Carbon sequestration in grassland systems

Abstract

Globally soils contain around twice the amount of carbon in the atmosphere and thrice in vegetations. Therefore, soil is both 'a source and a sink' for greenhouse gases and balance between the functions is very delicate. The gases move continuously from one pool to another maintaining balance in different pools of the ecosystem. Appropriate management of soil offers to the potential to provide solutions for each of the challenges related to food security and climate change. The estimated carbon sequestration potential of world soils lies between 0.4 to 1.2 Gt per year which includes 0.01-0.30 Gt per year from grasslands. Carbon sequestration can be enhanced in grasslands through grazing management, sowing favorable forage species, fertilizer application and irrigation, restoration of degraded grasslands etc. However, there are certain limitations that hinder in adopting the practices for enhancing carbon sequestration in grasslands. The limitations include continuous degradation of grasslands, changing climate, paucity of information on carbon stock of grasslands from developing countries, disagreement on systems for documenting carbon stock changes over a period of time, hindrance in policy implementations etc.

Full text

Range Mgmt. & Agroforestry 35 (2) : 173-181, 2014
ISSN 0971-2070
Carbon sequestration in grassland systems
P. K. Ghosh* and S. K. Mahanta
ICAR-Indian Grassland and Fodder Research Institute, Jhansi-284003, India
*Corresponding author email: ghosh_pk2006@yahoo.com
Abstract
Globally soils contain around twice the amount of carbon
in the atmosphere and thrice in vegetations. Therefore,
soil is both ‘a source and a sink’ for greenhouse gases
and balance between the functions is very delicate. The
gases move continuously from one pool to another
maintaining balance in different pools of the ecosystem.
Appropriate management of soil offers to the potential to
provide solutions for each of the challenges related to
food security and climate change. The estimated carbon
sequestration potential of world soils lies between 0.4 to
1.2 Gt per year which includes 0.01-0.30 Gt per year from
grasslands. Carbon sequestration can be enhanced in
grasslands through grazing management, sowing
favorable forage species, fertilizer application and
irrigation, restoration of degraded grasslands etc.
However, there are certain limitations that hinder in
adopting the practices for enhancing carbon
sequestration in grasslands. The limitations include
continuous degradation of grasslands, changing climate,
paucity of information on carbon stock of grasslands from
developing countries, disagreement on systems for
documenting carbon stock changes over a period of time,
hindrance in policy implementations etc.
Keywords: Carbon sequestration, Grasslands,
Management practices, Limitations
Grasslands cover around 3.5 billion ha area, represent-
ing 26 percent of the world land area and 70 percent of
the world agricultural area, and containing about 20 per-
cent of the world’s soil carbon stocks (Conant, 2010).
Many countries are rich in grassland resources (Table
1). People depend on these grassland resources for food
and forage production. Around 20 percent of the world’s
native grasslands have been converted to cultivated crops
but still a significant portion of the milk (27%) and meat
(23%) produced in the world comes from grasslands. In
fact, the livestock industry is primarily based on such
grasslands and provides livelihoods for about 1 billion of
the world’s poorest people and one-third of global prot-
-ein intake (FAO, 2006). In some developing countries,
the livestock sector even accounts for 50-80 percent of
GDP (World Bank, 2007). But these grasslands is now
under pressure to produce more and more livestock by
grazing more intensively, specially in rangelands of de-
veloping countries, which are vulnerable to climate
change and are expected nonetheless to supply a sub-
stantial quantity of the meat and milk in years ahead. As
a result of past practices, 7.5 percent of the world’s grass-
lands have already been degraded due to overgrazing.
Cultivation of native grasslands contributes to the trans-
fer of about 0.8 Mg of soil C to the atmosphere annually.
Soil organic matters lost due to conversion of native
grasslands to cultivation are also extensive. Removal of
large amounts of aboveground biomass, continuous
heavy stocking rates and poor grazing management prac-
tices are important human-controlled factors that influ-
ence grassland production and have led to the depletion
of soil carbon stocks. However, good grassland man-
agement can potentially reverse historical soil carbon
losses and sequester substantial amounts of carbon in
soils. Studies have indicated that improved grazing man-
agement can lead to greater forage production, more
efficient use of land resources, and enhanced profitabil-
ity and rehabilitation of degraded lands.
