, the national average of soil organic

    carbon per hectare in forest soil was estimated as

    183 Mg C ha-1 [33].

    There is a large error associated with estimating the

    mean carbon content of any ecologically or taxonomically

    based mapping unit [34]. Jobbagy and Jackson [22] noted

    that average coefficient of variation is commonly C60 %

    for the mean carbon content of a mapping unit. The large

    error reflects a number of factors, such as climate vari-

    ability within a map unit. In addition, variations in any

    other factors, such as topographic position on the land-

    scape, parent material, landscape age or biota, have large

    effects on estimate values of soil organic carbon [35].

    There have been tremendous improvements in the estima-

    tion of SOC stock of terrestrial ecosystems across the

    planet earth. However, still there is a need of studies on the

    estimation of SOC stock of small areas (acre level) to

    larger areas (square kilometre) of terrestrial ecosystem.

    These studies must take care about the important factors

    (e.g. bulk density, depth of soil) which would affect the

    estimation of SOC stock of different ecosystem.

    SOC Sequestration in Agricultural Soils

    Carbon sequestration in agricultural and degraded soils is

    in higher priority to restore the ecosystem and to mitigate

    global climate change. According to FAO, the total agri-

    cultural land area on earth is estimated about 1.6 billion

    hectares. This is 12 % of the total land on earth (13.2

    billion hectares) [36]. The total area of cultivated land has

    grown up by 159 million hectares since 1961 [36]. Globally

    agricultural soils sequestered 157, 210 and 248 Gigaton of

    carbon in one, two and third meter respectively [22]. The

    agricultural activities and land-use change contributes

    about one-third of the total greenhouse gas (GHG) emis-

    sions and are the largest sources of methane and nitrous

    oxide emissions [37, 38] (see more details in the effects of

    land use change in SOC section). The amount of carbon

    efflux from soil can be returned back to the soil by proper

    land management practices and agroforestry systems which

    Table 1 continued

    S.No Types of SOC research Major findings Research problem yet to be addressed References

    6. Calculation of turnover/mean

    residence time (MRT) of carbon

    by litter decomposition/carbon

    isotopes techniques

    MRT/turnover of SOC were calculated by

    first order kinetic model with the help of

    litter decomposition (rate) study

    Turnover/MRT varies in different types of

    soils and at different depths

    The turnover rate of active/labile pool of

    carbon is more than the stable/recalcitrant

    pool of carbon in soils

    Still there is a lack of understanding about

    the calculation of MRT and turnover of

    carbon in soils at different depths even by

    using carbon isotopes technique


    7. Mechanism of OC stabilization in

    different types of soils

    OC in soils is stabilized for long time by

    the processes of (1) chemical

    recalcitrance, i.e. selective preservation

    of certain compounds after

    decomposition (2) inclusion of organic

    matter into micro-aggregates, leading to

    physical protection of organic matter

    from microbial attack and (3) interaction

    with soil minerals especially silt ? clay

    But still there is a huge lacuna in the

    understanding of stabilization mechanism

    of OC in different taxonomic orders of

    soil at different depths under different

    vegetation cover

    [111, 149


    The Chronological Advancement of Soil Organic Carbon


  • are already included in the framework convention on cli-

    mate change [8]. The loss of carbon from agricultural soil

    can provide a reference level for carbon sequestration

    potential [39]. The estimated global potential of carbon

    sequestration in soil of croplands, degraded/desertified

    lands, irrigated soil and grazing/rang lands are 0.40.8,

    0.20.4, 0.010.03 Gt year-1 respectively [38]. In another

    study [40] the estimated rate of carbon sequestration

    potential in US cropland, Canada, European Union, China

    and India is 75208, 24, 9012, 105198 and

    3949 Tg C year-1. They also mentioned that the tropical

    agroforestry system has a high potential rate of carbon

    sequestration both in above and below ground at the rate of

    50 Mg C ha-1. Numerous studies have shown that soil

    carbon in cultivated lands can increase by adopting con-

    serving practices such as no tillage, no summer fallow and

    increasing residue inputs [4043]. The recommended

    management practices (RMPs) such as conservation tillage,

    agroforestry, use of manure, biosolids, grazing manage-

    ment and forest management can increase the rate of car-

    bon retention in the soil [38, 43].

    In India, Indo-Gangetic plains (IGP), (13 % of the total

    geographical area) are producing about 50 % of food

    grains and which is fed by almost 40 % of peoples in India.

    Bhattacharya et al. [44] successfully used the GEFSOC

    model in IGP areas and predicted about 1.27, 1.32 and

    1.27 Pg of carbon stored in 1990, 2000 and 2030 respec-

    tively. They also reported that soil of IGP may alone

    sequester about 1.1 Pg C in 2030. Bhattacharya et al. [45]

    estimated carbon stock (both organic and inorganic) of

    different agro climatic zones, bioclimatic systems and

    agro-eco sub regions of India. Bhattacharyya et al. [46]

    suggests that the agricultural management practices in

    India advocated through the national agricultural research

    system for the last 25 years did not cause any decline in

    SOC in the major crop growing zones of the country. Soil

    quality and productivity of IGP can be improved by the

    addition of organic amendments with inorganic fertilizer

    [47]. The long term application of organic amendments via

    farm yard manure (FYM) in rice fields of India could

    increase SOC by 10.7 % [48]. In another study the addition

    of NPK ? FYM alone has a good potential in carbon

    sequestration in Indian soil to mitigate GHG emission

    without any additional cost [49]. Thus proper management

    practices in Indian agricultural soil can have a high sink

    potential for carbon.

