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Research Note / Note de recherche
Variation in Plant Litter Decomposition Rates
across Extreme Dry Environments in Qatar
Mohammed H.S.A. Alsafran1, Judith Sarneel2, 3
and Juha M. Alatalo1
1Department of Biological and Environmental Sciences, College of Arts
and Sciences, Qatar University, P.O. Box: 2713 Doha Qatar
2Department of Ecology and Environmental Science, Umeå University,
SE-901 87 Umeå Sweden
3Ecology & Biodiversity Group and Plant Ecophysiology Group, Utrecht
University, Padualaan 8, 3584 CH Utrecht The Netherlands
Decomposition of plant litter is a key process for transfer of carbon and nutrients in
ecosystems. Carbon contained in decaying biomass is released to the atmosphere as
respired CO2, a greenhouse gas that contributes to global warming. To our knowl-
edge, there have been no studies on litter decomposition in terrestrial ecosystems in
the Arabian peninsula. Here we used commercial teabags (green tea, rooibos tea)
as standard substrates to study decomposition rates across contrasting ecosystems
in Qatar.
Teabags were buried under and beside Acacia tortilis trees, in depressions with
abundant grass vegetation, in saltmarsh without and with vegetation, under
Zygophyllum qatarense in drylands, in natural mangrove and in planted mangrove.
There were signicant site effects across ecosystems on decomposition rate (k),
litter stabilisation factor (S), nal weight of green tea and nal weight of rooibos
tea. Mangrove and depressions with grassland had the smallest amounts of
remaining green and rooibos tea after the incubation period (69-82 days), while
teabags buried under A. tortilis and in saltmarsh without vegetation had the largest
amounts. Thus decomposition rates differ among ecosystems in the desert environ-
ment. Further multi-year and site studies are needed to identify factors that inuence
decomposition rates across sites in extreme environments.
Keywords: Arabian Peninsula, carbon turnover, climate change, litter bags, green
tea, rooibos tea, teabag index, plant litter decomposition rates
Introduction
Plant litter decomposition plays an important role in the global carbon
cycle (Aerts 1997; Aerts 2006). It is estimated that more than 50% of net
primary production ends up in the soil (Wardle et al. 2004), while a large
The Arab World Geographer / Le Géographe du monde arabe Vol 20, no 2-3 (2017) 252-260
© 2017 Geo Publishing, Toronto Canada
Variation in Plant Litter Decomposition Rates across in Qatar 253
proportion is emitted to the atmosphere (Houghton 2007).
Decomposition rates of soil organic matter increase with temperature
and are more sensitive to changes at lower temperatures than at higher
temperatures (Kirschbaum 1995). This affects soil carbon content glob-
ally, with values typically being highest in cold regions (Lal 2004). Other
important factors affecting decomposition of plant litter are precipita-
tion, substrate, the quality of the plant litter and the organism performing
the decomposition (Cornelissen et al. 2007; Gavazov 2010). Models
suggest that climate and litter quality explain roughly 60-70% of global
litter decomposition rates (Parton et al. 2007). A global study has indi-
cated that in cold, dry regions, climate conditions are more important for
decomposition, while in warm, wet regions soil fauna are the main
controller (García-Palacios et al. 2013). However, that study did not
include any sites from the Arabian Peninsula with its extreme weather
conditions. Similarly, another global study (that also lacked data from
the Arabian peninsula) showed that climate (actual evapotranspiration)
was the best predictor of the decomposition constant (k-value) for litter
(Aerts 1997). Mean k-value was positively correlated with actual evapo-
transpiration, both being highest in the humid tropics (Aerts 1997). On a
global scale, climate change is likely to affect decomposition processes, as
increased levels of atmospheric carbon dioxide are likely to affect the
nitrogen content of litter and as warming will most likely increase decom-
position rates (Couteaux et al. 1995; Kirschbaum 1995; Cornelissen et al.
2007).
