Open Journal of Soil Science, 2012, 2, 116-122
http://dx.doi.org/10.4236/ojss.2012.22017 Published Online June 2012 (http://www.SciRP.org/journal/ojss)
Acacia trotilis and Calotropis procera: Do They
Substantially Promote Soil Carbon Sequestration?
Taoufik Saleh Ksiksi
Department of Biology, Faculty of Science, UAE University, Al-Ain, UAE.
Received October 23rd, 2011; revised November 30th, 2011; accepted December 18, 2011
ABSTRACT
Very little is known about the type and mix of desert plant species and their management to optimize carbon sequestra-
tion in desert ecosystems. Overgrazing is one important practice that affects soil carbon cycling and therefore sequestra-
tion. Improving soil carbon in desert ecosystems may be best through the use of native trees and shrubs. Acacia tortilis
and calotropis procera are two important species in the United Arab Emirates (UAE). The former is a native species that
improves biodiversity and the latter is not native and has been reported to be an indicator of overgrazing. The average
soil organic matter (SOM) content was higher in soils dominated by A. tortilis when compared to those dominated by C.
procera; 2.98 and 1.34; respectively (P < 0.05). Moreover, A. tortilis leaves had a higher OM content than C. procera
leaves (94.1% and 90.6%; respectively). The higher OM content of A. tortilis leaves explains the higher contribution of
this species to the overall soil organic matter inputs. There was also a significant effect of shrub species on total SOC (P
< 0.05). A total of about 14.7 tons of SOC were added per hectare in the areas dominated by A. tortilis. While only
about 6.6 tons of SOC were added to the areas dominated by C. procera. In short, it is believed that both species sub-
stantially promote soil carbon sequestration. Some significant superiority of the native A. tortilis has been shown. But
much has to be done to investigate the mix of plant species that promote the best soil carbon sequestration in the desert
areas. Further studies are required in order to assess temporal as well as spatial variations in soil carbon sequestration in
the UAE deserts. This will certainly help, in addition to other practices, in mitigating CO2 emission.
Keywords: Desert Soils; Nutrient Cycling; Soil Carbon
1. Introduction
Soil restoration and woodland regeneration are sound
strategies to increase soil carbon pool [1]. Increasing soil
carbon improves soil quality, productivity and long-term
sustainability. Equally important are the growing con-
cerns about global greenhouse gas emission issues, which
call for proper management of the terrestrial carbon pool.
Specifically, this calls for a better understanding of car-
bon sequestration and ways to optimize it. Especially that
desert lands are believed to contain small amounts of soil
carbon. In general terms, carbon sequestration in terres-
trial ecosystems can be defined as the net removal of car-
bon dioxide from the atmosphere into long-lived pools of
carbon. These pools can be living above-ground biomass,
wood products, living biomass in soils such as roots and
micro-organisms or recalcitrant organic and inorganic
carbon in soils [2]. In the UAE, for example, soil carbon
pools become more important and relevant if we consider
the vast areas covered by desert ecosystems, which oc-
cupy at least two-third of the country’s land area.
Fortunately, terrestrial carbon pools could be signifi-
cantly enhanced by adopting sound management prac-
tices in desert ecosystems. As in other agricultural sys-
tems, the potential of desert ecosystems to store carbon is
dependent on how adequately the soil-plant resources are
managed. For instance, promoting healthy perennial
plant species is a management option that could improve
the terrestrial carbon pool, through increased rooting
depth. This will be even more pertinent if native shrub
and tree species were used.
Unfortunately, very little is known about the type and
mix of desert plant species that optimize carbon stocks in
desert ecosystems. Additionally management principles
and practices which can maintain carbon stocks through
time are not well defined and adopted. Panicum, for in-
stance, offers an excellent carbon sequestration option
because of its deep rooting system and perenniality [3].
Understandably, plant species differ markedly in their
impact on soil carbon concentration and distribution,
mainly because of differences in their root systems. For
example, the mean carbon concentration in the top 10 cm
of soil in areas dominated by Panicum maximum was
3.31% compared to 1.89% and 0.74% in areas dominated

Acacia trotilis and Calotropis procera: Do They Substantially Promote Soil Carbon Sequestration? 117
by Themeda triandra and Aristida jerichoensis, respec-
tively [4]. Furthermore, the restoration of some types of
soils with Astrebla species has been attributed to a sub-
stantial increase in organic carbon concentration in the
top 5 cm of soil [5]. Such differences in soil carbon un-
der different species can be attributed to root systems’
characteristics, specifically root turnover, which is a cen-
tral component of ecosystem carbon and nutrient cycling
[6]. In areas where Prosopis and Acacia are adapted, 6.2
× 109 Mg of carbon would be sequestered [7]. These
types of carbon sequestration could offset CO2 emission
due to fossil fuel burning.