    Factors Affecting the Storage of SOC

    In the early eighties, scientists started to work on the fac-

    tors involved in the storage of organic carbon in different

    ecosystems. The following factors have a significant

    impact on the storage of carbon in different ecosystem are:

    Climate, Altitude and Topography

    Climate has long been believed to affect the accumulation of

    carbon in soil. Cool conditions and poor aeration in wet soils

    can impede decomposition as the result, soil carbon stocks

    are higher in cool and moist biomes and low in hot and dry

    biomes [50]. Temperature and moisture are two critical

    environmental factors influencing soil carbon storage. In

    general, SOC increases with increasing precipitation,

    decreasing temperature, and decreasing evapotranspiration/

    precipitation ratio [22]. The warm temperate and dry tropi-

    cal forests generally had the smallest soil carbon content,

    while wet boreal and tropical forests had the largest soil

    carbon values [19]. The storage relationship between pro-

    duction of organic matter in different type of tropical forests

    and climate is very important for better understanding of the

    global soil carbon pool changes.

    The allocation of carbon between vegetation and soil

    differs by latitude. An earlier study [51] reported that a

    large part of the vegetation (25 %) and soil (59 %) carbon

    pools are located in the high latitude forests. Low-latitude

    tropical forests are relatively heterogeneous and contain

    59 % and 27 % of global forest vegetation and soil carbon

    respectively [51]. Soil carbon densities in the low latitude

    forests are generally higher than those in the mid-latitude

    forests [19]. The high SOC stock in higher latitude is also

    influenced by permafrost dynamics and drainage [52]. In

    India carbon stock in different types of forests on Hima-

    layan mountain regions tended to decrease with increasing

    altitudes [53]. In addition to that altitude had a significant

    effect on species richness which declines to even a 100 m

    increase in latitude. The characteristic decline in vegetation

    with increasing altitude results in less accumulation of litter

    and low input of organic carbon in soils.

    Very few studies have reported the impact of topo-

    graphic aspects of soil carbon storage globally [54].

    Topographical aspects also induce local variation in tem-

    perature and precipitation, which along with chemical and

    physical composition of the substrate, are the main regu-

    lators of decomposition rate of SOM. Thus the topographic

    aspects affect the storage of SOC. Bhattacharyya et al. [55]

    extensively studied the formation and persistence of mol-

    lisols in the hills of humid tropical parts of India. They are

    found in the adverse climate only on the zeolitic Deccan

    basalt parent material which provided better water storage

    for the retention of organic matter in the soil. The forma-

    tion and development of different types of soils (soil

    orders) have a significant effect on SOC storage in a par-

    ticular region.

    J. Dinakaran et al.


  • Land Cover or Vegetation Cover

    The type of vegetation covering the earths surface is called

    as land cover. Globally, many studies on SOC dynamics

    have agreed that the amount and relative distribution of

    SOC is mainly associated with vegetation cover than cli-

    mate. Dominant plant life forms or community type (e.g.

    trees, shrubs, grasses and herbs) changes have an influence

    on soil carbon content because as plant life forms differ in

    litter chemistry, patterns of detrital input and rooting depth

    [56]. A few studies have also reported that plant life forms

    typically differs in the depth and distribution of their root

    systems [57, 58] which influences the amount and distri-

    bution of soil carbon. Plant species have the potential to

    influence soil carbon pool and its dynamics through vari-

    ation in carbon input (that is, net primary production) by

    influencing carbon losses, including SOM decomposition.

    Among tree species, organic horizon (O-horizon) pool

    sizes are largely controlled by the difference between

    inputs via litter fall and outputs via litter decomposition

    and thus should exhibit marked differences among species

    that vary in those attributes [59]. The influence of species

    on mineral horizon SOM dynamics is likely more complex.

    However recent study [60] reported the differences in SOC

    storage among different vegetal covers up to 1.5 m depth

    of tropical forests in India. This study also reported that

    deep rooting teak plant cover stored more amount of car-

    bon beyond 50 cm while the shallow root systems of

    bamboo cover stored very little. So, plant species have

    strong effects on storage of SOC in regional as well as

    global forest ecosystems.

    Soil Texture

    Soil systems consist of two different fractions of particles;

    (1) coarse size earth fractions ([2 mm in size) such asgravels, cobbles, boulders and other fragments of soil (2)

    fine earth fractions such as sand ([0.052 mm), silt([0.0020.05 mm) and clay (\0.002 mm). Numerousstudies have suggested that organic carbon sequestration in

    soil is texture dependent and highly correlated to the pro-

    portion of fine particles [61, 62]. Many studies defined the

    protective capacity of a soil as the maximum soil carbon

    can be associated with the clay and fine silt fractions [63,

    64]. Silt sized particles acted as a medium term sink for

    recently added organic carbon and sand sized fractions can

    be useful as sensitive indicators of changes in soil carbon

    status in response to land use management.

    Effects of Land Use Change on SOC

    Agricultural practices and land use significantly influence

    soil carbon storage. Historically, soils have lost

    4090 Pg C globally through cultivation and disturbance

    with current rates of carbon loss due to land use change of

    about 1.6 0.8 Pg C year-1, mainly in the tropics [65].

