REVIEW

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

cycle.

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

Introduction

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

e-mail: jesudina@gmail.com

123

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

ecosystems.

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

techniques.

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.

123

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

123

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

sediments)

[1949]

2 Factors involved in the storage of

carbon

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

inputs

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

ecosystem

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,

5098]

3. SOC pools separation and SOM

models

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

pools

No clarity of determining the active/labile

and stable/recalcitrant pool of carbon in

soils

Yet to quantify the labile and stable pool of

carbon in major soil ecosystems of the

world

There is no single widely accepted model

to tell the fate of sequestered carbon in

different types of soils under different

vegetation cover

[111128]

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

soils

Which pool (labile or recalcitrant) of OC is

more sensitive to rising global surface

temperature?

[19, 109,

129,

130]

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

depths

[131134]

J. Dinakaran et al.

123

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

[135148]

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

158]

The Chronological Advancement of Soil Organic Carbon

123

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.

123

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

123

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

carbon.

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

ecosystems.

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,

CESAR, SUMO2, SMART2, YASSO, CO2FIX and EFI-

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.

123

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

Techniques

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

123

(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

ecosystems.

J. Dinakaran et al.

123

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

123

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

sketchy.

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.

123

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

123

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.

References

1. Houghton JT, Ding Y, Griggs DJ, Noguer M, van der Linden PJ,

Dai X, Maskell K, Johnson CA (eds) (2001) IPCC, climate

change 2001: The scientific basis. Contribution of working

group I to the third assessment report of the intergovernmental

panel on climate change. Cambridge University Press, Cam-

bridge, p 881

2. Falkowsski P, Scholes RJ, Boyle E, Canadell J, Canfield D,

Elser J, Gruber N, Hibbard K, Hogberg P, Linder S, Mackenzie

FT, Moore B III, Pedersen T (2000) The global carbon cycle: a

test of our knowledge of earth as a system. Science 290:

291296

3. National Oceanic and Atmospheric Administration (NOAA)

(2012) Mauna loa monthly mean CO2. Earth Systems Research

Laboratory (ESRL), Boulder. http://www.ncdc.noaa.gov/sotc/

global/ and http://co2now.org/. Accessed 15 Sep 2012

4. Lal R (2004) Carbon sequestration, terrestrial. Encycl Energ

1:289298

5. Stern N (2007) The price of change: economics of climate

change. IAEA Bull 48(2):2528

6. Luo Y (2007) Terrestrial carbon cycle feedback to climate

warming. Annu Rev Ecol Evol Syst 38:683712

7. Reilly J, Mayer M, Harnisch J (2002) The Kyoto protocol and

non-CO2 greenhouse gases and carbon sinks. Environ Model

Assess 7:217229

8. IGBP Terrestrial Carbon Working Group (1998) The terrestrial

carbon cycle: implications for the Kyoto protocol. Science

280:13931394

9. Jandl R, Lindner M, Vesterdal L, Bauwens B, Baritz R,

Hagedorn F, Johnson DW, Minkkinen K, Byrne KA (2007) How

strongly can forest management influence soil carbon seques-

tration? Geoderma 137:253268

10. Feeley RA, Doney SC, Cooley SR (2009) Ocean acidification:

present conditions and future changes in a high-CO2 world.

Oceanography 22:3647

11. Kishwan J, Pandey R, Dadhwal VK (2009) Indias forest and

tree cover: contribution as a carbon sink, Technical Paper No.

