Journal of Environmental Protection, 2010, 1, 207-215
doi:10.4236/jep.2010.13025 Published Online September 2010 (http://www.SciRP.org/journal/jep)
207
Cropping Systems to Improve Carbon
Sequestration for Mitigation of Climate Change
Qingren Wang1,2, Yuncong Li1,2, Ashok Alva3
1Tropical Research and Education Center, Homestead, USA; 2Department of Soil and Water Science, University of Florida, Gainesville,
USA; 3USDA, ARS, Vegetable and Forage Crops Research, Prosser, USA.
Received April 23rd, 2010; revised May 20th, 2010; accepted May 25th, 2010.
ABSTRACT
The recent trend of an increase in the concentration of greenhouse gases (GHGs) in the atmosphere has led to an ele-
vated concern and urgency to adopt measures for carbon (C) sequestration to mitigate the climate change. Among all
GHGs, carbon dioxide (CO2) is the most important one which occurs in the greatest concentration and has the strong-
est radiative forcing among all. Reducing the release of CO2 to the atmosphere through “green energy” technologies or
fossil fuel energy alternatives, such as wind, solar and hydraulic energies, is a major challenge. However, removal of
atmospheric CO2 by terrestrial ecosystems via C sequestration and converting the sequestered C into the soil organic C
has provided a great opportunity for shifting GHG emission to mitigate the climate change. Soil is an ideal reservoir
for storage of organic C since soil organic C has been depleted due to land misuse and inappropriate management
through the long history. To optimize the efficiency of C sequestration in agriculture, cropping systems, such as crop
rotation, intercropping, cover cropping, etc., play a critical role by influencing optimal yield, total increased C seques-
tered with biomass, and that remained in the soil. As matter of fact, soil C sequestration is a multiple purpose strategy.
It restores degraded soils, enhances the land productivity, improves the diversity, protects the environment and reduces
the enrichment of atmospheric CO2, hence shifts emission of GHGs and mitigates climate change.
Keywords: Carbon Dioxide, Carbon Sequestration, Climate Change, Cropping System, Greenhouse Gas
1. Introduction
Rapid increase in carbon dioxide (CO2) concentration in
the atmosphere associated with other greenhouse gases
(GHGs), such as nitrous oxide (N2O) and methane (CH4),
since the industrial revolution is a major concern with
respect to its impact on climate change. Therefore, there
is an urgency to adopt effective measures for mitigating
the threat of global climate change [1]. The concentration
of CO2 in the atmosphere increased from 280 to 387
ppmv in 1750 to 2007, and continues to increase at the
rate of 1.5 ppmv per year. During the same period, N2O
was increased from 270 to 314 ppbv, and CH4 increased
from 700 to 1745 ppbv (Table 1) [2,3]. Increased con-
centration of GHGs impacts the temperature of the Earth
by absorbing and emitting radiation within the thermal
infrared range. The anthropogenic enrichment of GHGs
in the atmosphere and the cumulative radiative forcing
(factors affect the balance between incoming solar radia-
tion and outgoing infrared radiation within the Earth’s
atmosphere) has led to a substantial increase in global
surface temperature. The major sources to enrich the at-
mospheric GHGs are fossil fuel combustion and land use
changes. For instance, about 25% of CO2, 50% of CH4
and up to 70% of N2O released globally through human
sources [4]. Increased frequency of natural disasters,
such as floods, tsunami, hurricane, etc., during the recent
years might be attributed to the climate change associ-
ated to increased accumulation of GHGs in the atmos-
phere. The global surface temperature increased by 0.6℃
since the late 19th century with a current average warm-
ing rate of 0.17℃ per decade [2]. Such temperature in-
crease would considerably alter the distribution of pre-
cipitation, e.g., 0.5-1% of precipitation increase per dec-
ade in the most of Northern Hemisphere and 0.3% in-
crease in the tropics and sub-tropics [1]. Consequently,
land productivity, biomass accumulation, biodiversity,
and the whole environmental system would be negatively
impacted. The US EPA has released its final findings on
greenhouse gases and has declared that “GHGs threaten

Cropping Systems to Improve Carbon Sequestration for Mitigation of Climate Change 208
the public health and welfare of the American people”
[5]. Therefore, it is urgent to adopt practical and effective
approaches to controlling the GHG emission for mitigat-
ing global climate change for a sustainable development
of the environment. The objective of this review is to
briefly elucidate the main sources of GHG emission and
particularly address the sustainable development of crop-
ping systems for carbon sequestration to mitigate the
threat of the global climate change.