    When natural vegetation is converted to cultivated crops,

    rapid decline in soil organic carbon occurs due to lower

    fraction of non-soluble material in more readily decom-

    posed crop residues and tillage leads to mixing and stirring

    soil, break up aggregates and exposes organomineral

    surfaces to decomposers [66]. Agricultural production has

    changed the natural ecosystems and disturbed the soil

    environment, leading to a significant decline of soil carbon,

    with most of carbon loss occurring in the first several years

    [67]. The SOC loss is accentuated when inputs of organic

    carbon in cultivated systems are lower, and losses due to

    mineralization and erosion are higher than in natural sys-

    tems [68]. The rate of SOC loss due to conversion from

    natural to agricultural ecosystems is more drastic in the

    tropics than in the temperate-region soils [69]. There is

    sufficient evidence that rapid and significant decline of

    SOC has occurred as a result of land use change most

    notably when natural ecosystems are converted to agri-

    cultural systems. Globally, land use change and soil culti-

    vation are estimated to have caused the loss of 136 Pg of

    soil C to the atmosphere since 1750 [68]. Globally agri-

    cultural activity reduces the original soil carbon content by

    *30 % [34]. In another report [66], SOC losses occur asmuch as 50 % in surface soil up to a depth of 20 cm after

    cultivation for 3050 years. The average loss of soil carbon

    was about 40 % in the plough layer to a depth of 30 cm

    [70]. Finally, management activities can influence the

    labile fraction of the SOC stock [71], and affect soil quality

    and productivity. Thus, Lal [68] suggested that imple-

    menting a program to increase SOC storage requires gov-

    ernments mandate no-till agriculture or provide financial

    incentives to farmers. Adoption of management practices

    like stubble retention and reduced tillage are found to

    potentially increase carbon in agricultural soils.

    Salinity, Sodicity and Soil Erosion

    Increasing soil salinity and sodicity are an important global

    land degradation issues in arid or semiarid regions and also

    predicted to increase in the near future [72]. Arid regions

    and drylands, of the world, are about 45 % of the earths

    land surface, have the highest potential of sequestering

    carbon at the rates similar to those of pine forests located in

    Europe [73]. These dry lands have the potential of SOC

    sequestration of about 1 Pg C year-1 [74]. These lands

    also home to 38 % of the total global population. Car-

    bonate minerals are common in the soil of dry regions of

    the world. The excessive amount of salts and sodicity in

    soil has adverse effects on soil properties by the excessive

    formation of calcium carbonate (CaCO3) [46, 75]. Soil

    The Chronological Advancement of Soil Organic Carbon


  • inorganic carbon (SIC) content is two to three times more

    than SOC in arid-semiarid region up to a depth of 1 meter

    [76]. An almost 229 million hectare soil of our country is

    calcareous in nature and the role of SIC in carbon

    sequestration is crucial to maintain soil fertility [75]. SIC

    stock of Indian bioclimatic zones such as semi-arid, sub-

    humid and humid in IGP is 124.48, 12.28 and 24.11 Tg/

    lakh ha respectively [46]. Interestingly an earlier study [77]

    found an indirect relationship between the SIC and the

    formation of SOC. The low levels of SOC and soil

    microbial biomass in salt affected soil is due to low sub-

    strate additions and decomposition rate [78]. Initial esti-

    mation of SIC in arid regions of our country would

    stimulate the future research on the SIC role in carbon

    sequestration in soil.

    Soil erosion is the main land degradation process that

    releases soil carbon. Lal [79] estimated about 1,094 Mha of

    land, globally, is severely affected by water and wind

    erosion. Soil erosion is an important compartment while

    calculating the global carbon budget. The estimated

    amount of soil carbon from the eroded land by water and

    wind is 46 Pg C year-1 [79]. In India the estimated

    amount of carbon eroded by water is 6 Tg C year-1 [28]. It

    is very important to understand the loss of carbon in Indian

    soil caused by water/floods. Though there is a need of

    management action plan for the land covered by forest and

    crops in India. Every year there is a huge amount of soil

    which is eroded by water in Himalayan and other mountain

    regions in India during monsoon season. However till date

    there is no clear understating about the source (from the

    mountains) and sink (terrestrial lands/river/sea) concept of


    Litter Decomposition and Microbial Population

    Plant litter decomposition is a dominant process in the flow

    of carbon and nutrients in most terrestrial ecosystems [80].

    Litter decomposition is regulated by three factors: (1) cli-

    mate (2) litter quality, and (3) the nature and abundance of

    microbial communities [81]. Climate has a direct effect on

    litter decomposition due to the effects of temperature and

    moisture [82]. Temperature and precipitation on litter

    decomposition rates are stronger in the early stage [83].

    Although, early decomposition rates are strongly affected

    by litter chemical components [84]. Feng et al. [85]

    reported that lignin derived SOM is susceptible to

    enhanced degradation under warmer temperatures. Thus

    climate is affecting the storage of SOC through litter

    decomposition processes. The bulk of litter comprises

    structural components of plant cell walls (cellulose, hemi-

    cellulose and lignin) and hence carbon is always in much

    larger concentrations than nutrients [86]. In addition to

    structural polymers, litter also contains water soluble

    fractions, such as simple sugars and aminoacids, oils,

    waxes, simple and complex phenolic compounds and cu-

    tins [87, 88]. Many studies demonstrated the relationship

    between the litter quality characteristics and decomposition

    rates for a large number of plant species [89, 90]. The C:N

    ratio is accepted as a general index of quality [91]. Min-

    eralization rate tends to decrease with increasing C:N ratio.

    The early stages of decomposition are dominated by the

    easily decomposable carbohydrates, while at later stages

    lignin exerts the major control on decomposition rate [92].

