THE CARBON CYCLE IN DRYLANDS
Penelope Serrano-Ortiza Oscar Pérez-Priegob, Enrique P. Sánchez-Cañetea, Ana
Werea, Francisco Domingoa, Cecilio Oyonartec and Andrew S. Kowalskib.
a
Estación Experimental de Zonas Àridas (EEZA, CSIC), 04120 Almería, Spain
Dept. Física Aplicada, Universidad de Granada, 18071 Granada, Spain
c Dept. Edafología & Química Agrícola, Universidad de Almería, 04120 Almería, Spain
b
1. Introduction
Drylands are characterised by patches of vegetation and bare soil exposed to
erratic rainfall events producing water-stressed vegetation during the drought period
(Domingo et al. 1999). These water-limited ecosystems exist on every continent and
comprise nearly a third of the total land surface corresponding to 60 million km 2 (Okin
2001; Schlesinger 1990). Dryland rangelands support about 50% of the world's livepool
and provide forages for both domestic animals and wildlife (Puigdefábregas 1998).
Although drylands withstand extreme climatic conditions, they are very sensitive to
perturbations such as drought, fires or climate change, leading to desertification (Mouat
and Lancaster 2006). Globally, the organic and inorganic carbon (C) storage in such
water-limited systems is about 20-30% of the terrestrial global total (Eswaran et al.
2000; Rasmussen 2006; Safriel et al. 2005). In this context, the main characteristic of
drylands is the capacity to store about 95% of the soil inorganic C (SIC) globally via
“caliche formation” (Marion et al. 2008). However, the role of the SIC pool in relation to
climate change is less well understood (Lal and Kimble 2000). In addition, due to arid
conditions and the large percentage of bare soil and carbonate rocks, some other
processes besides photosynthesis and respiration contribute to C sequestration or C
gaseous emissions to the atmosphere. These include weathering processes, formation
of secondary carbonates and subsoil ventilation that can even dominate the ecosystem
C exchange during the dry season. Such contributions limit the use of biological
models to provide estimates of C pool in drylands. This work reviews measurements of
annual C sink capacities together with analysis of the main drivers controlling the
potential C sequestration capacity and principal processes involved.
2. Materials and Methods
This work analyzes the published literature concerning the carbon cycle over drylands
using the eddy covariance technique.
The eddy covariance technique is one of the most important methods used all over the
world and has the potential to quantify how whole ecosystems respond to a spectrum
of climate regimes (Baldocchi et al. 2001). The use of this powerful tool requires
micrometeorological expertise and technological advances (Aubinet et al. 2000).
Fluxes of CO2 and H2O are estimated using an infrared gas analyzer to measure
densities of CO2 and H2O and a sonic anemometer to measure wind speed at a
frequency higher than 10 Hz.
3. Results and Discussion
3.1 Main natural processes involved in carbon sequestration and loss
3.1.1 Biological processes
The organic C stored in soils and biomass is mainly due to the balance between
photosynthesis (net CO2 uptake) and respiration (net CO2 release via decomposition,
degradation and diffusion processes). Such biological processes are mainly
responsible for annual NEEs in most ecosystems (forest, wetlands, cropland, etc.).
Thus, the FLUXNET community (Baldocchi et al. 2001) interprets CO2 fluxes measured
using micrometeorological techniques (Dabberdt et al. 1993) as a biological flux
neglecting non-biological processes (Falge et al. 2001; Reichstein et al. 2005; Stoy et
al. 2006; Valentini et al. 2000). However, many CO2 flux measurements over drylands
indicate contributions of abiotic processes to the NEE (Emmerich 2003; Ferlan et al.
2011; Hastings et al. 2005; Inglima et al. 2009; Mielnick and Dugas 2000; Were et al.
2010) (Wohlfahrt et al. 2008; Xie et al. 2008).These processes can dominate the flux
during the dry season (Kowalski et al. 2008) with annual contributions >50% depending
mainly on meteorological conditions (Serrano-Ortiz et al. 2009).
3.1.2 Weathering processes
Some soils are derived from carboniferous (calcareous) rocks and/or include additional
carbonates (secondary carbonates or caliche) as a result of weathering and
precipitation processes (Eq. 1).
H 2O( aq)  CO2( g )  CaCO3( s )  2HCO3  Ca(2aq ) (1)
In terms of NEE, Eq. (1) specifies that for each molecule of CaCO 3 dissolved a
molecule of atmospheric CO2 is consumed. During precipitation, one molecule of CO2
is released for every molecule of CaCO3 deposited to the surface. Thus, either water
lost by evapotranspiration or additional sources of Ca2+ would enrich aqueous
concentrations and enhance the deposition of CaCO3 from the aqueous solution, and
release CO2 to the atmosphere (Eq. (1) to the left).