In the UAE, desired and undesired species are becom-
ing part of the desert ecosystems. An undesired species
that is prevalent in many parts of the UAE deserts is
colotropis procera (Aiton) W.T. Aiton. It is common in
many parts of the UAE desert as it is an indicator of
overgrazing [8]. Desired species—such as acacia toritil-
lis—are also an integral part of the UAE deserts. Ac acia
toritillis promotes floral diversity as well as provides
feed sources for the majority of wildlife as well as live-
stock species in the country [9]. But do C. procera and A.
toritillis have a potential to substantially improve soil
carbon sequestration? What is the extent of such seques-
tration in the UAE soils? Those are some of the questions
that the present endeavor will try to address.
2. Materials and Methods
2.1. Study Site
The study was conducted in the surrounding area of Al-
Ain city in the (UAE). The average minimum tempera-
ture in Al-Ain is 22˚C while the average maximum is
35.8˚C (Table 1). The annual average long term rainfall
is 119.7 mm (Table 2) and the humidity is 58% (Table
3). Three locations were selected where the two plant
species grow. The aim was to collect pair samples with
similar soil characteristics. The soils were characterized
as sandy to sandy loam.
2.2. Sample Collection
Soil samples—about 150 grams—were collected from
the various locations from the top soil layer (5 cm) and at
10 cm deep. A total of 72 samples (2 Species × 2 Posi-
tions × 2 Depths × 9 Replicates) were collected during
Spring and during Winter of 2009-2010. Samples were
collected from underneath the shrub canopy and away
from the shrub canopy; referred to as in and out; respec-
tively. Leaf samples were collected from each tree to
assess percent OM and OC. Percent dry matter loss and
percent moisture losses were also assessed. All samples
were then transported to the UAE University labs for
analyses.
2.3. Sample Analyses
Soil and leaf samples were first air dried for 48 hours.
Soil samples were sieved to remove coarse material.
Crucibles were then used to oven-dry each sample at 105
degrees C for 72 hours. Moisture content was calculated
for the soils samples following this step (formula a).
Combustion was performed for 3 hours on the soil and
leaf samples to estimate organic matter (formula b). Per-
cent organic carbon was calculated as a fraction of OM
(formula c).
a) Moisture content: Sample loss/Dry Weight of Sam-
ple.
b) Percent Soil Organic Matter (SOM): Sample loss in
combustion/Dry Weight of Sample.
c) Percent Soil Organic Carbon (SOC): Organic matter
× 0.58.
In order to assess the total bulk SOC that the two spe-
Table 1. Monthly variation in the air temperature (˚C) (1965-2001) of ten meteorological stations in Al-Ain UAE. Minimum
(top row) and maximum (bottom row) (Ministry of Agriculture and fisheries UAE, 1965-2001).
Station Jan. Feb.Mar. Apr. MayJun. Jul. Aug. Sep. Oct. Nov. Dec.Mean
14.8 14.918.5 20.4 23.927.1 30 30.2 26.6 23 18.9 15.822.0
Al-Ain 23.3 26.930.8 35.4 40.443.5 43.544.9 42.0 37.6 32.4 28.835.8
Table 2. Monthly variation in the annual rainfall (mm) during (1965-2001) of ten meteorological stations in Al-Ain UAE
(Ministry of Agriculture and fisheries UAE, 1965-2001).
Station Jan. Feb. Mar. Apr. May.Jun. Jul. Aug. Sep. Oct. Nov. Dec.Total
Al-Ain 68.1 45.7 2.7 Trace0.0 0.0 0.0 0.0 0.0 0.0 0.0 3.2 119.7
Table 3. Monthly variation in the relative humidity (%) during (1965-2001) of ten meteorological stations in Al-Ain UAE
(Ministry of Agriculture and fisheries UAE, 1965-2001).