    Soil microbes play a pivotal role in litter decomposition

    processes. They release degradative enzymes that attack

    dead organic matter (OM), breaking it down into digestible

    units that they can take up [93]. Microbial population liv-

    ing in soil contains carbon less than 4 % of SOC. Microbial

    population adds to carbon in soil by the continuous process

    of growth and death [94]. They are sensitive to land

    management practices such as cultivation practices, forest

    degradation and afforestation. Recently people have been

    interested in the role of microbial necromass (longer turn

    over time) in the stabilization of SOC [94, 95]. It is men-

    tioned that at most of the times two different groups of

    microbes (autochthonous and zymogenous) participate in

    decomposition/mineralization [96, 97] processes. These

    two groups of microbes actively depend upon the avail-

    ability of substrates [97]. A zymogenous group of microbes

    flourishes with the availability of fresh inputs and mainly

    metabolizes the labile fractions (cellulose and hemicellu-

    loses). It becomes dormant when the fresh supply stops

    [96]. An autochthonous group of microbes will survive

    even if the fresh supply is stopped. They mainly metabolize

    the recalcitrant fraction of SOM [96, 98]. After all these

    actions, the remaining SOM becomes stable for a longer

    period of time. Thus microbial community types affect the

    SOC storage/stabilization via litter decomposition process.

    Still there is a paucity of information on the microbial

    contribution to the long lived carbon pool in different


    SOM Models

    Carbon sequestration in soils of various ecosystems can be

    estimated from direct measurements of the carbon input to

    the soil by litter fall, root decay and release by net min-

    eralization [99] or carbon sequestration in soil can be cal-

    culated based on dynamic SOC models. Innumerable

    models exist on SOC dynamics based upon their unique

    objectiveness. They are CENTURY-C, ROTH-C, ANIMO,


    SCEN [100107]. These models need carbon inputs to the

    soil by litter fall and root decay and carbon release by net

    mineralization to allow the calculation of soil carbon

    J. Dinakaran et al.


  • sequestration. The widely accepted SOC models are

    CENTURY-C and ROTH-C. Initially these two models are

    used mainly for agricultural soils and later on have also

    been applied for forest soils [107]. The CENTURY-C

    model divided SOM into three different pools with varying

    residence times, e.g., active (15 years), slow

    (2040 years) and passive (4002,000 years). While the

    ROTH-C model has four active SOC compartments and a

    small amount of inert organic matter (IOM). The four

    active compartments are decomposable plant material

    (DPM), resistant plant material (RPM), microbial biomass

    (BIO) and humified organic matter (HUM). Each com-

    partment decomposes by a first-order process with its own

    characteristic rate. But the IOM compartment is resistant to

    decomposition. An earlier study [99] used three different

    approaches (Limit value, N-balance and SMART-2) to

    measure the net sequestration rate of SOC. The limit-value

    approach calculated the highest carbon sequestration

    ranging from 160 to 978 kg ha-1 year-1, followed

    by the N-balance method which ranged from 0 to

    535 kg ha-1 year-1, the SMART-2 calculated the lowest

    carbon sequestration from 30 to 254 kg ha-1 year-1. The

    most common model used to describe the dynamic behavior

    or turnover of SOM, especially the forest, is the first-order

    kinetic model which assumes a constant zero-order input

    with constant proportional mass loss per unit time [108].

    This model considers that the SOM is a single pool and

    decomposed at similar rates. After that, Giardina and Ryan

    [109] calculated the turnover time of the mineral soil carbon

    by first-order decay model. Subsequently, Thornley and

    Cannell [110] reported a four pool soil organic matter model

    accounting for carbon flow with linear kinetics model; they

    are fast, intermediate, slow and stabilized. All the conceptual

    carbon pools used in these models are not directly measur-

    able, because current methodologies are unable to selec-

    tively isolate functional carbon pools with specific residence

    time and stabilization mechanism.

    SOC Pools: Separation by Various Fractionation


    SOM consists of various functional pools that are stabilized

    by specific mechanisms and have certain turnover rates

    [111]. There are three main soil fractionation techniques

    (physical, particle size and chemical) which are used to

    separate the SOC pools from the whole soil/aggregates to

    understand the dynamics of carbon.

    Physical Fractionation

    Physical fractionation involves the application of various

    degrees of disaggregating treatments (dry and wet sieving,

    slaking) dispersion (ultrasonic vibration in water), density

    separation and sedimentation [111]. The detailed methods

    and procedures have been published by Elliott and Cam-

    bardella [112] and Christensen [113, 114]. Thus, Six et al.

    [63] extensively used the physical fractionation techniques

    for measuring and understanding the carbon storage of

    afforested soils. For this, he fractionated the soil samples

    into four size of aggregates ([2,000,[250500,[53250,\53 lm), then each size aggregate was subjected to isolatethe light and heavy fractions of carbon and finally the

    heavy fractions of all the aggregates were further frac-

    tionated into four carbon pools by sieving method. Mic-

    roaggregates (\250 lm) are bound together to formmacroaggregates ([250 lm) with organic compounds ofdifferent origin and stability. Physical fractionation of soil

    particles into size and density classes can provide infor-

    mation on the importance of interactions between organic

    and inorganic soil components and the turnover of SOM.

    Particle Size Fractionation

    Particle size fractionation technique is based on the concept

    that SOM remains associated with particles of different

    sizes such as sand ([0.052 mm) silt ([0.050.002 mm)and clay (\0.002 mm). In this technique, the soil is sepa-rated into sand, silt and clay with the help of sieves and

    sedimentation processes [115]. SOM within the sand

    fraction is allocated to the active pool and SOM in silt and

    clay fractions to the intermediate and passive pool. Gen-

    erally about 5075 % of total organic carbon was associ-

    ated with clay sized particles (\0.002 mm), about 2040 %with silt sized particles ([0.050.002 mm) and \10 %with sand sized particles ([0.05 mm) in temperate soil[114]. Fractionation by particle size provides a rough dif-

    ferentiation between young (active) and old (Intermediate

    and passive) SOM. In general, SOC associated with sand

    particles represents the active pool and the SOC associated

    with silt ? clay particles forms stable pools.