Global carbon cycle and grasslands
Carbon dioxide (CO2) is one of many greenhouse gases
that keep warm the atmosphere by trapping heat radiating
from earth. This trapped heat is called the greenhouse
effect and without it the earth would have been about 33
0C colder. In the recent past there is increase in
concentration of carbon dioxide as a result of
anthropogenic activities. As we burn more and more fossil
fuels to power our vehicles, keep our industry running
and make our homes more comfortable, we are
increasing concentrations of greenhouse gases in the
atmosphere. At present, human activity adds about seven
billion tones of carbon dioxide into the air every year.
Indeed, the global carbon cycle is closely linked to the
greenhouse effect. Carbon moves continuously among
Invited article

Carbon sequestration
174
air, plants, and soils and changes to any of these three
components invariably affects the other components.
Through the process of photosynthesis, plants capture
atmospheric carbon dioxide and store the carbon in their
living tissue, both above and below the ground. Some of
this organic carbon becomes part of the soil as plant
parts die and decompose, and some is lost back to the
atmosphere as gaseous carbon emissions through plant
respiration and decomposition. Herbaceous grassland
plants contribute to grassland carbon stores primarily
by the growth and sloughing of roots, a cyclical process
in the case of perennial species and especially when
grazed. When such a plant is pruned back, as with
grazing, a roughly equivalent amount of roots dies off
(adding carbon to the soil) because the remaining top-
growth can no longer photosynthesize enough food to
feed the plant’s entire root system. If given adequate
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
Australia
Russian federation
China
United States
Canada
Kazakhastan
Brazil
Argentina
Mongolia
Sudan
Angola
Mexico
South Africa
Ethiopia
Congo Dem. Rep.
Iran
Nigeria
Namibia
Tanzania, United Republic
Mozambique
Chad
Mali
Central African Republic
Somalia
India
Zambia
Botswana
Saudi Arabia
Oceania
Europe
Asia
North America
North America
Asia
South America
South America
Asia
Sub-Saharan Africa
Sub-Saharan Africa
C. America & Crib.
Sub-Saharan Africa
Sub-Saharan Africa
Sub-Saharan Africa
Middle East & N. Africa
Sub-Saharan Africa
Sub-Saharan Africa
Sub-Saharan Africa
Sub-Saharan Africa
Sub-Saharan Africa
Sub-Saharan Africa
Sub-Saharan Africa
Sub-Saharan Africa
Asia
Sub-Saharan Africa
Sub-Saharan Africa
Middle East & N. Africa
7,704,716
16,851,600
9,336,856
9,453,224
9,908,913
2,715,317
8,506,268
2,781,237
1,558,853
2,490,706
1,252,365
1,962,065
1,223,084
1,132,213
2,336,888
1,624,255
912,351
825,606
945,226
788,938
1,167,685
1,256,296
621,192
639,004
3,090,846
754,676
579,948
1,958,974
6,576,417
6,256,518
3,919,452
3,384,086
3,167,559
1,670,581
1,528,305
1,462,884
1,307,746
1,292,163
1,000,087
944,751
898,712
824,795
807,310
748,429
700,158
665,697
658,563
643,632
632,071
567,140
554,103
553,963
535,441
526,843
508,920
502,935
85.36
37.13
41.98
35.80
31.97
61.52
17.97
52.60
83.89
51.88
79.86
48.15
73.47
72.85
34.55
46.08
76.74
80.63
69.67
81.58
54.13
45.14
89.20
89.69
17.32
69.81
87.75
25.67
Grassland
area (%)
Total land
area (km2)
Total
grassland
area (km2)
Rank Country Region
Table 1. Countries rich in grassland resources
Source: Murphy-Bokern (2009)
rest from grazing, both roots and top-growth recover and
the cycle begins again. With good grazing management,
perennial plants can live and reproduce for many years
with an ongoing cycle of pruning, root-sloughing, and
regeneration, contributing more and more carbon to the
soil indefinitely.
Globally, there is about two times as much carbon in soil
organic matter (SOM) than there is in the atmosphere
and as a result, a relatively small shift in soil organic
matter can have a large impact on carbon dioxide in the
air (Janzen et al., 2002). To stop rising concentrations of
carbon dioxide in the atmosphere, countries around the
world are actively seeking means and ways to increase
carbon storage on land. The large amount of land area
covered by grasslands as well as the relatively unexplored
potential for grasslands soils to store carbon has incre-

Ghosh & Mahanta
-ased interest in the carbon cycles of these ecosystems.