Globally, and over long time scales, weathering processes are balanced with
respect to CO2 (Berner 2003; Lasaga et al. 1994). However, at annual and seasonal
scales the predominance of dissolution or precipitation processes may be relevant to
local NEEs (Serrano-Ortiz et al. 2010) and contribute to the observed CO2 fluxes
(Emmerich 2003; Mielnick and Dugas 2000). In addition, although precipitation
processes imply CO2 release at short time scales, caliche formation can be considered
a net atmospheric CO2 sink over long timescales: every two molecules of bicarbonate,
previously formed during the growing season by dissolution of two molecules of CO 2,
react with one molecule of additional Ca2+ to form one molecule of calcite and release
one molecule of CO2. The formation of this secondary C form depends on land use and
soil/crop management systems (Lal 2004). Addition of biomass, whose decomposition
increases the partial pressure of CO2 in the soil, together with irrigation, increases SIC
in agricultural soils (Entry et al. 2004). While its contribution to the total annual
atmospheric CO2 sink may be less than 10% (Eswaran et al. 2000; Liu and Zhao
2000), it is unclear how SIC responds to rainfall and temperature changes predicted
under the climate change scenarios (Rasmussen 2006).
3.1.3 Subsoil ventilation processes
Drylands over carbonate rocks with cracks, pores and cavities, together with soils with
deep vadose-zones, show a high capacity to store CO2 belowground. Since such CO2
storage may represent as much as 60% of the annual atmospheric sink (Serrano-Ortiz
et al. 2011), the subsurface can be considered a temporal depot for CO2 coming from
different processes (mainly weathering and respiration) (Serrano-Ortiz et al. 2010). In
addition to diffusion processes, such soils have the potential to emit the stored CO2 via
ventilation (Sanchez-Cañete et al. 2011) and contribute to ecosystem CO2 exchange
observed. Ventilation is a transport process due to net movements of air in and out of
an enclosed space. The behaviour of ventilation processes, in caves for example, is
controlled by the degree of connection between the cavities and the aboveground
system (Cuezva et al. 2011) and, thus, such processes have only been detected when
the soil is dry (Cuezva et al. 2011; Sanchez-Cañete et al. 2011; Serrano-Ortiz et al.
2009; Were et al. 2010). The main meteorological drivers controlling soil CO2
ventilation due to pressure pumping are wind speed and turbulence (Jassal et al. 2005;
Lewicki et al. 2010). Therefore, the non-negligible role of subsurface as a temporal
depot of CO2, along with seasonal ventilation can contribute to the annual net
ecosystem carbon balance (Serrano-Ortiz et al. 2010).
3.2 Carbon sink capacity at ecosystem level
The processes mentioned above (weathering, ventilation and erosion processes) act
together in drylands and contribute to the measured annual net C exchange (Table 1).
For deserts located in southwest of the U.S.A. and Baja California, published studies
have determined that the most important driver controlling CO2 fluxes is not the amount
of rainfall but mostly its timing (Hastings et al. 2005; Mielnick et al. 2005; Wohlfahrt et
al. 2008). However, published annual net C exchanges are not in agreement. While
two desert shrublands located in the Mojave Desert and Baja California, with similar
annual precipitation, act as annual net C sinks of 106 ± 70 and 52 g C m-2 yr-1,
respectively, the Chihuahuan Desert site emitted 145 g C m-2 annually. Grasslands
located in North America (New Mexico, Arizona and California) and Europe (Southeast
Portugal and Southeast of Spain) are C sources ranging from 141 (source) to -190
(sink) g C m-2 yr-1 depending mostly on the total amount of rainfall (Aires et al. 2008;
Anderson-Teixeira et al. 2011) (Emmerich 2003; Ma et al. 2007; Scott et al. 2006) and
also wind speed for the particular site located in Southeast of Spain (Rey et al. 2011).