    Density Fractionation

    Density fractionation is totally based on the isolation of

    SOM which is not firmly associated with mineral part of

    the soil [116]. This technique can separate the SOM into

    light (LF) and heavy fraction (HF). Light fraction is defined

    as a plant-like and less stable fraction with high carbon

    concentration while the heavy fraction is a more stable and

    high-density organomineral fraction having lower carbon

    concentration [117]. SOM associated with mineral surfaces

    like phyllosilicates are most often characterized by a den-

    sity greater than 1.62 g cm-3 [111]. The lighter fractions

    with a density of 1.62 g cm-3 consist mostly of pieces of

    plant residues, as either particulate SOM (POM) or free

    The Chronological Advancement of Soil Organic Carbon


  • (fPOM) or occluded in aggregates (oPOM) [113]. Histor-

    ically density fractionation relied on organic liquids like

    tetrabromoethane C2H2Br4, tetrachloromethane CCl4, and

    inorganic salts such as sodium iodide (NaI), ZnBr2 and

    sodium polytungstate which are increasingly used to sep-

    arate LF and HF fractions [63, 116]. Six et al. [118] used

    sodium polytungtate (SPT) to isolate light and heavy

    fractions of SOM rather than by using organic liquids or

    solutions of ZnBr2 or NaI, because of their high viscosity at

    high concentration and their use to produce wide range of

    densities (1.03.1 g cm-3).

    Chemical Fractionation

    Unlike physical and particle size fractionations, chemical

    fractionations use many chemicals (inorganic bases such as

    NaOH and acids especially hydrochloric acid and sulfuric

    acid) also for the extraction of sugars and heavy fractions

    of OM to extract SOM pools [119, 120]. Normally

    extraction with 0.5 M NaOH removed more carbon and N

    from coarser fractions than from fine fractions [111]. OM

    extracted with NaOH is not a homogeneous soil fraction,

    because the extraction procedure simultaneously affects

    organomineral and organoorgano interactions. Resi-

    dence time of alkaline extracts reported generally varies

    and is dependent on soil type [121]. Acid hydrolysis is one

    of the chemical methods used to separate the older carbon

    from the whole soil by removing proteinaceous and poly-

    saccharide type of organic materials [122]. Hydrochloric

    acid (HCl) and sulphuric acid (H2SO4) are used to separate

    the old/recalcitrant from the whole soil. However, recently

    deposited lignin and waxes may resist to acid hydrolysis.

    Thus, the acid hydrolysis approach is able to concentrate

    on old carbon pool but may produce biased results [123].

    The SOC associated with the coarse fraction is commonly

    less decomposed material and has high C/N ratio than that

    associated with humic nature with low C/N ratio [114]. In

    general agreement, the total SOC contains at least the

    fractions like active or labile, intermediate, slow and pas-

    sive pools [110, 124126].

    The number of SOC pools and the techniques which

    have been used for sequestration varied from author to

    author. All these methods are having their own limitations.

    Therefore the information available on fractionation of

    SOC pool is still scanty and the techniques have not yet

    been standardized in various types of soils. The ultimate

    aim of all these fractionation methods is to separate the

    discrete labile and recalcitrant pool of carbon and to

    measure the definite size and turnover rate of these pools,

    because, the active/labile pool is more sensitive to land use/

    land cover change and temperature. The stable SOC pool is

    termed as a fraction of total SOC pool with a slow turnover

    time (may be hundreds of years). It is resistant to microbial

    attack during decomposition and is relevant as a long-term

    storage of sequester carbon in soils. Although global esti-

    mates of the fast-cycling or labile SOC fractions range

    from 250 to 530 Pg as compared to the total SOC pool of

    2,500 Pg [119]. This clearly indicates that a major part of

    the SOC stored in soil comprises the stable pool. Labile

    SOM are recognized as potential indicators of soil quality

    changes in a short-term due to variation in management

    practices such as tillage, fertilizer and manure applications,

    and crop rotation [127, 128]. Special emphasis has been

    placed on SOM fractions with rapid turnover rate because

    these respond more rapidly to changes in management or

    environmental conditions if any change in these pools

    would ultimately affect the level of atmospheric CO2. In

    general, the proportion of stable SOC pool increases with

    an increase in soil depth [60, 123]. The trend was observed

    in the dry deciduous forest soil profile of teak, bamboo and

    mixed vegetal cover [123, 128] in Gujarat, India. However,

    there is a need for information about the proportion of

    stable SOC pool in other types of forest soils to understand

    the dynamics of stable carbon with response to global

    climate change. This knowledge may be critical for future

    long term SOC sequestration.

    Temperature Sensitivity of SOC

    About 60 % of total carbon is stored in below ground in

    tropical and temperate forest ecosystems of the world,

    whereas 90 % of carbon in below ground at boreal forests

    predominantly in the form of organic detritus [19]. On the

    other hand the higher temperature may result in enhanced

    organic detritus decomposition and release more CO2 into

    the atmosphere. The study of Giardina and Ryan [109]

    showed that decomposition of old, recalcitrant SOM or

    organic carbon in mineral soil is less sensitive to tem-

    perature than labile carbon. However the recent researches

    on natural grassland revealed a phenomenon of acclima-

    tion whereby the sensitivity of soil respiration to warming

    decreases after the ecosystem is exposed to experimental

    warming for a certain time. According to the Fang group

    [129] the temperature sensitivity of the decomposition of

    SOM does not change with soil depth, sampling method,

    and incubation time. The temperature sensitivity of slow

    carbon pools is even higher than that of the fastest pools.