Areas where more carbon is absorbed than given off are
referred to as carbon sinks and include areas such as
native grasslands, forests and agro-ecosystems. In global
context, grasslands store around 34% of the global
terrestrial stock of carbon while forests store around 39%
and agro-ecosystems around 17 percent (World
Resources Institute, 2000). Unlike forests where the
vegetation is the primary source of carbon storage, most
of the grassland carbon is stored in the soil.
Carbon in grasslands
Tropical and temperate natural grasslands play a
significant role in the global carbon cycle but poorly
recognized. Grassland soil carbon stocks amount to at
least 10% of the global total, but other sources indicated
it is up to 30% of world soil carbon. Comparisons of SOM
stocks between biomes and different studies are
complicated by divergent definitions and procedures, but
at least it can be said that grassland soils represent a
significant carbon pool, of the order of 200-300 Pg
(Scurlock and Hall, 1998). In fact, grassland ecosystems
are far from uniform, ranging from the natural savannas
of Africa to the prairies and steppes of North America and
Russia, from the derived savannas found on many
continents to the sown pastures of Europe and Latin
America.
One of the reasons for the intensive use of grasslands is
the high natural soil fertility. Grasslands characteristically
have high inherent SOM content, averaging 333 Mg/ha.
SOM, an important source of plant nutrients, influences
the fate of organic residues and inorganic fertilizers,
increases soil aggregation, which can limit soil erosion,
and also increases cat ion exchange and water holding
capacities. It is a key regulator of grassland ecosystem
processes. Thus, a prime underlying goal of sustainable
management of grassland ecosystems is to maintain
high levels of SOM and soil carbon stocks. Primary
production in overgrazed grasslands can decrease if
herbivory reduces plant growth or regeneration capacity,
vegetation density and community biomass, or if
community composition changes. If carbon inputs to the
soil in these systems decrease because of decreased
net primary production or direct carbon removal by
livestock, soil carbon stocks will decline. Similar to carbon
sequestration in forests or agricultural land,
sequestration in grassland systems primarily, but not
entirely in the soils, is brought about by increasing carbon
inputs. It is widely accepted that continuous excessive
grazing is detrimental to plant communities and soil
carbon stocks. When management practices that deplete
soil carbon stocks are reversed, grassland ecosystem
carbon stocks can be rebuilt, sequestering atmospheric
CO2.
Grasslands as carbon sinks
An estimate has indicated that globally, 0.2 - 0.8 Gt CO2
per year can be sequestered in grassland soils by 2030.
Although both fertilization and fire management can
contribute to carbon sequestration, most of the potential
regression equation developed between TOC and
biomass production of twelve legumes showed
significantly high R2 (0.61) value except Alysicarpus
ragouts and Clitoral ternate (Rai et al., 2013).
Adoption of agroforestry practices: Agroforestry like
silvipasture system enhances carbon uptake by length-
ening the growing season, expanding the niches from
which water and soil nutrients are drawn and, in the case
of nitrogen (N)-fixing species, enhancing soil fertility.
When silvipasture systems are introduced in suitable
locations, carbon is sequestered in the tree biomass
and tends to be sequestered in the soil as well (Table 3).
Improved management in existing agroforestry systems
can sequester 0.012 Tg C per year, while conversion of
630 million ha of unproductive or degraded croplands
and grasslands to agroforestry can sequester as much
as 0.59 Tg C annually by 2040. Indeed, the C sequestra-
tion potential of agroforestry system has been found to
vary between 12 and 228 Mg/ha with a median value of
95 Mg/ha. Agro forestry systems are believed to have a
higher potential to sequester C than pastures or field
crops (Kirby and Potvin, 2007). This hypothesis is based
on the assumption that introduction of trees in croplands
and pasture would result in greater net aboveground as
well as below ground C sequestration (Haile et al., 2008;
Singh and Gill, 2014). Abundant litters and/or pruning
biomass returned to the soil combined with the decay of
roots, contribute to the improvement of soil physical and
chemical properties. The land-use systems ranked in
terms of their SOC content are in the order of forests>agro
forestry>tree plantations> arable crops.
