<i>Acacia trotilis</i> and <i>Calotropis procera</i>: Do They Substantially Promote Soil Carbon Sequestration?

doi:10.4236/ojss.2012.22017

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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.

Email: [email protected]

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

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.

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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