    Thus the stabilised nature of lignin derived SOM is more

    susceptible to enhanced degradation under warmer tem-

    peratures [130]. Therefore a high heat or warmer tem-

    perature may reduce the storage of recalcitrant carbon and

    ultimately affect the storage of SOC in various


    J. Dinakaran et al.


  • Chemical Characterization of SOC

    Diverse groups of chemical compounds present in the SOC

    are mostly derived from plants, animal tissues and micro-

    bial derivatives. Major components of plant tissues are

    polysaccharides, lignin, tannins and proteins, while soil

    bacteria and fungi consist mainly of homo and hetero-

    polysaccharides (chitin, peptidoglycan, lipopolysaccha-

    rides), and animals contribute carbohydrates, proteins and

    lipids [131]. The technique of 1H1 and13C nuclear mag-

    netic resonance spectroscopy with cross-polarization and

    magic-angle spinning (CPMAS NMR) has been used to

    characterize organic carbon in soil, litter and humus layer

    [132134]. These studies divided spectra of litter, soil and

    humus residues into chemical shift regions which are 050

    alkyl carbon (C) (for 1H1 NMR spectra it is 0.52 ppm),

    6093 ppm O-alkyl C (for 1H1 NMR spectra it is

    35 ppm), 112140 ppm aromatic C (for 1H1 NMR spectra

    it is 78 ppm) and 165190 ppm carboxyl C (for 1H1 NMR

    spectra it is 1012 ppm). Assessment of the chemical

    characterisation of labile and recalcitrant pool of carbon is

    very important in soil carbon dynamics studies [128, 131,

    132]. Biochemically recalcitrant SOM fractions are enri-

    ched by an alkyl group of carbon structures and the pro-

    portion of alkyl carbon and the mean age of SOM increases

    with an increase in soil depth [133, 134]. Consequently,

    understanding the chemical nature of the SOC pools is very

    important particularly in the stabilization process of carbon

    in deeper layers.

    Carbon Isotopes in the SOC Dynamics Study

    For the last one decade more researchers used the radio-

    carbon dating method along with any one of the fraction-

    ation techniques (physical, particle and chemical) to

    measure the age of the SOM pools of boreal, temperate and

    tropical forests [135137]. Carbon isotopes (12C, 13C and14C) are very useful tracers in the study of the soil carbon

    cycle. Because, plants take carbon in the form of carbon

    dioxide (12C, 13C and 14C) during photosynthesis by two

    different mechanisms, which are named the C3 and C4pathways depending on whether CO2 is initially coupled to

    a 3-C or a 4-C compound [138]. When plants shed their

    leaves, twigs, flowers, barks or whole plant dies the

    amounts of 12C and 13C contained in the dead matter

    remain the same because they are both stable isotopes. But

    the amount of 14C decreases over time as the isotope

    decays predictably into its new form. C3 plants discrimi-

    nate more against 13C than C4 plants during the photo-

    synthetic uptake of CO2. Relatively small but significant

    variations in the 13C/12C ratio result from these different

    photosynthetic pathways. Thus, terrestrial plants with the

    C3 (Calvin cycle) pathway have d13C values in the range

    -35 to -20 % while the plants with the C4 (Hatch-Slack)pathway have higher d13C values ranging from -19 to-9 % [139, 140]. The isotopic composition of SOC clo-sely resembles the isotopic composition of the vegetation

    carbon from which it has been derived [139]. The use of

    d13C enables a distinction between separate organic carbonpools derived from the original and new vegetation,

    respectively as long as one of the vegetation communities

    (original or new) is predominantly of the C3 and the other

    of the C4 type [141143]. Therefore the stable isotope

    technique will give us more insight into the SOC dynamics

    associated with changes in the vegetation cover change and

    land use. Earlier studies [144, 145], by using the stable

    carbon isotope in short as well as long term incubation

    experiments, provided some new insight into the carbon

    sequestration in agricultural soil and suggest that the

    hydrophobic nature of humic substances may reduce the

    mineralization of labile carbon. An earlier study [146]

    estimated the turnover time of forestpasture soil by using

    d13C values integrated with three different SOC models. Itwas estimated that of the carbon in the top 20 cm ofthese soils has turnover time less than 30 years. Radio-

    carbon (14C) is unstable, and decays by emission of an

    electron to form 14N, with a half-life of 5,726 years. This is

    useful for studying to estimate the age of the SOM pools

    [135]. Thus 14C dating of bulk SOM gives 14C ages which

    range from over-modern (containing 14C produced by

    atmospheric nuclear weapons testing) to [10,000 years[147]. Thus many SOM based studies use 14C with frac-

    tionation to measure the size and active ? passive SOM

    pools of different forest ecosystems [135, 137]. An earlier

    study [148] used the combination of 14C labelling with 13C

    natural abundance to distinguish the three carbon sources

    in CO2, microbial biomass and dissolved organic carbon

    (DOC) and also evaluate the mechanism and sources of

    priming effect. Still there is a lack of understanding the

    concept of Priming effect by addition of fresh litter in

    different types of soils at different depths under different

    types of vegetation cover in natural field conditions.

    SOC Stabilization

    SOC consists of various functional pools that are stabilized

    by specific mechanisms and have certain turnover rates

    [111]. Organic carbon in soils is stabilized for long time

    ([1,000 years) by the processes of (1) chemical recalci-trance, i.e. selective preservation of certain compounds

    after decomposition like lignin etc. (2) inclusion of organic

    matter into micro-aggregates, leading to physical protec-

    tion of organic matter from microbial attack and (3)

    interaction with soil minerals especially silt ? clay [149].