Restoration of degraded lands and introduction of
grasses on arable lands: Restoring degraded lands
enhances production in areas with low herbage
productivity, increasing carbon inputs and sequestering
carbon. It is now well documented that practices like
silvipastures, hortipastures, improved grazing
management etc could lead to greater forage production,
more efficient use of land resources, and enhanced
175

sequestration in non-degraded grasslands is due to
changes in grazing management practices. Estimated
rates of carbon sequestration per unit are lower than
those for sequestration on agricultural land, but
sequestration potential is comparable to that of croplands
because grasslands cover such a large portion of the
earth’s surface. Management practices used to increase
livestock forage production also have the potential to
augment soil carbon stocks, thus sequestering
atmospheric carbon in soils. Methods of improved
management practices include intensive grazing
management, fertilization, irrigation, and sowing of
favourable forage grasses and legumes. Improved
grazing management (management that increases
production) leads to an increase of soil carbon stocks by
an average of 0.35 Mg C /ha/yr.
Grazing management: It is now proven fact that well
managed grazing stimulates growth of herbaceous
species and improves nutrient cycling in grassland
ecosystems. Increased early season photosynthesis (as
measured by chamber CO2 exchange rates) was
observed on grazed mixed-grass prairie compared to
ungrazed prairie/exclosures (LeCain et al., 2000).
Similarly it was reported that grazing stimulates
aboveground production (McNaughton et al., 1996) and
increases tillering and rhizome production (Schuman et
al., 1990). Besides, grazing may stimulate root respiration
and root exudation rates (Dyer and Bokhari, 1976).
Livestock defecation and urination also significantly affect
nutrient cycling and relocation in grazing systems. All of
these factors might have contributed to the observed
increases in soil C storage. The grazing process also
affects the rate of turnover/decomposition of the
aboveground component of the plant community (litter,
standing dead). Under light and heavy grazing shoot
turnover was found to be 36 and 39% compared to 28%
in ungrazed exclosures. It was concluded that animal
traffic might enhance the physical breakdown, soil
incorporation and rate of decomposition of litter and
standing dead plant material (Schuman et al., 2002).
Again grazing management can lead to decreased
carbon removal if grazing intensities are reduced or if
grazing is deferred while forage species are most actively
growing. Sustainable grazing management can thus
increase carbon inputs and carbon stocks without
necessarily reducing forage production. Grazing
management can also be used to restore productive
forage species, further augmenting carbon inputs and
soil carbon stocks.
Fertilizer application and irrigation: In many cases
grasslands were found to be deficient in N and they have
exhibited increased production and increased water-use-
efficiency in response to N fertilizations. Fertilizer
application stimulated litter production, thereby
enhancing soil C storage. N fertilizer application
increased plant production of the tall grass prairie and
resulted in an increase in soil C of 1.6 Mg/ha (Rice, 2000).
Application of 40 kg N/ha caused significant increase in
dry forage yield and total organic carbon (TOC) content of
the soil in natural pasture (Rai et al., 2013). The rate of
TOC buildup was 1.5 times more than the natural
grassland (0.74 g/kg/yr). Application of other nutrients,
where they were deficient, also enhanced organic C
storage (Conant et al., 2001). However, the benefits of
increased soil C sequestration must be compared to the
C costs of fertilizer production in order to determine the
net effect on the atmosphere (Schuman et al., 2002).
Similarly, application of water/irrigation can enhance water
and nitrogen balances leading to increase in plant
productivity and carbon inputs.
Fire management: In some grassland, fire management
also influence the amount of C stored in biomass by
altering the density or encroachment of woody species.
Burning of biomass produces charcoal, a form of C very
resistant to decomposition, which can account for a
significant portion of the C stored in some grassland
soils. Annual burning and grazing on the tall grass prairie
were found to increase in soil C storage of 2.2 Mg/ha
after 10 years, which accounted for an increase of 0.22
Mg/ha/year (Rice, 2000).
Sowing of favourable forage species: Introduction or
promoting improved and favourable forage species that
are better adapted to local climate, more resilient to
grazing, more resistant to drought and able to enhance
soil fertility can lead to increased biomass production.
Enhancing production ultimately results in greater carbon
inputs and carbon sequestration. It was observed that
introduction of Macroptilium lathyroides in natural
grassland resulted in 1.29 times increase in TOC as
compared to natural grassland. Other legumes also
showed increased mixed biomass production and soil
TOC (Table 2). Maximum increase in TOC and soil organic
carbon (SOC) buildup rate was observed with
micropitilum lathyroildes (42%) followed by Stylosanthes
guianensis. Except Alysicarpus ragouts and Clitoris
tenanted the rate of TOC buildup was higher in legume
incorporated grassland than the natural grassland. The
176
Carbon sequestration

Table 2. Effect of range legumes on forage yield of natural
grassland and organic carbon in the soil
Source: Rai et al.( 2013)
profitability and rehabilitation of degraded lands and
restoration of ecosystem services. Silvipasture system
had a great role in rehabilitating the degraded land. Eight
different tree species were tested as component of
Cenchrus ciliaris based silvipasture for their relative
efficacy on herbage production against Cenchrus ciliaris
as pasture (Table 4). Photosytnthetically active radiation
(PAR) reaching to the ground affected the performance
of the understory vegetation under different tree species.