Station Jan. Feb. Mar. Apr. May.Jun. Jul. Aug. Sep. Oct. Nov. Dec. Mean
Al-Ain 66 64 59 53 50 53 53 54 56 58 63 66 58

Acacia trotilis and Calotropis procera: Do They Substantially Promote Soil Carbon Sequestration? 118
cies add to the soils, it has been decided to estimate bulk
SOC based on an approximate number of shrubs growing
in the study sites. It has been assumed that an average 20
shrubs and A. tortilis and 5 shrubs of C. procera were
growing in the site. It was also estimated that the age of
shrubs was 15 and 10 years for A. to rtilis and C. procera;
respectively. The total amount of SOC per square meter
was calculated following the method (formula d) re-
ported by [10].
d) Soil C (g·m−2) = z × pb × c × 10.
where z = thickness of each sample depth (cm), pb = bulk
density (1.7 g·cm−3) of each sample depth and c = carbon
concentration (g·C·kg−1 soil) of each sample depth. The
results will be reported in tons per hectare (tons·Ha–1).
2.4. Statistical Analyses
ANOVA analyses were performed to compare main ef-
fects (season, shrub species, soil depth and position in
relation to shrub canopy) and all interactions. SYS-
TAT11 was used to perform all analyses [11].
3. Results
3.1. Soil Organic Matter
3.1.1. The Effects of Tree Species
Soil organic matter (SOM) was different in soils domi-
nated with A. tortilis than in those dominated with C.
procera (P < 0.05; Table 4; Table 5). The average SOM
content was higher in soils dominated by A. tortilis when
compared to those dominated by C. procera; 2.98 and
1.34; respectively (Table 4). When comparing canopy
positions (i.e. underneath or away from the shrub) in ar-
eas dominated by either A. tortilis or C. procera, no dif-
ferences were detected (P > 0.05). The above findings do
not agree with much of what was reported elsewhere.
Results from the sonorant desert suggested higher soil
fertility underneath live plants, regardless of species and
phenology [12]. Except for cactus, which was suggested
to use nutrients and remove fertility areas around them.
There was an improvement in soil characteristics under-
neath A. tortilis when compared to open grassland area,
away from the shrub [13]. Their findings restrict these
differences, however, to lightly grazed sites. Additionally,
significant differences in soil characteristics, underneath
vs away from the tree/shrub canopies, in lightly grazed
areas were found [14].
While in our study, the grazing history is unknown but
heavy camel grazing has been a wide spread practice in
the area. The increase in camel populations within the
UAE during the past few decades is also an indicator of
the pressure on the desert ecosystems [15]. It is strongly
believed, therefore, that while the dominant shrub species
contribute to SOM, but a substantial part of this contribu-
tion may be the result of the associated species that grow
in soils where A. tortilis or C. procera dominate. The
associated species contributed to soil fertility in soils
dominated with A. tortilis in Ethiopia [14]. Acacia tortilis
has also been reported to improve associated species in
Table 4. Summary averages for percent soil organic matter (OM) and soil organic carbon (OC) for the two seasons at two
different soil depths under Acacia tortilis and Calotropis procera species growing in the deserts of the UAE.
Soil Depths
5 10
Shrub Species under/out of Shrub % OM % OC % OM % OC Average % OM Average % OC
Spring 2.13 1.23 2.02 1.17 2.07 1.20
Acacia 2.77 1.61 2.70 1.56 2.73 1.59
In 2.94 1.71 2.85 1.65 2.90 1.68
Out 2.60 1.51 2.54 1.47 2.57 1.49
Calotropis 1.49 0.86 1.34 0.78 1.41 0.82
In 1.38 0.80 1.36 0.79 1.37 0.79
Out 1.60 0.92 1.31 0.76 1.45 0.84
Winter 2.46 1.43 2.04 1.19 2.25 1.31
Acacia 3.52 2.04 2.93 1.70 3.23 1.87
In 3.66 2.12 2.90 1.68 3.28 1.90
Out 3.37 1.96 2.96 1.72 3.17 1.84
Calotropis 1.40 0.81 1.15 0.67 1.28 0.74
In 1.34 0.78 1.24 0.72 1.29 0.75
Out 1.46 0.85 1.06 0.62 1.26 0.73
grand total 2.29 1.33 2.03 1.18 2.16 1.25

Acacia trotilis and Calotropis procera: Do They Substantially Promote Soil Carbon Sequestration? 119
Table 5. ANOVA analysis percent soil organic matter (SOM) for the two seasons at two different soil depths under Acacia
tortilis and Calotropis procera species growing in the deserts of the UAE.