    The Chronological Advancement of Soil Organic Carbon


  • Chemical Recalcitrance

    Plant tissues may vary widely in their litter chemistry and

    thereby in their decomposition rate. The chemical constit-

    uents of litter include concentrations of various classes of

    organic compounds and mineral elements in relation to

    plant characteristics and availability. These compounds can

    be broadly categorized as labile metabolic compounds

    (such as sugars and amino acids), moderately labile

    structural compounds (such as cellulose and hemicellulose)

    and recalcitrant structural material (such as lignin and

    cutin) [111]. During litter decomposition the labile com-

    ponents are degraded first, while the recalcitrant ones

    (waxes, polyphenolics, lignin) remain, as has been known

    for many years. Lignin is known to have low degradability

    as compared to other leaf components and consequently is

    of major importance in soil humus formation [150]. Lignin

    has long been preserved in soil due to its chemical recal-

    citrance. Lignin was also considered as a major component

    of SOM influencing its pool size and its turnover. However,

    certain white rot fungi and termites have a capacity to

    degrade the lignin molecule [150]. This stabilization of

    carbon for longer time is due to the selective preservation

    of lignin compounds in soils. But the fate of lignin in soil

    with full of white rot fungi and other organisms is still


    Physical Protection (Through Microaggregates)

    Most studies divide soil aggregates into three classes based

    on their size and there is some variability across the studies

    in the size classes used [63, 113, 114]. Organic particles are

    associated with soil matrix to form macroaggregates

    ([250 lm) and microaggregates (\250 lm) duringdecomposition and also sequester SOC [151]. The amount

    of plant residues and the degree of SOM decomposition are

    vital factors to the formation and stabilization of aggre-

    gates. Plant roots, hyphae and residues are the primary

    organic skeleton to enmesh the soil particles together and

    build aggregates [152]. Generally OM found on the exte-

    rior of soil aggregates is physically far more accessible to

    degradation than carbon compounds physically protected

    on the interior of these aggregates. The SOC sequestration

    in the soil is controlled by the degree of physico-chemical

    stabilization of SOM inside the aggregates. The degree of

    adherence of these primary particles as a function of

    organic matter and clay mineralogy determines whether the

    aggregates are stable or unstable [153]. Soil aggregation is

    an important process of carbon sequestration and perhaps a

    useful strategy to mitigate the increase in the concentration

    of atmospheric CO2. The amount of residue (input) incor-

    porated to soil ecosystem and the degree of soil disturbance

    has its impact on aggregate and SOC dynamics. But still,

    there is a paucity of information on how the fresh organic

    materials enter into micro or macro aggregates in different

    types of forests under different types of soils.

    Interaction with Soil Minerals

    Clay minerals are characterized by the small grain size and

    large surface area. Because of large surface area it greatly

    adsorbs the organic matter. It is typically associated with

    soil containing higher amount of clay [154]. The strong

    bonds between organic anion and clay crystals due to the

    action of the polyvalent cations prevent the organic mate-

    rials from microbial decomposition [151]. Also it is

    believed that the importance of clay in holding SOC across

    soils is influenced by mineralogical composition. The oxide

    of Fe and Al on clay mineral surface gives clay minerals a

    capacity to adsorb OM [154]. Several studies [155, 156]

    also reported that Fe oxides play an important role in

    carbon stabilization in different soils. An earlier study

    [123] found the direct relationship between HF soluble

    fraction (mineral associated fraction, stable fraction) and

    Fe oxides which enriched with clay fraction. SOC associ-

    ated with different clay minerals with respect to different

    types of soils and the relative importance of such clay

    minerals for stabilising SOC remains unclear. Recent

    researches have shown that (1) carbon in subsoil horizons

    is stabilised by mineral interactions [123], (2) its stability

    may be due to the scarcity of fresh plant litter input, which

    leads to an energy limitation of the soil microbial biomass

    [157] and/or (3) to the microbes inability to physically

    access the organic matter in deep soil [158]. Questions still

    remain on how OC interacts physically as well chemically

    with the soil matrix for stabilizing the sequester carbon.

    Further researches must need to understand the mechanism

    responsible for the stabilization of sequester carbon in soil.

    Promotion of SOC Sequestration Services

    There have been a number of scientific evidences demon-

    strating that climate change is the key factor directly

    altering the ecosystem functioning and leading to biodi-

    versity loss [159, 160]. Estimation of the impacts of cli-

    mate change on any ecosystem requires changes in the

    ecosystem services. SOC sequestration is an important

    strategy to mitigate the rising level of global atmospheric

    CO2 and also to the stakeholders for developing an accu-

    rate policy for biodiversity conservation [161163]. The

    appropriate policy and regulatory measures especially with

    regard to measuring the changes in carbon, residence time

    of carbon (with the existing knowledge of SOC research)

    and trading of carbon credits in any ecosystem with respect

    to land use change is important in near future. There is an

    J. Dinakaran et al.


  • alarming increase of land degradation by deforestation,

    continuous agriculture and shifting cultivation in many

    states of India. Thus, there is a need of promotion of SOC

    sequestration services and/or research in regional to global

    scale level.