Relative radiation infiltration through tree species varied
between 58-78%, lowest being in Acacia nilotica and
highest in Eucalyptus sp. Variation in litters fall and
biomass production under different trees affected the
TOC content of the surface soil. In the same period
Leucaena leucocephala caused 3.4 times increase in
the TOC over the initial values. TOC content was
maximum in Leucaena leucocephala followed by
Albizzia lebbeck, Dichrostachys mutants, Albizzia
proceura and others.
It has been observed that different arable land-use
systems result in very rapid declines in SOM. Much of
this loss in soil organic carbon can be attributed to
reduced inputs of organic matter, increased
decomposability of crop residues, and tillage effects that
decrease the amount of physical protection to
decomposition. Including grass in the rotation cycle on
arable lands can increase production return organic
matter (when grazed as a forage crop), and reduce
disturbance to the soil through tillage. Thus, integrating
grasses into crop rotations can enhance carbon inputs
and reduce decomposition losses of carbon, each of
which leads to carbon sequestration (Conant, 2010).
Based on present management practices majority of the
grasslands in temperate regions are considered as C
sinks (Abberton et al., 2010; Acharya et al., 2012). It was
observed that the land-use change from arable cropping
to grassland results in an increase of soil C of 30 g C/m2/
year. Direct measurements of soil C indicated a C
sequestration of 45–80 g C/m2/year. In France, meta
analysis has shown that on average, for a 0–30 cm soil
depth, C sequestration reached 44 g C/m2/year over 20
years. This is approximately half the rate (95 g C/m2/s/
year) at which C is lost over a 20–year period following
conversion of permanent grassland to an annual
cropland (Soussana et al., 2004). Temperate pastures
in the northeast USA are highly productive, they can
potentially act as significant C sinks. However, these
pastures are subjected to relatively high biomass
removal as hay or through consumption by grazing
animals. Consequently, for the first eight years after
conversion from ploughed fields to pastures, they were
a small net sink for C at 19 g C/m2/year but when biomass
removal and manure deposition were included to
calculate net biome productivity, the pasture was a net
source of 81 g C/m2/year. It was concluded that heavy
use of biomass produced on grasslands prevents them
from becoming C sinks. C sequestration potential for
managed grasslands in USA was found to be 10–90 g
C/m2/year depending on the level of change. Based on
the Kyoto Protocol on Climate Change for Europe, C
sequestrations were 52 g C/m2/year for established
grassland and 144 g C/m2/year for conversion of arable
land to grassland. Country estimates varied from a source
of 4.5 g C/m2/year for Portugal to sequestration of 40.1 g
C/m2/year for Switzerland. However, estimates based on
the full GHG balance for grasslands across Europe
indicated that most grassland areas are net sources for
GHGs in terms of their total global warming potential
because the beneficial effect of sequestering C in soils
is outweighed by the emissions of N2O from soils and
CH4 from livestock (Levy et al., 2007).