2. Major Sources of GHGs and their
Contributions
A major source of CO2 in the atmosphere is fossil fuel
combustion and cement production. Total emission from
the above source increased from 5.4 ± 0.3 to 7.9 Pg C
yr-1 in the global scale in 1980s; 6.3 ± 0.4 Pg C yr-1 from
the same source in the 1990s; and up to 7.9 Pg C yr-1
from 1980 to 2005 [1,6]. Over 70% of the total emission
is from combustion of liquid and solid fuels. Land use
change, such as deforestation, land degradation, etc. also
contribute to anthropogenic CO2 emission [1,7,8], which
has been constant at about 1.7 ± 0.8 Pg C yr-1 during
1980s through 2005. The global emission of carbon is
estimated at 270 ± 30 Pg due to fossil fuel combustion
and 136 ± 55 Pg due to land use change and soil cultiva-
tion during the last 150 years [1,9-11]. The CO2 emission
rate has increased dramatically since 2000, as evident
from an increase from 1.1% during 1990-1999, to > 3%
since 2000. This is attributed to increased energy demand
with an increase in gross domestic product (GDP) [6].
Besides CO2, there are some other gases that can con-
tribute the global climate change. However, the contribu-
tion to the greenhouse effect by different gases is deter-
mined by the characteristics of the gas and its abundance.
For instance, CH4 is about 8 fold stronger than CO2 on a
molecule-for-molecule basis, however the net contribu-
tion of CH4 to the greenhouse effect is much smaller be-
cause its lower concentration than that of CO2. From the
radiative forcing of the main GHGs, CO2, CH4 and N2O
(Table 1), it is also evident that CO2 is the predominant
GHG. Similarly, the other three GHGs named in the
Kyoto Protocol, hydrofluorocarbons, perfluorocarbons,
and sulfur hexafluoride (US EPA, 2009) may impact the
climate change but their radiative forcing is considerably
low due to their very low concentrations as compared to
that of CO2. Therefore, it is important to control the con-
centration of atmospheric CO2 by reducing its emission
by using fossil fuel more efficiently than ever before, and
by adoption of “green-energy technologies”, such as fos-
sil fuel alternatives, solar, wind, hydraulic energies, etc.
On the other hand, terrestrial plants play a critical role to
remove the atmospheric CO2 via their photosynthesis and
assimilation of CO2 to produce plant biomass.
3. Carbon Sequestration for Shifting GHG
Mitigation
Carbon sequestration by terrestrial vegetation, as one of
the most effective options for shifting the GHG emission
has been identified by the Intergovernmental Panel on
Climate Change [2]. Terrestrial ecosystems associated
with land use and soil management play an important
role in the global C budget [1]. For example, the current
terrestrial sink for carbon is estimated to hold 550-700 Pg
of carbon in the world’s vegetation and 1200-1600 Pg of
soil organic carbon [8]. This has shown a great potential
to offset the total amount of C emitted and accumulated
in the atmosphere through all possible sources.
Therefore, the United Nations (UN) Framework Con-
vention on Climate Change (UNFCC) has setup 3 major
conventions to combat desertification, land degradation,
and improving biodiversity. Furthermore, the Kyoto
Protocol, negotiated in 1997, provides the framework for
activities aimed at reducing emissions of GHGs. The
protocol contains a joint commitment of the industrial-
ized countries to reduce their GHG emissions by at least
5% below the levels of 1990, over the period of 2008-
2012 [12].
The removal of atmospheric CO2 by increasing the as-
similation of CO2 with terrestrial vegetation, retaining C
and enhancing the transformation of atmospheric C to
plant biomass and soil organic matter along with reduc-
ing GHG emission has become a worldwide strategy to
Table 1. Increase of dominant greenhouse gases and their radiative forcing [3].