Source Sum-of-Squares df Mean-Square F-ratio P
Season 1.165 1 1.165 1.747 0.189
Taxa 96.154 1 96.154 144.220 0.000
Depth 2.499 1 2.499 3.748 0.055
Season*Taxa 3.525 1 3.525 5.287 0.023
Season*Depth 0.827 1 0.827 1.240 0.267
Taxa*Depth 0.149 1 0.149 0.223 0.637
Season*Taxa*Depth 0.381 1 0.381 0.572 0.451
Error 90.674 136 0.667
desert ecosystems [9,13]. Some other desert species were
also reported to improve soil characteristics beyond their
canopy [16].
3.1.2. The Effects of Season
SOM was highest during winter collection than during
spring (P < 0.05). The difference between the two shrub
species is more pronounced during winter. During which
A. tortilis had an average SOM of 3.23% while C. pro-
cera had an average SOM of 1.28%. During spring the
average SOM for A. tortilis and C. procera was 2.73%
and 1.41%; respectively.
3.1.3. The Effects of Soil Depth
The average SOM was highest in the top 5 cm (P =
0.055). SOM was 2.29% and 2.03% at 5 cm and 10 cm
soil depth; respectively. For A. tortilis, SOM was 3.14%
and 2.81% for 5 cm and 10 cm soil depths; respectively.
While for C. procera SOM was 1.82% and 1.63% for the
two depths, respectively. Marked differences in the top 5
cm of the soil profile were reported by [14].
3.1.4. Leaf Content
Overall A. tortilis leaves had a higher OM content than C.
procera leaves (94.1% and 90.6%; respectively) at P <
0.05 (Figure 1). But little fluctuations were observed
during the 28-day period (data not shown). The higher
OM content of A. tortilis leaves explains the higher con-
tribution of this species to the overall soil organic matter
inputs. This is another reason to encourage the planta-
tions of such native species in the UAE deserts.
As for the leaf moisture content, A. tortilis contained a
slightly lower level (52.2% and 84.6%; respectively) and
lost moisture at a relatively faster rate than C. procera
(Figure 2). But toward the end of the 28-day period, both
species had comparable moisture contents (9.7% and
11.3%; respectively).
3.2. Soil Organic Carbon
Percent SOC was transformed into bulk tons of SOC per
hectare. Please see the methodology section for more
details. There was significant effect of shrub species on
SOC (P < 0.05). An estimated total of about 14.7 tons of
SOC were added per hectare in the areas dominated by A.
tortilis (Table 6). While about 6.6 tons of SOC were
added to the areas dominated by C. pr ocer a.
Variations between soil depths was also detected (P <
0.05). The average SOC at 5 and 10 cm depths was 15.5
tons·Ha–1 and 13.9 tons·Ha–1 for A. tortilis; while SOC
was 7.1 tons·Ha–1 and 6.1 tons·Ha–1 for C. procera; re-
spectively.
Many other studies also report positive SOC seques-
tration but many disagree on the estimated amount per
hectare; mainly because of ecosystem differences and
variations in the adopted experimental protocols. An av-
erage of about 26 tons·Ha–1 of SOC in the grazing lands
of Ethiopia was reported [17]. While others [10] reported
about 14.7 tons·Ha–1 of SOC in the top 10 cm of the soil
profile semiarid acacia woodland. The accumulation of
SOC at 0 - 10 cm depth was estimated to be a staggering
61.2 tons·Ha–1 [18]. Some of the changes to SOC were
attributed, and rightly so, to land management practices
[19] such as overgrazing [20]. In the UAE, and across
much of the region, overgrazing has been reported as one
of the main threats facing desert environments [21].
For A. tortilis, we can estimate an annual SOC addi-
tion of about 0.98 tons·Ha–1 would be added to the desert
soils of the UAE. While an estimated annual addition of
0.66 tons·Ha–1 of SOC in soils dominated by C. procera.
Restoring grasslands to woody grasslands, where A. tor-
tilis was growing in the sahel, would add about 0.8
tons·Ha–1 of SOC annually [22]. The huge differences
between the estimates in our study and those in many
other studies may be attributed to floral understory. In
the UAE, the understory of grass species is much less
when compared to the Ethiopian grasslands, for instance.