While ecosystem C sink capacity in grasslands located in Northern Asia also depend
on optimal temperature in summer (10-20ºC) (Kato et al. 2006; Wang et al. 2008). For
savannas, Scott et al. 2009 measured an annual net C release ranging from 14 to 95 g
C m-2 yr-1 in a semiarid savannah in southern Arizona, while Ma et al. (2007) measured
an annual net C uptake ranging from 56 to 155 g C m -2 yr-1 in a savannah site located
in California with higher annual precipitation and lower temperature. Finally, shrublands
in drylands act mostly as small sinks for atmospheric CO2 (uptake from 2 to 75 g C m-2
yr-1) (Anderson-Teixeira et al. 2011; Luo et al. 2007; Serrano-Ortiz et al. 2009; Zhao et
al. 2006), with some exceptions. In riparian areas where shrubs and woody plants have
the capacity to exploit water resources by growing deep roots (Domingo et al. 1999),
annual NEE can be higher than 200 g C m-2 yr-1 (Scott et al. 2006). On the contrary, the
annual C loss estimation for the Lucky Hills site located in Arizona was 144 g C m -2 yr-1.
The source of this C appears to be from the large inorganic carbon pool in these soils
(Emmerich 2003).
In summary, according to published studies, the carbon sink
capacity at ecosystem level in drylands is very variable depending on the ecosystem
type, inorganic C pool and mostly on rainfall timing and temperature during growing
season.
Table 1 Annual net ecosystem C exchange (g C m-2) measured mostly using the eddy
covariance technique, together with annual temperature and rainfall. Negative sign (-)
for net ecosystem exchange indicates a C sink
Vegetation
Desert and
bare soil
Location
Experimental Site
Mean annual
temperature
(ºC)
20
Mean annual
precipitation
(mm)
210
Net ecosystem
C exchange
(g C m-2 yr-1)
-106±70
Mojave Desert
(USA)
Desert on the Nevada Test Site, 120
km northwest of Las Vegas
Wohlfahrt et al.
2008
Chihuahuan Desert
(USA)
About 40 km northeast of Las
Cruces, New Mexico
Mielnick et al.
2005
–
272
145*1
Baja California
(Mexico)
15 km west of the city of La Paz
and 1.5 km from the Bay of La
Paz (CIBNOR)
“Cabo de
Gata Natural Park” Almería
(Andalucía)
Hastings et al.
2005
24*2
147
197
-39
-52
Rey et al. 2011
17
210
251
294
66
144
92
Southeast Portugal
Monte do Tojal, Évora in Southern
Portugal
Aires et al. 2008
14.7
364
751
49
-190
New Mexico (USA)
Sevilleta LTER in Central New
Mexico
13
244
30
Arizona (USA)
The Kendall grassland Agricultural
Research Service Walnut Gulch
Experimental Watershed
AndersonTeixeira et al.
2011
Scott et al. 2010
14.5
Emmerich 2003
17
313
312
274
246
162
356
-69
-98
-55
-47
21
126*1
Foodplain terraces along the San
Pedro River
Scott et al. 2006
17
234
-63
Foothills of the Sierra Nevada
Ma et al. 2007
17±1
562±193
Southwest Spain
Grassland
California (USA)
3
(-88)–141
Qinghai-Tibetan
Plateau (China)
Alpine meadow
Kato et al. 2006
-0.65
-0.91
-1.53
561*
-79
-92
-173
Northern China
Inner Mongolia Autonomous
region
Wang et al. 2008
Central Mongolia
Arizona (USA)
Hentiy province of Mongolia
The Santa Rita mesquite savanna
site located on the Santa Rita
Experimental Range (SRER)
Li et al. 2005
Scott et al. 2009
California (USA)
Southwest Spain
Foothills of the Sierra Nevada
Mediterranean plateau, 25 km from
the coast
Ma et al. 2007
Serrano-Ortiz et
al. 2009
2.5
1.3
1.7
1.2
19
20
20
19
17±1
12
297
174
215
196
285
335
289
330
562±193
475
10
30
-15
-41
60
14
95
30
(-155) – (-56)
-2±23
New Mexico (USA)
Sevilleta LTER in Central New
Mexico
14
244
-30
San Diego (USA)
Southern California, 75 km east of
Pacific Ocean
AndersonTeixeira et al.
2011
Luo et al. 2007
15
349
-52
Arizona(USA)
Foodplain terraces along the San
Pedro River
Scott et al. 2006
17
234
-212
Lucky Hills, Agricultural Research
Service Walnut Gulch
Experimental Watershed
Emmerich 2003
17
256
144*1
Alpine Potentilla fruticosa at the
Haibei Research Station
Zhao et al. 2006
2.3
2.2
542
493
-59
-75
Savanna
Shrubland
Reference
Qinghai-Tibetan
Plateau (China)
*1Data not measured by the eddy covariance technique
*2 Estimated data using
information of Figure 5.8.1 of the cited reference *3 Annual average precipitation for
1981-2000
Running projects
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Balance de carbono en el olivar: efecto de la presencia de la cubierta vegetal
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