Limitations of carbon sequestration in grasslands
All ecosystems namely forest ecosystem, agro-
ecosystem, grassland ecosystem, etc. take up
atmospheric CO2 and mineral nutrients and transform
them into organic products (Conant, 2010). In grasslands,
carbon assimilation is directed towards the production
177
Ghosh & Mahanta
Forage
DM
yield
(Mg/ha)
TreatmentsSOC
buildup
rate
(g/kg/yr)
TOC
(g/kg)
Natural grassland
Alysicarapus rugosus
Atylosia scarabaeoides
Clitoria ternatea
Dolichos lablab
Desmodium tortusum
Glycine javanica
Macroptilium atropurpureum
Macroptilium lathyroides
Mimosa invisa
Stizolobium deeringianum
Stylosanthes guianensis
Stylosanthes humilis
Vigna luteola
3.3
4.2
4.1
4.4
4.7
4.2
3.8
4.1
4.9
3.7
4.0
4.2
4.0
4.2
7.78
7.55
9.22
7.47
10.07
9.72
8.58
7.99
11.05
8.91
8.10
10.53
8.22
9.15
0.74
0.67
1.22
0.64
1.51
1.39
1.01
0.81
1.83
1.12
0.85
1.66
0.89
1.20

Agro forestry (Pseudotsuga menzeiscii + Trifolium subterraneum)
Agrisilviculture (Gmelina arborea + field crops)
Silvopastoral system: (Acacia mangium + Arachis pintoi)
Silvopastoral system: (Brachiaria brizantha + Coradia alliodora +
Guazuma ulmifolia)
Alley cropping system: Erythrina poeppigiana + maize and bean
(Phaseolus vulgaris)
Fodder bank (Gliricidia sepium, Pterocarpus lucens and P. erinaceus)
Tree based pastures: slash pine (Pinus elliottii) + bahiagrass
(Paspalum notatum)
Gliricidia sepium + maize (Zea mays)
11
5
10-16
10-16
19
6-9
8-40
10
0-45
0-60
0-100
0-100
0-40
0-100
0-125
0-200
95.89
27.4
173
132*
1.62
33.4
6.9-24.2
123
Agro forestry system/species Age (yr) Soil C
(Mg/ha)
Soil
depth
(cm)
Table 3. Soil carbon sequestration in different agroforestry systems
*Carbon-sequestration potential, which is based on C-stock estimates; Source: Nair et al. (2009); Rai et al. (2013)
Table 4. Influence of tree species on forage yield, leaf litter and TOC content of soil in Cenchrus ciliaris silvipasture
Albizzia lebbeck
Albizzia procera
Acacia tortilis
Acacia nilotica
Leucaena leucocephala
Dichrostachys nutan
Hardwikia binata
Eucalypytus sp.
Open (without tree)
Initial*
5.63
4.21
4.36
4.47
5.28
5.46
5.34
3.87
5.95
0.62
3.40
2.30
0.70
2.20
3.20
2.00
1.50
2.00
-
-
9.31
8.37
7.44
7.98
10.37
9.04
7.18
6.91
6.91
3.05
3.04
2.73
2.43
2.61
3.39
2.96
2.35
2.26
2.26
Dry forage
yield (Mg/ha)
Leaf litters
(Mg/ha)
TOC
(g/kg soil)
Relative
increase
Tree
*Initial vegetation was comprised of Aristida sps., Eromopogon faveolatus and Heteropogon contortus; Source: Hazra (1995)
of fibre and forage by manipulating species composition
and growing conditions. These ecosystems are a major
source and sink for the three main biogenic greenhouse
gases (GHG); CO2, nitrous oxide (N2O) and methane
(CH4). In undisturbed ecosystems, the carbon balance
tends to be positive: carbon uptake through
photosynthesis exceeds losses from respiration. Thus
the basic processes governing the carbon balance of
grasslands are similar to those of other ecosystems:
the photosynthetic uptake and assimilation of CO2 into
organic compounds and the release of gaseous carbon
through respiration (primarily CO2 but also CH4). However,
biomass in grassland systems, being predominantly
herbaceous (i.e. non- woody), is a small, transient carbon
pool (compared to forest) and hence soils constitute the
dominant carbon stock. Grassland systems thus can be
productive ecosystems, but have certain limitations as
discussed below (Mengistu and Mekuriaw, 2014):
Changing climate: Grasslands are highly vulnerable to
climate change. Primary production in natural grasslands
is relatively low, varies considerably from place to place,
and is strongly limited by precipitation. Even where
rainfall is high (upto 900 mm of precipitation per year),
almost all of the precipitation falls during distinct rainy
seasons and evapotranspiration demands exceed
precipitation during most of the year. Hence, precipitation,
and thus production, varies considerably from year to
year, with coefficients of variation averaging 33%, and as
high as 60% in some of the drier areas. But grassland
management practices that sequester carbon tend to
make systems more resilient to climate variation and
climate change. Increased SOM (and carbon stocks)
improves yields, enhances soil fertility, reducing reliance
on external nitrogen inputs. Surface cover, mulch and
SOM all contribute to a decrease in inter-annual variation
in yields; and practices that diversify cropping systems,
178
Carbon sequestration

such as grass and forage crops in rotation, sequester
carbon and enhance yield consistency.