Greenhouse gases Preindustrial level Current level Increase Radiative forcing
(W/m2)
CO2 280 ppmv 387 ppmv 107 ppmv 38% 1.46
CH4 700 ppbv 1745 ppbv 1045 ppbv 149% 0.48
N2O 270 ppbv 314 ppbv 44 ppbv 16% 0.15
CFC-12* 0 533 pptv 533 pptv - 0.17
*CFC: chlorofluorocarbon

Cropping Systems to Improve Carbon Sequestration for Mitigation of Climate Change 209
mitigate climate change. However, the efficiency of C
sequestration by various vegetations and management in
various systems differs greatly due to their physiological
characteristics, growth rates, biomass accumulation, and
environmental factors. Therefore, it is important to opti-
mize ecosystems based on various climates and geo-
graphical characteristics to efficiently and effectively
sequester C from the atmosphere and for shifting the mi-
tigation of the climate change.
Regarding the C source and sink in the global ecosys-
tems, there have been a lot of controversial arguments.
Forest is commonly considered as a C sink because the
storage of organic C accumulated in trunks and major
branches of a tree can last longer and the C cycling is
slower than that in annual plants. However, it is only a
“time” issue because the C slowly sequestered in forests
through a long term period might be easily returned to
the atmosphere either through deforestation or through
climate-change induced emissions. As indicated by
Turner et al. [13], significant losses of C are associated
with harvesting either for biomass energy or for wood
products even for intensely managed forests. In the case
of biomass energy, C is lost in one-way emission as a
source through direct fuel combustion of wood. For
wood products, only about 23% of merchantable wood
can be harvested [14] and noncommercial parts of the
tree are burned as slash or left to decompose. Also, a
large fraction of the merchantable wood may become
products with lifetimes of less than 5 years [13]. Wild-
fires, often occurs in forest, can cause an abrupt emission
of CO2 to the atmosphere. In addition, prevailing affore-
station projects may often not be desirable from a social
point of view and it might compete for land use with
food production in agriculture to meet people’s need as
population grows. Afforestation may not be desirable
either from the point of view in improving the biodiver-
sity. Nevertheless, the Kyoto Protocol, in its original
form focuses on forestry activities, such as afforestation
and management, to improve the C sink. However, there
may be a number of problems difficult to be solved for
what are related to such forestry activities, particularly
monitoring and verification, permanence, leakage and
environmental effects in C sequestration by forest. In
addition, safeguarding C stored in aboveground biomass
of forests is difficult due to economic pressure that en-
courages logging for income returns, which definitely
stimulates litter decomposition and CO2 release.
4. Potential and Prospects of Cropping
Systems for C Sequestration
4.1. Soil Organic C Stock for C Sequestration
There are advantages in promotion of degraded agro-eco-
systems as a potential C sink because agriculture occu-
pies a larger portion of global land area (about 35%) than
any other land use (Table 2). Soil organic C (SOC) in
cultivated soils, where it contributes to soil fertility,
might be less tempting to release through overexploita-
tion due to slow decomposition. As matter of fact the
prospects of good crop yields in the future would be jeo-
pardized if the soil fertility cannot be maintained prop-
erly [12]. However, in agro-ecosystems that account a
main proportion of the whole ecosystems, soil fertility is
an important contribution to improve the biomass pro-
duction and as in turn, increases the SOC accumulation
by various vegetations. However, the conversion of the
plant sequestered C to soil organic C, which forms recal-
citrant C, plays a crucial role since the soil C can have a
very stable and long residence time, hundreds and even
thousands of years under most circumstances [15].
Agricultural soils under appropriate management can
contain substantial amounts of soil C in the form of soil
organic matter (SOM). Soils, excluding carbonated rocks,
constitute the largest carbon pool, approximately 1500 Gt,
which is almost three fold greater than that stored in the
terrestrial biomass and twice the amount stored in the
atmosphere [16]. The SOM contributes to plant available
nutrients, buffers environmental stress, improve wa-
ter-holding capacity, and reduce erosion. In addition,
agricultural soils possess potential to restore a consider-
able quantity of sequestered C since a significant amount
of SOC has been lost from the system due to land degra-
dation and mismanagement. Most croplands have lost
30-40 Mg C ha-1, and most degraded soils may have lost
40-60 Mg C ha-1 [17]. Restoration of such quantity of
soil C via C sequestration in agro-ecosystems can, apart
from removing CO2 from the atmosphere, improve the
sustainable production of the agriculture. In addition,
compared to C passively stored in a forest, the SOC in
agricultural soils can actively benefit food production
and improve the agricultural sustainability. However, the
historical C loss from the soil cannot be ignored since
Table 2. Total area of land uses and their distributions
worldwide (adopted from FAO, 2001).