130, Indian council of forestry research and education (ICFRE),

pp 112

12. Kleber M, Johnson MG (2010) Advances in understanding the

molecular structure of soil organic matter: implications for

interactions in the environment. Adv Agron 106:77142

13. Post WM, Izaurralde RC, Mann LK, Bliss N (2001) Monitoring

and verifying changes of organic carbon in soil. Clim Change

51:7399

14. Schlesinger WH (1990) Evidence from chronosequence studies

for a low carbon storage potential of soils. Nature 348:232234

15. Ramachandra TV, Shwetmala (2012) Decentralised carbon

footprint analysis for opting climate change mitigation strategies

in India. Renew Sustain Energ Rev 16:58205833

16. Trumper K, Bertzky M, Dickson B, van der Heijden G, Jenkins

M, Manning P (2009) The natural fix? The role of ecosystems in

climate mitigation. A UNEP rapid response assessment. United

Nations Environment Programme, UNEPWCMC, Cambridge

17. Sombroek WG, Nachtergaele FO, Hebel A (1993) Amounts,

dynamics and sequestering of carbon in tropical and subtropical

soils. Ambio 22:417426

18. McDowell N (2002) Developing countries to gain from carbon-

trading fund. Nature 420:4

19. Post WM, Emmanuel WR, Zinke PJ, Stangenberger AG (1982)

Soil carbon pools and world life zones. Nature 298:156159

20. Eswaran H, Berg EVD, Reich P (1993) Organic carbon in soils

of the world. Soil Sci Soc Am J 57:192194

21. Brown S, Lugo AE (1990) Tropical secondary forests. J Trop

Ecol 6:132

22. Jobbagy EG, Jackson RB (2000) The vertical distribution of soil

organic carbon and its relation to climate and vegetation. Ecol

Appl 10:423426

23. Jenny H, Raychaudhuri SP (1960) Effect of climate and culti-

vation on nitrogen and organic matter reserves in Indian Soils.

ICAR, New Delhi, p 126

24. Gupta RK, Rao DLN (1994) Potential of wastelands for

sequestering carbon by reforestation. Curr Sci 66:378380

J. Dinakaran et al.

123

http://www.ncdc.noaa.gov/sotc/global/http://www.ncdc.noaa.gov/sotc/global/http://co2now.org/

25. Bhattacharya T, Pal DK, Mandal C, Velayutham M (2000)

Organic carbon stock in Indian soils and their geographical

distribution. Curr Sci 79:655660

26. Velayutham M, Pal DK, Bhattacharyya T (2000) Organic carbon

stock in soils of India. In: Lal R, Kimble JM, Stewart BA (eds)

Global climatic change and tropical ecosystems. Lewis pub-

lishers, Boca Raton, pp 7196

27. Bhadwal S, Singh R (2002) Carbon sequestration estimates for

forestry options under different land-use scenarios in India. Curr

Sci 83:13801386

28. Lal R (2004) Soil carbon sequestration in India. Clim Change

65:277296

29. Ramachandran A, Jayakumar S, Haroon RM, Bhaskaran A,

Arockiasamy DI (2007) Carbon sequestration: estimation of

carbon stock in natural forests using geospatial technology in the

Eastern Ghats of Tamil Nadu, India. Curr Sci 92:323331

30. Forest Survey of India (FSI) (2009) India state of the forest

report. Ministry of state environment and forests, New Delhi,

pp 1199

31. Kumar GP, Murkute AA, Gupta S, Singh SB (2009) Carbon

sequestration with special reference to agroforestry in cold

deserts of Ladakh. Curr Sci 97:10631068

32. Kaul M, Mohren G, Dadhwal VK (2010) Carbon storage and

sequestration potential of selected tree species in India. Mitig

Adapt Strategy Glob Change 15:489510

33. Jha MN, Gupta MK, Alok S, Rajesh K (2003) Soil organic

carbon store in different forests in India. Indian For

129:714724

34. Amundson R (2001) The carbon budget in soils. Annu Rev

Earth Planet Sci 29:535562

35. Amundson R, Jenny H (1997) On a state factor model of eco-

systems. Bio Sci 47:536543

36. FAO (2011) The state of the worlds land and water resources

for food and agriculture (SOLAW)managing systems at risk.