There have been relatively few decomposition studies in arid envi-
ronments, despite the fact that these cover about 12% of the total soil
surface on earth (Goodall et al. 2009). This is a serious omission, as
existing models, which to a large extent focus on precipitation, cannot
account for the rapid turnover of organic material in these environments
(Vossbrinck et al. 1979; Montaña et al. 1988; Steinberger et al. 1990;
Austin and Vivanco 2006). Instead, in arid environments photodegrada-
tion (Austin and Vivanco 2006; Barnes et al. 2015; Liu et al. 2015; Huang
et al. 2017), soil-litter mixing (Lee et al. 2014; Barnes et al. 2015; Hewins
and Throop 2016; Joly et al. 2017) and precipitation pulses (Hamadi et al.
2000; Hewins and Throop 2016; Joly et al. 2017) have been suggested to
be important for decomposition. The extreme arid environments of the
Arabian Peninsula are particularly under-represented in litter decompo-
sition studies. Mangrove wetlands are somewhat better studied, e.g. a
study on litter decomposition in a mangrove in Qatar found an initial
mass loss of 68% within the rst 108 days, after which the decomposition
rate declined (Mahasneh 2001). This is in line with ndings by others that
early-stage decomposition is faster, with leaching of soluble compounds
The Arab World Geographer / Le Géographe du monde arabe Vol 20, no 2-3 (2017)
254 Mohammed H.S.A. Alsafran, Judith Sarneel & Juha M. Alatalo
and decomposition of non-lignied cellulose and hemicellulose (Berg and
McClaugherty 2003). However, to our knowledge there are no previous
studies on litter decomposition in the extreme arid environment of terres-
trial ecosystems in the Arabian Peninsula. Therefore, in the present study
we measured the initial mass loss of organic matter and calculated the
stabilisation factor Sand decomposition rate kacross different ecosys-
tems in Qatar, using commercial green tea and rooibos tea as model
litters. The teabag index that can be derived from these measurements
represent a uniform decomposition value can be compared across ecosys-
tems (Keuskamp et al. 2013; Didion et al. 2016). Our specic objective
was to determine whether decomposition rates vary across different
ecosystems in the extreme arid environment of Qatar, as warm dry envi-
ronments may not result in great differences in decomposition rates.
Methods and Study Sites
Study Sites
Qatar lies within the desert belt extending from North Africa to Central
Asia and is one of the most arid countries in the world, with average
annual rainfall of 78.1 mm, so water is a major limiting ecological factor.
The climate is classied in the Köppen system as BWhsn (hot desert
climate with winter rainfall and high relative humidity). Sporadic rainfall
events occur from October to May. Temperature ranges between 35°C
and 45°C in summer, and between 15°C and 30°C in winter (December-
February).
The test teabags were buried at 10 sites (Table 1) that represent
common landscape and vegetation types in the Arabian Peninsula. These
were: Acacia tortilis (acacia) drylands (two sites), grassland, saltmarsh
without vegetation, saltmarsh with vegetation (two sites, dominated by
Salsola spp.), drylands with the succulent Zygophyllum qatarense, two
natural mangrove sites (Avicennia marina Forsk.) and planted mangrove
(A. marina) (Table 1). The Acacia tortilis drylands had sparse cover of A.
tortilis trees without undercover vegetation (which sprouts after sporadic
rainfall events), so teabags were buried under A. tortilis trees and in
patches of bare ground just beside the trees. The grassland site was
located in a landscape depression, where grasses are abundant after rain-
fall events, after which they wilt slowly as the soil dries out.
Methods
We followed the teabag index protocol (Keuskamp et al. 2013), and used
commercial Lipton green teabags (EAN: 87 22700 05552 5) and Lipton
rooibos teabags (EAN: 87 22700 18843 8), made from nylon with a mesh
The Arab World Geographer / Le Géographe du monde arabe Vol 20, no 2-3 (2017)
Variation in Plant Litter Decomposition Rates across in Qatar 255
size of 0.25 mm, which allows microorganisms and mesofauna access to
the tea but excludes macrofauna (Setälä et al. 1996). Green tea has a high
cellulose content and faster decomposition than rooibos tea, which has a
high lignin content (Keuskamp et al. 2013).