Land use Area (Mha) %
Permanent crops 132 0.9
Arable land 1,369 9.7
Permanent pasture 3,460 24.5
Forest and woodland 4,172 29.6
Agricultural land 4,961 35.2
Total land area 14,094 100.0

Cropping Systems to Improve Carbon Sequestration for Mitigation of Climate Change 210
about 20% of the anthropogenic emissions of CO2 was
contributed by agriculture and land-use change [18].
Much of the historic C loss (about 66-90 Pg C) from the
soil can be restored via C sequestration in 25-50 years [1]
with appropriate land management. Indeed, soil has pos-
sessed a promising potential for C sequestration and C
storage. A summary of soil C sequestration rates in the
crop land of major countries is listed in Table 3, which
shows that the cropland can sequester about 75-208 Tg C
yr-1 in US, 24 Tg C yr-1 in Canada, 90-120 Tg C yr-1 in
the European Union, 105-198 Tg C yr-1 in China and
39-49 Tg C yr-1 in India [4].
4.2. Cropping Systems for C Sequestration
The great potential of C sequestration in cropland has
provided a promising approach to reducing the atmos-
pheric concentration of CO2 for mitigating climate change.
However, this approach depends on cropping systems,
which may be defined as an operating system for growers
to follow in their practices for crop production. An ideal
cropping system for C sequestration should produce and
remain the abundant quantity of biomass or organic C in
the soil.
The organic C concentration in the surface soil (0-15
cm) largely depends on the total input of crop residues
remaining on the surface or incorporated into the soil. It
decreases soil C greatly to remove crop top from the soil
by cleaning up the land [19]. Therefore, to improve C
sequestration, it is critical to increase the input of plant
biomass residues. Biomass accumulation can be en-
hanced by an increase in cultivation intensity, growing
cover crops between main crop growing seasons, reduc-
ing fallow period of land, crop rotations, and intercrop-
ping systems. Biomass return to the soil can be improved
by elimination of summer or winter fallow, and main-
taining a dense vegetation cover on the soil surface,
which can also prevent soil from erosion for SOC loss.
The major strategies in developing cropping systems are
discussed below.
Table 3. Potential of soil C sequestration in cropland of
major countries [4].
Country (region) Potential rate of C sequestration
in cropland (Tg C yr-1)
U.S.A. 75-208
China 105-198
European Union 90-120
India 39-49
Canada 24
4.2.1. Crop Rotation
Crop rotation can improve biomass production and soil C
sequestration, especially rotations with legumes and
non-legumes. Growing legumes can substantially reduce
the nitrogen input as chemical fertilizers, which in turn
can reduce the fossil fuel consumption in manufacturing
fertilizers [20,21]. Conversely, without appropriate crop
rotation, soil productivity and biomass production will
decrease due to an infestation increase in weeds, diseases,
and insects. Increase in cropping intensity or cropping
more frequently by reducing the frequency of bare land
fallow in the crop rotation is another effective approach
to improve biomass production and soil C sequestration.
In addition, increase cropping intensity can decrease or-
ganic matter decomposition rate and mineralization/oxi-
dation of SOC [22]. A long term (15 yrs) study with corn
and soybean cropping systems indicated that the
corn-soybean rotation system had the greatest productiv-
ity and returned the largest crop residues to the soil
compared to monoculture of corn or soybean [23]. The
above study implies that application of low carbon-to-
nitrogen residues to maintain soil fertility in the major
corn-soybean growing region in the U.S. would increase
soil C sequestration by 13-30 Tg yr-1. This is equal to
1-2% of the estimated annual C released into the atmos-
phere from fossil fuel combustion in the U.S. (1.4 Pg C
yr-1) [24].
4.2.2. Intercropping
Intercropping can improve the crop productivity due to
increased efficiency of utilization of sunlight with an
adequate spatial distribution of various plant architec-
tures. Intercropping systems include row intercropping,
strip intercropping, mixed cropping and relay intercrop-
ping, which depends mainly on the characteristics of
various crops in spatial distribution and cropping goals.