Continuous degradation of grasslands: Grassland
degradation is continuously occurring under all climates
and farming systems, and is generally related to a
mismatch between livestock density and the capacity of
the pasture/grassland to be grazed and trampled.
Mismanagement is common. Ideally the land/livestock
ratio should becontinuously adjusted to the conditions
of the pasture, especially in dry climates where biomass
production is erratic, yet such adjustment is rarely
practiced. This is particularly the case of arid and semi-
arid regions where communal grazing is prevalent. In
these areas, increasing population and encroachment
of arable farming on grazing lands have severely
restricted the mobility and flexibility of the herds, which
enabled this adjustment. Grassland degradation results
in a series of environment problems, including soil
erosion, degradation of vegetation, carbon release from
organic matter decomposition, loss of biodiversity owing
to habitat changes and impaired water cycles.
Limited information on carbon stock of grasslands
from developing countries: There is paucity of
information/data from developed countries which limits
to the creation of robust accounting systems that offer
the same utility for quantifying soil carbon sequestration
in developed and developing countries. In fact, systems
that integrate measurement and mechanistic modeling
require robust sources of data that reflect the range of
potential management practices. A variety of efforts are
under way across the developed world to build up, test
and implement such systems. Lack of accurate
information on these aspects can lead to greater
uncertainty in estimates of soil carbon stock changes,
and ultimately result in climate-driven bias because
majority of studies from developed country are related to
temperate regions.
Disagreement on systems for documenting carbon
stock changes: Soil carbon stocks of an ecosystem vary
as a function of soil texture, landscape position, drainage,
plant productivity and bulk density, all of which vary
spatially, and create heterogeneity that makes it difficult
to quantify changes in soil carbon stocks over time.
However, methods for analyzing soil carbon concentration
of a given sample are well established and easily carried
out with high precision and minimal analytical error.
During quantifying soil carbon stock over time, sampling
error can be large and the cumulative effects of managing
small net sinks to mitigate fossil-fuel emissions will have
to be understood, analyzed, monitored, and evaluated in
the context of larger, highly variable, and uncertain
sources and sinks in the natural cycle. Thus, the main
challenge in documenting plot-level changes in soil
carbon stocks is not in measuring carbon, rather
designing an efficient, cost-effective sampling and
carbon stock estimation system, which also need to be
agreed by different stakeholders.
Policy implementation issues: In spite of win–win
situations in which practices that sequester carbon in
grasslands also lead to enhanced productivity, policies
to encourage adoption of practices that sequester carbon
in grasslands lag behind policies for forest and
agricultural lands. This is particularly true for practices
that promote increased primary productivity or livestock
production and practices that arrest grassland
degradation. Reducing emissions from grassland is not
only likely to pay dividends in maintaining carbon stocks,
but also in sustaining the livelihoods of people making a
living from grasslands. Again smallholder households
from developing countries represent a serious limitation
for documenting carbon sequestration from grasslands.
In many countries pastoralists also occupy substantial
portions of the land area with the potential to sequester
carbon in grasslands. However, pastoralists are often
socially marginalized and with insecure land tenure rights,
making it very difficult for participation in carbon markets.
Moreover, the strength and ability of government
institutions required to implement such schemes is often
insufficient in the countries and areas where they are
most needed.
Conclusion
The rates of carbon sequestration and soil organic carbon
(SOC) values were found to vary among the grassland
systems, but it is understandable that grassland systems
provide valuable carbon storage. Increasing SOC
storage through land use changes and land
management is a low cost and environmentally beneficial
way of sequestering substantial amounts of atmospheric
CO2 and need to be practiced. However, further research
is needed across multiple locations addressing key
ecological processes and mechanisms to determine the
principal drivers affecting C sequestration (Derner and
Schuman, 2007). The continued development of
sophisticated in-situ and laboratory equipment to
accurately detect small but ecologically-important
changes in soil C and its components will open new
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horizons for future experimentation and verification of C
change due to management options or climatic variances.
There is a need to move from the basic approach of soils
and soil ecology to a more fundamental and functional
understanding of the processes and mechanisms that
affect SOC dynamics and how they are influenced by
land management, environment and their interaction.
Newly emergent fields of soil microbial ecology should
provide additional in sight into microbial function and
processes that affect C sequestration under normal and
the widely fluctuating precipitation patterns found in arid
and semi-arid environments. Hence, as better research
information becomes available, a more thorough and
accurate estimation of C sequestration potential of
grasslands can be achieved in near future.
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