All teabags were weighed (air-dry, to 0.0001 accuracy) and 46 replicate
pairs of green and rooibos bags were buried 8 cm deep in holes at each
location, with each bag in a separate hole. The bags were buried between
12 and 20 March 2017 and retrieved between 28 May and 6 June 2017
(incubation duration 68-82 days).
The retrieved teabags were cleared of soil and roots and oven-dried
(70°C for 48 h). Due to the fact that the silty soil entered the bags, weight
loss of the teabags was then determined by deducting loss on ignition
(overnight at 550°C). Four bags had to be removed from the analysis due
to damage from invertebrates and foxes, leaving 42 tea bags for analysis.
Stabilisation factor S(i.e. the fraction of the labile material that is not
decomposed, but stabilises after three months) and decomposition rate,
k, were calculated according to Keuskamp et al. (2013) based on the
equation:
W(t)= ae-k1t +(1- a)*K2= ae-k1t +(1- a)
where W is weight, a is decomposable fraction (where S+ 1 - a), t is time
(days incubated), K1is decomposition rate of the labile fraction (steep
part of the curve) and K2 is decomposition rate of the recalcitrant frac-
tion, assumed to be 0 during short incubations of less than 3 months. a
was determined initially for green tea:
S= 1- agreen/Hgreen
and, assuming that stabilization was similar for green and rooibos
tea, aroo was calculated as:
aroo= Hroo (1 - S).
The Arab World Geographer / Le Géographe du monde arabe Vol 20, no 2-3 (2017)
TABLE 1
List and location of sites in Qatar included in the litter decomposition study.
Location codes LAT LONG Soil type Vegetation type
Grassland depression 25.37446 51.51617 Sandy loam Grasses
Saltmarsh with vegetation 25.72942 51.57562 Silty loam Salsola spp.
Saltmarsh with vegetation 25.69819 51.55212 Silty loam Salsola spp.
Saltmarsh no vegetation 25.65812 51.54709 Silty loam No vegetation
Acacia vegetated 25.51005 51.41388 Sandy loam Acacia tortilis
Acacia bare ground 25.40923 51.45926 Sandy loam No vegetation
Mangrove 25.73607 51.57624 Marine alluvium Avicennia marina
Mangrove 25.69730 51.55065 Marine alluvium Avicennia marina
Planted mangrove 25.66118 51.54853 Marine alluvium Avicennia marina
Zygophyllum 25.37446 51.51617 Sandy loam Zygophyllum qatarense
256 Mohammed H.S.A. Alsafran, Judith Sarneel & Juha M. Alatalo
where Hgreen and Hroo are the hydrolysable fraction of green and
rooibos tea, respectively, determined in laboratory extractions
(Keuskamp et al, 2013).
Subsequently, kcan be calculated from:
W(roo) = aroo e-kt +(1- aroo)
Statistical Analyses
The data were not normally distributed, so we used the conservative non-
parametric Kruskal-Wallis test to analyse the effect of site on stabilisa-
tion factor S, decomposition rate k, mass loss of green tea and mass loss
of rooibos tea. All analyses were performed in IBM © SPSS © Statistics
Version 24.
Results and Discussion
Across ecosystems, we found a signicant site effect on decomposition
rate (p=0.011), litter stabilisation factor (p=0.001), nal weight of green
tea (p=0.001) and nal weight of rooibos tea (p0.001) (Figure 1). The
mangrove and grassland depression sites had the smallest amounts of
remaining green and rooibos tea after the incubation period in soil, while
teabags buried under A. tortilis and in saltmarsh without vegetation had
the largest amounts. Rapid litter turnover and high kvalues in mangrove
have also been reported in other studies (Tam et al. 1998; Li and Ye 2014).