For example, row intercropping corn or sorghum with
vine crops, such as climbing beans or sweet potato, can
improve the productivity of the latter crop since their
vines can climb on the former plants to take the advan-
tage of space and sunlight. In which, the former plants
usually may be expected to produce optimal yield be-
cause of their sacrifices to support the latter plants. For
the convenience of harvests for different crops, espe-
cially applying combined harvest machines, strip crop-
ping is preferred. Selecting crops or varieties with vari-
ous maturity dates may help staggered harvest. In India,
the sorghum and pigeonpea intercropping is a common
practice. In this intercropping system, sorghum domi-
nates the early stage of growth and mature in about 4
months, and the slow-growing pigeonpea flowers and
ripen after the harvest of sorghum, which efficiently util-
ize the time and space for an optimal productivity of both

Cropping Systems to Improve Carbon Sequestration for Mitigation of Climate Change 211
crops [25].
Intercropping can improve the crop productivity con-
siderably. For example, based on the land equivalency
ratio (LER) [LER = (intercropping crop1/pure crop1) +
(intercropping crop 2/pure crop 2)], the total yields of
sweet corn and southern peas with intercropping systems
at different densities showed that 30-48% more land is
required to produce the same yield (Table 4) in south
Carolina [26].
There are many examples applying intercropping sys-
tem to improve total crop yields and incomes. For in-
stance, in Iowa, a strip cropping system with oats, corn
and soybean on ridge-till rows showed that net returns
with strip intercropping can be increased by 38% ($ 188
vs. $ 136 ha-1) compared with same crops in monoculture
[27]. Another example is that intercropping oat, corn and
soybean increased oat yield by 5%, corn yield by12-15%,
soybean yields dropped by 10% on the border rows due
to the shading impact but the yield in the middle rows
were much higher than that at the border to offset such a
yield loss. As a result, the total soybean yield in this sys-
tem was greater than that in the monoculture [28]. The
total yield increase and benefit improvement from such
intercropping system can be attributed to mutual benefits
or synergetic effects of various crops. For instance, in the
above intercropping system, the early-maturing oats effi-
ciently utilized sunlight, soil nutrients and water to pro-
duce yields before corn fully developed to create shading
and competition impacts on water and nutrients, and the
corn strips can provide wind protection for oats. Soybean
can fix nitrogen by rhizobium bacteria to supplement
compensation of nutrient uptake by corn, and corn strips
can provide an effective windbreak to protect soybean. In
addition, strip intercropping can efficiently reduce the
infestation by insects and pathogens of the host plants. In
Yunnan province of China, the blast disease of rice was
Table 4. Yields of sweet corn and southern peas with inter-
cropping at different densities [26].
Plant density* Corn yield
(Mg ha-1)
Peas (Mg
ha-1)
Land
equivalency
requirement
(LER)
Full corn 6,272 - -
Full peas - 1,344 -
Low corn 4,704 896 1.41
Medium corn 5,152 896 1.48
High corn 5,600 560 1.30
*low corn: 2,714 plants ha-1; medium corn: 3,848 plants ha-1; high corn:
4,820 plants ha-1; and peas were 12,879 plants ha-1 in all intercropping plots.
successfully controlled by adopting a mixture of two
different rice varieties instead of a typical pure stand of a
single variety. This in turn decreased the need for che-
mical fungicides [29].
Mixed cropping is also an effective approach in the
intercropping system to optimize the ecosystem for
maximum plant production by planting two or more
plants in a mixture. The benefits of mixed cropping are to
balance the input and output of soil nutrients, suppress
weeds and insects, control plant disease, resist climate
extremes, such as wet, dry, hot and cold, and to increase
the overall productivity with limited resources [30]. The
classic example of mixed cropping is that the American
“three sisters”, corn, beans and curcurbits (squash and
pumpkins). These plants, domesticated at different times,
were together an important component of Native Ameri-
can agriculture. In the history, all these three plants were
seeded in the same hole. The corn provides a stalk for the
beans to climb on, the beans are nutrient-rich to offset
what taken up by corn, and the squash or pumpkin grows
low to the ground to keep weeds down and to prevent
water from evaporation. With these mutual benefits, an
overall optimal productivity with corresponding quantity
of biomass of both underground and aboveground can be
reached, which shows a potential for biomass return and
soil C sequestration.
In the modern agriculture, such mixed cropping sys-
tem has to be modified for the convenience of manage-
ment and harvest with machinery. In addition to grazing
pastures, there are a number of successful selections for
the mixed cropping system in agriculture, such as wheat
and chickpea; soybean and pigeon pea; peanut and sun-
flower; sorghum and pigeon pea; barley and chickpea;
wheat and mustard; and cotton and peanut, etc. [31].