4. Discussion
Acacia tortilis is an important native species to the UAE
and needs to be grown in large areas as part of the cur-
rent attempts to re-vegetate the desert. Assuming an av-
erage success rate of 20 individuals per hectare, we can

Acacia trotilis and Calotropis procera: Do They Substantially Promote Soil Carbon Sequestration? 120
87
88
89
90
91
92
93
94
95
96
97
Acacia Calotropis
PERCEN TORGANI CMATTER
SHRUBSPECIES
Figure 1. Percent leaf organic matter content (%OM) for both Acacia tortilis and Calotropis procera species growing in the
deserts of the UAE.
0
10
20
30
40
50
60
70
80
90
Fresh 7days14days 21days 28days
PERCENTMO ISTURE
DAY S SINCEFRESHLEAVESWERECOLL ECTED(OVENDRIED)
Acacia
Calotr o pis
Figure 2. Percent moisture loss for both Acacia tortilis and Calotropis procera species growing in the deserts of the UAE.
Table 6. Average soil organic carbon (SOC) at two different soil depths under and aw ay from the canopies of Acacia tortilis
and Calotropis procera species growing in the deserts of the UAE.
Soil Organic Carbon Canopy Position
Taxa Soil Depth Under Away Average
Acacia 5 16282.22 14728.61 15505.42
10 14180.83 13571.67 13876.25
Acacia Average 15231.53 14150.14 14690.83
Calotropis 5 6696.11 7536.67 7116.39
10 6417.50 5855.55 6136.53
Calotropis Average 6556.80 6696.11 6626.46
Grand Average 10894.17 10423.12 10658.64
sequester 19.6 Mt in the next 10 years if we plant 2000
hectares of A. tortilis. The other equally important bene-
fits of improved species diversity and soil improvement
are to be taken into consideration.
For A. tortilis, based on the above numbers, we can es-
timate an annual SOC addition of about 0.98 tons·Ha–1 to
be added to the UAE desert ecosystem. While an esti-
mated annual addition of 0.66 tons·Ha–1 of SOC in soils
dominated by C. procera. This highlights the importance
of re-vegetating our desert ecosystems using species that

Acacia trotilis and Calotropis procera: Do They Substantially Promote Soil Carbon Sequestration? 121
promote carbon sequestration. The direct benefits are the
greening of these ecosystems, while indirect benefits
may include the creation of islands of fertility underneath
these shrubs and the improvement of the vegetative cover
in the floral understory. Islands of fertility in the Sonoran
Desert underneath mesquite canopies played important
ecosystem functions [22].
Furthermore, SOC inputs enrich soil characteristics.
An increase of 1 ton of soil carbon of degraded cropland
soils may increase crop yield by 20 to 40 kg·Ha–1 for
wheat, 10 to 20 kg·Ha–1 for maize, and 0.5 to 1 kg·Ha–1
for cowpeas. In addition to enhancing food security, car-
bon sequestration has may possibly offset fossil fuel
emissions by 0.4 to 1.2 gigatons of carbon per year, or 5
to 15% of the global fossil-fuel emissions [1].
Finally and to address the question stated in the project
title, it is believed that both species substantially promote
soil carbon sequestration. It is important to note that one
species is a native wanted species (A. tortilis) and the
other is an introduced unwanted plant species (C. pro-
cera). Some significant superiority of the native A. tor-
tilis has been shown. As for the extent of soil carbon se-
questration, some evidence of vertical as well as horizon-
tal variability was shown. Much has to be done, however,
to investigate the mix of plant species that promote the
best soil carbon sequestration in desert areas. Besides
enhancing food security, soil carbon sequestration offsets
global fossil fuel emission by up to 15% [1]. The extent
of such quantitative estimates within the region and more
specifically within the UAE is unknown. More detailed
studies are to be initiated in order to assess temporal as
well as spatial variations in soil carbon sequestration in
the UAE deserts.
5. Acknowledgements
The investigator would like to express his sincere appre-
ciation to the Research Affairs at the United Arab Emir-
ates University for the financial support of this project
under fund Grant # 01-04-2-11/09. The support from the
Biology Department and the Faculty of Science is also
much appreciated. The investigator would also like to
express his gratitude to Drs Mohamed T. Moussa and
Nael Fawzi for their assistance in data collection.
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