As expected due to differences in the ‘litter’ quality, rooibos tea (high
lignin content) had higher remaining mass after the incubation period
than green tea at all sites. The faster decomposition of green tea (high
cellulose content) across all sites is in line with the suggestion that litter
quality is one the major controllers of litter decomposition (Cornwell et al.
2008; Zhang et al. 2008; Li et al. 2011; Bradford et al. 2016). Water avail-
ability has also been suggested to be an important controller of litter
decomposition on local scale in deserts and semi-arid regions (Couteaux
et al. 1995; Zhang et al. 2008; Joly et al. 2017), with drier sites having lower
kvalues (Gholz et al. 2000). A previous study in the Al Barriyya Desert
(Palestine) found that decomposition rates were signicantly higher
during the short rainy winter period than in the dry period (Hamadi et al.
2000). Experimental water addition in semi-arid Inner Mongolia also
signicantly increased kvalues (Liu et al. 2006), while warming and exper-
imental rainfall reduction signicantly decreased litter decomposition in
dry Mediterranean grassland (Almagro et al. 2015). In Qatar, water avail-
ability is likely to be an important controller of decomposition, as the
country has an extreme arid environment with only sporadic rainfall
events and with no permanent freshwater bodies. This was reected in our
The Arab World Geographer / Le Géographe du monde arabe Vol 20, no 2-3 (2017)
Variation in Plant Litter Decomposition Rates across in Qatar 257
data, as drier acacia sites had lower kvalues than mangrove sites, which
were wetter. While sporadic local rainfall events could thus be expected to
temporarily increase the decomposition rate at drier sites in Qatar, global
warming accompanied with decreased rainfall could potentially decrease
litter decomposition in the longer term. Our study was conducted during
the dry spring-early summer period, so further long-term studies are
needed to capture sporadic rainfall events.
The initial study in which the teabag index was developed was able to
distinguish differences in decomposition rate (k) and stabilisation factor
(S) between ecosystems on a global scale after an incubation period of ca.
90 days (Keuskamp et al. 2013). Here we show that the method also
works well on a smaller regional scale, as we were able to distinguish
differences in both kand Sbetween different ecosystems in Qatar. In fact,
The Arab World Geographer / Le Géographe du monde arabe Vol 20, no 2-3 (2017)
FIGURE 1
Box plots of decomposition rate k, stabilisation factor S and mass loss of green and
rooibos tea for different ecosystems in Qatar, where k represents the short-term
dynamics of litter decomposition and S is indicative of long-term carbon storage.
Calculations were based on incubation periods of between 69 and 82 days. Labels indi-
cate ecosystems. Box plots show the 10th -90th percentiles of the data.
258 Mohammed H.S.A. Alsafran, Judith Sarneel & Juha M. Alatalo
our results show that there is considerable variation in k, Sand mass loss
of litter between local dryland ecosystems, and that the teabag index is
suitable for studies on litter decomposition in arid environments. The k
and Svalues for mangrove and the arid Acacia and Zygophyllum sites
were in line with values reported for mangrove in Florida and for desert
soils in China (Keuskamp et al. 2013). However, in order to gain a better
understanding, experimental multi-year and multi-site studies are needed
to identify factors that inuence decomposition rates across ecosystems
in the extreme environments of the Arabian peninsula.
Acknowledgements
The authors wish to thank Mara Abdelhameed Dafaalla and Mariana
Tavelin-Sjöberg for their assistance during laboratory work. The study
was partly funded by a grant from Qatar University to JMA (grant
QUUG-CAS-DBES-15/16-5). JS conducted her work within the
strategic theme Sustainability, sub-theme Water, Climate, and
Ecosystems, at Utrecht University and was funded by the Swedish
Research Council, Vetenskapsrådet.
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