    The Conundrum of SOC Sequestration

    The literature provided in this review provides a knowl-

    edge that plant species/vegetation cover have the potential

    to influence the SOC pools and their dynamics through

    variation in carbon inputs (through litter fall) and carbon

    losses (via soil respiration). Figure 1 shows the general

    view of soil carbon storage mechanism of terrestrial

    ecosystems and the areas to be explored seriously across

    different types of ecosystems. Also there is a need of a

    widely accepted method to separate the labile and recal-

    citrant pool of OC in different types of soils under dif-

    ferent types of forests. Scientists are far away to reach/

    answer the following questions; (1) How does the carbon

    from aboveground litter enter into the soil and gets stored

    in different pools of carbon? (2) What are the mecha-

    nisms/processes that control the entering of carbon into

    labile or recalcitrant pool in different types of forest and

    agriculture ecosystem? (3) What are the environmental

    factors that indeed control the stabilization of SOC in

    different types of ecosystems? (4) What are the roles of

    soil texture/mineralogy in protection of OC in different

    types of soils? (5) How long the sequestered carbon stays

    in different types of soils under different types of vege-

    tation cover with respect to global climate change? (6)

    How much carbon sequestered in different types of soils

    is microbially derived? and (7) How it is managed to

    increase the storage of carbon in arid and semi-arid

    regions, especially calcareous soils, covered by crops in

    India? In order to develop a framework that how these

    SOC pools would react with the land use/land cover

    change associated with climate change. First of all it is

    needed to separate and measure definite size of labile/

    recalcitrant pool of carbon in different types of soils under

    different types of vegetation cover across the world.

    Fig. 1 A general view ofcarbon storage mechanism in

    soils of terrestrial ecosystem. (1)

    Atmospheric carbon entered

    into terrestrial vegetation via

    photosynthesis process and

    stored it in leaves (dynamic),

    stems and roots (collectively

    called as litter) (2) Litter

    shedding from the terrestrial

    vegetation seasonally and/or die

    off from vegetation due to

    senescence process (3) Litter

    decomposition on the soil floor

    that has been controlled by

    various factors (4) OC storage

    during litter decomposition

    process in four different discrete

    pools [110] with different

    turnover rates (5) Soil

    respiration is the outcome of

    litter decomposition processes

    that has also been controlled by

    various factors. Also the figure

    shows the major important

    factors that have been affecting

    the sequestration of carbon and

    SOC stabilization. The question

    mark in figure corresponding to

    the area has to be studied clearly

    to understand the mechanism of

    stabilization of sequestered

    carbon in different types of soils

    under different types of

    vegetation cover

    The Chronological Advancement of Soil Organic Carbon


  • Conclusion

    Understanding SOC pools and the processes/mechanisms

    involved in the transformation of sequestered carbon into

    labile/stabilized pool in different types of soils under dif-

    ferent vegetation cover is quite a challenging task. Since

    there is no common or standardized fractionation technique

    to separate the labile and recalcitrant pool of carbon in

    different types of soils under different vegetation covers,

    understanding the sensitivity of labile and/or recalcitrant

    pool of carbon in soils to various climatic factors is quite

    contentious one. Hence the combination of various frac-

    tionation techniques would be tried in different types of

    soils to solve these problems. The combination of NMR

    and carbon isotopes technique will be the best choice to

    understand the labile or recalcitrant nature of carbon and

    their turnover rates. Among developing nations, India is

    more vulnerable to the current global climatic change due

    to the rapid development of industrialization and urban

    settlement in fertile forest/agricultural land. The increasing

    rate of inorganic carbon in Indian agricultural soils besides

    arid and semiarid zones needs a special attention by

    researchers in India. The future research agenda for carbon

    sequestration in soil or other ecosystems will integrate

    many different disciplines and have the common goals like

    measuring the size, dynamics and turnover rate of labile

    and recalcitrant pool of carbon (sequestered by forests,

    agriculture lands in different climatic zones and planta-

    tions) in all different ecosystems than simply estimating the

    carbon stock in different ecosystems. The promotion of

    research on SOC sequestration services in degraded lands

    of India would be beneficial for the government and other

    stakeholders to improve the biodiversity, food security and

    carbon credits trading. Also understanding their sensitivity

    to various factors like physical, biological and anthropo-

    genic is very important as the soilplant system is an

    essential part of the planet earth.

    Acknowledgments Authors are thankful to DST (SERB) forfinancial assistance through the DST fast track scheme for young

    scientist (SR/FT/LS-59/2012), University of Delhi through a scheme

    to strengthen R & D doctoral research program and long term funding

    by the Tropical Soil Biology and Fertility Program (UNEP/GEF).

    Logistic support from Head, Department of Botany, University of

    Delhi is thankfully acknowledged.


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    The Chronological Advancement of Soil Organic Carbon Sequestration Research: A ReviewAbstractIntroductionCarbon SequestrationAbiotic StrategiesConsequences of Abiotic Carbon Sequestration

    Biotic StrategiesChronological Progress of SOC Research Across the WorldSOC stocksSOC Sequestration in Agricultural SoilsFactors Affecting the Storage of SOCClimate, Altitude and TopographyLand Cover or Vegetation CoverSoil TextureEffects of Land Use Change on SOCSalinity, Sodicity and Soil ErosionLitter Decomposition and Microbial Population

    SOM ModelsSOC Pools: Separation by Various Fractionation TechniquesPhysical FractionationParticle Size FractionationDensity FractionationChemical Fractionation

    Temperature Sensitivity of SOCChemical Characterization of SOCCarbon Isotopes in the SOC Dynamics StudySOC StabilizationChemical RecalcitrancePhysical Protection (Through Microaggregates)Interaction with Soil Minerals

    Promotion of SOC Sequestration ServicesThe Conundrum of SOC SequestrationConclusionAcknowledgmentsReferences


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