Food and Agriculture Organization of the United Nations,

Rome, Earthscan

37. Follett RF (2001) Soil management concepts and carbon

sequestration in cropland soils. Soil Tillage Res 61:7792

38. Lal R (2004) Soil carbon sequestration impacts on global cli-

mate change and food security. Science 304:16231627

39. Paustian K, Andren O, Janzen HH, Lal R, Smith P, Tian G,

Tiessen H, Van Noordwijk M, Woolmer PL (1997) Agriculture

soils as a sink to mitigate CO2 emissions. Soil Use Manag

13:230244

40. Hutchinson JJ, Campbell CA, Desjardins RL (2007) Some

perspectives on carbon sequestration in agriculture. Agric For

Meteorol 142:288302

41. Ingram JSI and Fernandes ECM (2001) Managing carbon

sequestration in soils: concepts and terminology. Agric Ecosyst

Environ 87:111117

42. West TO, Post WM (2002) Soil organic carbon sequestration

rates by tillage and crop rotation: a global data analysis. Soil Sci

Soc Am J 66:19301946

43. Freibauer A, Rounsevell MDA, Smith P, Verhagen J (2004)

Carbon sequestration in the agricultural soils of Europe. Geo-

derma 122:123

44. Bhattacharyya T, Pal DK, Easter M, Batjes NH, Milne E, Ga-

jbhiye KS, Chandran P, Ray SK, Mandal C, Paustian K, Wil-

liams S, Killian K, Coleman K, Falloon P, Powlson DS (2007)

Modelled soil organic carbon stocks and changes in the Indo-

Gangetic Plains, India from 1980 to 2030. Agric Ecosyst

Environ 122:8494

45. Bhattacharyya Y, Pal DK, Chandran P, Ray SK, Mandal C,

Telpande B (2008) Soil carbon storage capacity as a tool to

prioritize areas for carbon sequestration. Curr Sci 95:482494

46. Bhattacharyya Y, Chandran P, Ray SK, Pal DK, Venugopalan

MV, Mandal C, Wani SP (2007) Changes in levels of carbon in

soils over years of two important food production zones of India.

Curr Sci 93:18541863

47. Ghosh S, Wilson B, Ghoshal S, Senapati N, Mandal B (2012)

Organic amendments influence soil quality and carbon seques-

tration in the Indo-Gangetic plains of India. Agric Ecosyst

Environ 156:134141

48. Mandal B, Majumder B, Bandyopadhyay PK, Hazra GC,

Gangopadhyay A, Samantara RN, Mishra AK, Chaudhury J,

Saha MN, Kund S (2007) The potential of cropping systems and

soil amendments for carbon sequestration in soils under long-

term experiments in subtropical India. Glob Chang Biol

13:357369

49. Pathak H, Byjesh K, Chakrabarti B, Aggarwal PK (2011)

Potential and cost of carbon sequestration in Indian agriculture:

estimates from long-term field experiments. Field Crop Res

120:102111

50. Schimel DS (1995) Terrestrial ecosystems and the carbon cycle.

Glob Change Biol 1:7791

51. Dixon RK, Solomon AM, Brown S, Houghton RA, Trexier MC,

Wisniewski J (1994) Carbon pools and flux of global forest

ecosystems. Science 263:185190

52. Hobbie SE, Schimel JP, Trumbore SE, Randerson JR (2000)

Controls over carbon storage and turnover in high latitude soils.