Furthermore, mixed cropping has a long history and it
has been practiced in India, China and many other coun-
tries. For instance, Horrocks et al. [32] revealed the
mixed cropping system in early New Zealand; Jahansooz
et al. [33] reported the yield increase of wheat and
chickpea in mixed cropping compared with sole cultiva-
tion in Australia; Gunes et al. [34] demonstrated the mu-
tual benefits in mineral nutrients and soil moisture by
mixed cropping in Europe; and Sahile et al. [35] showed
that mixed cropping can promote proactive integrated
disease management because mixed cropping of faba
bean with cereals (barley and corn) can contribute to the
slowing of chocolate spot epidemics and increase grain
yield of faba bean in Ethiopia.
Relay intercropping, such as planting soybean into
standing winter wheat between 20 and 30 days prior to
wheat harvest, can efficiently take spatial and time ad-
vantages for optimal yield and eliminate the fallow pe-
riod to conserve the soil and reduce water evaporation.

Cropping Systems to Improve Carbon Sequestration for Mitigation of Climate Change 212
The key to succeed in relay intercropping is timing and
wheat row spacing for both plants to develop. If soybean
is planted too early, it will become very tall and spindly
due to lack of sufficient sunlight, and too late will delay
the soybean development, and the spacing of 25 to 38 cm
in wheat row of width is appropriate at Ohio State [36].
Too narrow wheat row spacing will limit the develop-
ment of soybean plants and too wide will sacrifice the
wheat yield. Since the relay intercropping can capture
and utilize as much sunlight as possible, it has profound
effect on the growth of intercropped soybean. Such a
cropping system increases the net return and overcomes
the risk of over production of one commodity and price
fluctuations. Meanwhile, the land can be well covered
and natural resources, especially the sunlight and soil, are
efficiently utilized to produce economic yield and im-
prove biomass accumulation.
4.2.3. Cover Cropping
Growing cover crops is another effective approach to
improve C sequestration and SOC storage. In the tem-
perate region, winter cover crops, such as rye, ryegrass,
oats, pea, vetch, clover, are commonly grown in fall,
survived through the mild winter and grow again in
spring to cover the bare lands during the off season. The
biomass production of vetch and rye winter cover crops
in biculture often ranges 5.7 to 8.2 Mg ha-1 in the above-
ground, and 372 to 880 kg ha-1 belowground, which re-
sult in a total C input to the soil ranged from 6.8 to 22.8
Mg ha-1 by cover crops, cotton and sorghum in rotation
[37]. Sainju et al. [37] also reported that SOC increased
by 6-8% with cover crops at 0 to 10 cm, and by 0.4%
with rye in monoculture and 3% with vetch and rye in
biculture at 0-30 cm. However, in the tropical or sub-
tropical region, summer cover crops, such as sunn hemp,
velvetbean, sorghum sudangrass, are prevailing species
grown during the hot and humid summer to cover the
bare land conserving soil and water and those summer
cover crops, especially sunn hemp can produce as much
as 15 Mg ha-1 of aboveground biomass and 3.5 Mg ha-1
belowground biomass, combined contributes to 8 Mg ha-1
of organic C input into the soil within 3 months [38,39].
Therefore, cover cropping system provides an excellent
strategy to improve C sequestration for mitigation of
climate change.
4.2.4. Companion Cropping
In organic farming for vegetable production system,
companion cropping system is often practiced. For ex-
ample, the use of permanent beds of companion crop
grown alongside the vegetable crops (e.g., lettuce, cab-
bage, etc.) has been developed under various conditions,
which is perceived as a possible alternative in organic
crop production. Companion crops also have the poten-
tial to reduce the impact of pests and weeds to benefit the
vegetable crops because of the biodiversity. However,
the vegetable crop may benefit from the companion crop
through a number of channels [40], for example:
Trapping effects: an excellent example is the use of
collards to attract the diamondback moth (Plutella xylos-
tella) away from cabbage because the former plant is
more attractive to the pests [41].