Glob Change Biol Suppl 6:196210

53. Sheikh MA, Kumar M, Bussmann RW (2009) Altitudinal vari-

ation in soil organic carbon stock in coniferous subtropical and

broadleaf temperate forests in Garhwal Himalaya. Carbon Bal-

ance Manag. doi:10.1186/1750-0680-4-6

54. Yimer F, Ledin S, Abdelkadir A (2006) Soil organic carbon and

total nitrogen stocks as affected by topographic aspect and

vegetation in the Bale Mountains, Ethiopia. Geoderma

135:335344

55. Bhattacharyya T, Pal DK, Lal S, Chandran P, Ray SK (2006)

Formation and persistence of Mollisols on zeolitic Deccan basalt

of humid tropical India. Geoderma 146:609620

56. Gill RA, Burke IC (1999) Ecosystem consequences of plant life

form changes at three sites in the semiarid United States. Oec-

ologia 121:551563

57. Jackson RB, Canadell J, Ehleringer JR, Mooney HA, Sala OE,

Schulza ED (1996) A global analysis of root distributions of

terrestrial biomes. Oecologia 108:389411

58. Nepstad DC, Carvalho CR, Davidson EA, Jipp PH, Letebvre

PA, Negreiros GH, Silva ED, Stone TA, Trumbore SE, Vieira S

(1994) The role of deep roots in the hydrological and carbon

cycles of Amazonian forests and pastures. Nature 372:666669

59. Binkley D, Giardina C (1998) Why do trees affect soils in

temperate and tropical forests? The warp and woof of tree/soil

interactions. Biogeochemistry 42:89106

60. Dinakaran J, Krishnayya NSR (2008) Variations in type of

vegetal cover and heterogeneity of soil organic carbon in

affecting sink capacity of tropical soils. Curr Sci 94:11441150

61. Bosatta E, Agren GI (1997) Theoretical analyses of soil texture

effects on organic matter dynamics. Soil Biol Biochem

29:16331638

62. Hassink J (1997) The capacity of soils to preserve organic C and

N by their association with clay and silt particles. Plant Soil

191:7787

63. Six J, Callewaert P, Degryze S, Morris SJ, Gregorich EG, Paul

EA, Paustian K (2002) Measuring and understanding carbon

storage in afforested soils by physical fractionation. Soil Sci Soc

Am J 66:19811987

64. Ratnayake RR, Seneviratne G, Kulasooriya SA (2008) Charac-

terization of clay bound organic matter using activation energy

The Chronological Advancement of Soil Organic Carbon

123

http://dx.doi.org/10.1186/1750-0680-4-6

calculated by weight loss on ignition method. Curr Sci

95:763766

65. Smith P (2008) Land use change and soil organic carbon

dynamics. Nutr Cycl Agroecosyst 81:169178

66. Post WM, Kwon KC (2000) Soil carbon sequestration and land-

use change: processes and Potential. Global Change Biol

6:317328

67. Luo Z, Wang E, Sun OJ (2010) Soil carbon change and its

responses to agricultural practices in Australian agro-ecosys-

tems: a review and synthesis. Geoderma 155:211223

68. Lal R (2004) Soil carbon sequestration to mitigate climate

change. Geoderma 123:122

69. Lal R (2005) Forest soils and carbon sequestration. For Ecol

Manag 220:242258

70. Davidson EA, Ackerman IL (1993) Changes in soil carbon

inventories following cultivation of previously untilled soils.

Biogeochemistry 20:161193

71. Ellert BH, Gregorich EG (1995) Management-induced changes

in the actively cycling fractions of soil organic matter. In: Mcfee

WW, Kelly JM (eds) Carbon forms and functions in forest soils.

SSSA, Madison, pp 119139

72. Pathak H, Rao DLN (1998) Carbon and Nitrogen mineralisation

from added organic matter in saline and alkali soils. Soil Biol

Biochem 30:695702

73. Shekhawat NS, Phulwaria M, Harish, Rai MK, Kataria V,

Shekhawat S, Gupta AK, Rathore NS, Vyas M, Rathore N,

Vibha JB, Choudhary SK, Patel AK, Lodha D, Modi R (2012)

Bioresearches of fragile ecosystem/desert. Proc Natl Acad Sci

India B 82:319334

74. Lal R (2004) Carbon sequestration in dryland ecosystems.

Environ Manag 33:528544

75. Pal DK, Dasog GS, Vadivelu S, Ahuja RL, Bhattacharyya T

(2000) Secondary calcium carbonate in soils of arid and semi-

arid regions of India. In: Lal R, Kimble JM, Eswaran H, Stewart

BA (eds) Global climate change and pedogenic carbonates.

Lewis Publishers, Boca Raton, pp 149185

76. Sahrawat KL (2003) Importance of inorganic carbon in

sequestering carbon in soils of the dry regions. Curr Sci

84:864865

77. Sahrawat KL, Bhattacharyya T, Wani SP, Chandran P, Ray SK,

Pal DK, Padmaja KV (2005) Long-term lowland rice and arable

cropping effects on carbon and nitrogen status of some semi-arid

tropical soils. Curr Sci