Biochemical pest suppression: Some plants exude
chemicals from roots or aerial parts that can suppress or
repel pests and protect the neighboring plants. For in-
stance, the African marigold (Tagetes erecta), which can
release thiopene, a nematode repellent, making it a good
companion for a number of garden crops. Allelochemi-
cals, such as juglone found in black walnut, can suppress
the growth of many plants, which can be used for weed
control. The use of mown-killed grain rye as a mulch can
prevent weed germination but do not affect transplanted
tomatoes, broccoli, or many other vegetables.
Nursing effects: Tall and dense-canopied plants may
protect more vulnerable species through necessary shad-
ing (e.g., ginger plant) or by providing windbreak. For
instance, oats have been long used to help the establish-
ment of alfalfa and other forages. In some cases, the
nurse effect can act simply as a physical-spatial interac-
tion function to benefit the main crop.
4.2.5. Ratoon Cropping
Ratoon cropping is a technique allowing a crop to pro-
duce two or more harvests for yield from one planting.
The basic requirements in ratoon cropping are that the
crop has to have a well developed root system, earlier
maturity and a perennial nature. Ratoon cropping has
obvious advantages for crop production and soil C se-
questration. For instance, ratoon cropping reduces the
cost of production via savings in land preparation and
planting; it has a better use of the growing season; effi-
ciently utilize the sunlight energy; higher yields and
biomass per unit area can be reached in a given period of
time; less use of irrigation water and fertilizer than the
main or original crop because of a shorter growth period;
prevent soil and water erosion and nutrient leaching; and
more productive economically compared to conventional
cropping system. Ratooning sorghum [Sorghum bicolor
(L.) Moench] or sugarcane (Saccharum officinarum L.) is
successful [42], and the main crop should be cut at about
2.5-10 cm above the ground level after its maturity. Okra
(Abelmoschus esculentus) is another ideal ratooning ve-
getable crop in tropics or subtropics, for which such ra-
tooning can be conducted two or three times [43].
4.2.6. Cropping Practices
Appropriate cropping practices, such as fertilization to
adjust nutrient balance, appropriate water supply, etc.,

Cropping Systems to Improve Carbon Sequestration for Mitigation of Climate Change 213
are important factors to optimize biomass production,
improve crop growth and development. However, con-
ventional tillage, especially the moldboard plowing, can
result in rapid mineralization of SOC, which leads to
SOC depletion rather than sequestration. Therefore, to
enhance C sequestration in the soil, increased amount of
plant residues must be returned to the soil and the soil
must be kept a minimum disturbance. In addition, it is
important to transfer the sequestered C into a physically
or chemically stable form, such as recalcitrant C or soil
organic C via slow humification or carbonization proc-
ess.
The stability of organic C in plant residues or in soil
pool depends largely on environmental changes, such as
soil types, temperature, and moisture. However, the plant
components play a major role for its organic C stability
against its decomposition rate. For example, usually there
are two major compartments of organic C in plants, ac-
tive and inert, which might refer to labile and recalcitrant
pools, respectively, in two-pool models proposed by
McLauchlan and Hobbie [44]. The active organic C con-
sists of 4 sub-components, decomposable organic C, re-
sistant organic C, microbial biomass organic C, and hu-
mified organic C [45]. The physiological and chemical
characteristics in plant residues, such as C:N ratio and
lignin content, may affect the distribution of those dif-
ferent organic C compartments, which consequently in-
fluence the decomposition rates. There are a number of
reports on C sequestration or SOC accumulation in crop-
lands through integrated cropping systems and cropping
practices, such as conservation tillage; cover cropping,
crop rotation; land use restoration or shifting cultivation,
and fertilization, etc. [4,16,19,37,46-49]. Obviously, soil
organic C pool has a great potential to store sequestered
C and integrated cropping systems associated with crop-
ping practices has displayed the promising prospects in C
sequestration from the atmosphere and shifting the miti-
gation of climate change.
5. Acknowledgements
The research was financially supported via a USDA-ARS
collaboration research program.
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Unit conversion:
Mg (megagram) = 1 × 106 g or million gram;
Gg (gigagram) = 1 × 109 g or billion gram;
Tg (teragram) = 1 × 1012 g or trillion gram or million ton;
Pg (petagram) = 1 × 1015 g or billion ton; ppmv (parts per
million by volume) = 1 × 10-6 liter;
ppbv (parts per billion by volume) =1 × 10-9 liter;
pptv (parts per trillion by volume) = 1 × 10-12 liter.