carbon flux in deserts depends on soil cover type-a case study in the gurbantunggute desert, north...

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Carbon ux in deserts depends on soil cover type: A case study in theGurbantunggute desert, North China

Y.G. Su, L. Wu, Z.B. Zhou, Y.B. Liu, Y.M. Zhang*

Key Laboratory of Biogeography and Bioresource in Arid Land, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, South Beijing Road 818, Urumqi,

Xinjiang 830011, China

a r t i c l e i n f o

Article history:

Received 22 April 2012Received in revised form19 November 2012Accepted 14 December 2012Available online 2 January 2013

Keywords:

Arid and semiarid regionsBiological soil crustsCarbon sinkRainfallVascular plants

a b s t r a c t

Carbonux represents carbon uptake from or release to the atmosphere in desert ecosystems, yet thechanging pattern of carbon ux in desert ecosystems and its dependence on soil cover type andrainfall amount are poorly understood. We measured net carbon uxes (NCF) in soil with four covertypes (moss crusted soil, cyanobacteria/lichen crusted soil, bareland and semishrubEphedra distachya-inhabited site) from April to October of 2010 and 2011, and NCF and dark respiration (DR) after fourrainfall amounts (0, 2, 5, and 15 mm) in cyanobacteria/lichen crusted soil, bareland and theE. distachya-inhabited site. NCF in the E. distachya-inhabited site differed signicantly from those of theother three soil cover types, while no difference was observed between the moss and cyanobacteria/lichen crusted soils or between the two crusted soils and bareland on most measurement occasions.NCF ranged from 0.28 0.14 to 1.2 0.07 mmol m2 s1 in the biologically crusted soils, and from2.2 0.27 to 0.46 0.03 mmol m2 s1 at theE. distachya-inhabited site. Daily NCF in the biologicallycrusted soils and bareland showed carbon release at most times and total carbon production rangedfrom 48.8 5.4 gC m2 yr1 to 50.9 3.8 gC m2 yr1, while the E. distachya-inhabited site showeda total carbon uptake of57.0 9.9 gC m2 yr1. Daily variances in NCF were well-explained byvariances in surface soil temperature, and seasonal NCF showed a signicant linear relationship withsoil moisture in the two biologically crusted soils and bareland when soil volumetric water contentwas less than 3%. Rainfall elicited intense carbon release in cyanobacteria/lichen crusted soil, barelandand at the E. distachya-inhabited site, and both NCF and DR were positive in the rst two days afterrainfall treatments. Mean NCF and DR were not different between rainfall amounts of 2, 5 and 15 mmin cyanobacteria/lichen crusted soil and bareland, while they were signicantly higher after 15 mmrainfall treatment compared with 2 mm and 5 mm treatments at the E. distachya-inhabited site. MeanNCF and DR in the rst two days increased logistically with rainfall amount. Based on our ndings, wesuggest that E. distachya-inhabited sites contribute to carbon uptake in the Gurbantunggute Desert,while biologically crusted soils exhibit carbon release for most of the year. Even though photosynthesisimmediately following rainfall can be stimulated, carbon uptake effect in biologically crusted soil islikely intermittent and conned to periods when moisture is available.

2012 Elsevier Ltd. All rights reserved.

1. Introduction

Accurate assessment of carbon budget in variable ecosystems isvital to our understanding and accurate estimation of the globalcarbon balance (Schime et al., 2001;Korner et al., 2003), especiallyconsidering the ever-increasing nature of carbon emissionsresulting from fossil fuel usage and land use changes (IPCC, 2007).Compared with other ecosystems, deserts have received much less

attention in this regard because of their lower vascular plantbiomass (Whittaker, 1975) and soil carbon storage (Ojima et al.,1993;Jobbgy and Jackson, 2000). Indeed, profound variances inthe vegetation pattern and large spatial heterogeneity in availableresources in desert ecosystems contribute to signicant uncertaintyin evaluating their roles as carbon sources or sinks (Schlesingeret al., 1996;Jasoni et al., 2005;Wohlfahrt et al., 2008;Xie et al.,2009;Liu et al., 2012). Most studies conducted so far have showneither that deserts are a source of carbon or that they are carbonneutral (Schimel et al., 2001). However, some recent studies haveshown evidences of carbon sinks in deserts, and these ndingsmake deserts potentially valuable in efforts to mediate climate

* Corresponding author. Tel./fax: 86 991 7823149.E-mail address:[email protected](Y.M. Zhang).

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Soil Biology & Biochemistry

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Soil Biology & Biochemistry 58 (2013) 332e340


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change compared to temperate forests or grassland ecosystems(Jasoni et al., 2005;Wohlfahrt et al., 2008;Xie et al., 2009). Forexample, in a sandy desert ecosystem containing deciduous semi-shrubs (Caragana korshinskiiand Artemisia ordosica), slight carbonxation of 13.87e23.36 g C m2 yr1 has been observed. Moreover,

in the Mojave desert, which is dominated by the evergreen shrub(Larrea tridentata (DC.) Cov.) and droughtedeciduous shrubs(Lycium andersonii (A. Gray) and Ambrosia dumosa (A. Gray)),carbon uptake of 102 67e185 15 g C m2 yr1 has beenobserved (Jasoni et al., 2005;Wohlfahrt et al., 2008).

The species composition and vegetation cover of desertecosystems differ greatly (Noy-Meir, 1973). In mobile deserts,vegetation cover is usually around 1% or less, and xeric shrubs orsemi-shrubs occur in these areas (Noy-Meir, 1973). By contrast, inxed and semi-xed deserts, vegetation cover increases dramati-cally with precipitation, and can even exceed 40% when ephem-eroid and ephemeral herbs are present under favorable soilmoisture conditions. Moreover, biological soil crusts develop andare widely spread on the soil surface, where no vascular plantsdevelop in these regions (Belnap and Lange, 2003). In recent years,articial planting in mobile sandy dunes (Li et al., 2003;Zhao et al.,2010) and increasing annual rainfall (Wohlfahrt et al., 2008) havechanged vegetation structure, biomass as well as soil properties ofdeserts. Such changes may potentially alter the magnitude ofcarbonux in desert ecosystems. In addition, vegetation structure,which differs profoundly among deserts, interacts with environ-mental factors that control carbon cycling and affects carbonuxindeserts (Raich and Schlesinger, 1992). For instance, greater vege-tation cover usually induces more ecosystem carbon uptakethrough increased photosynthesis (Wohlfahrt et al., 2008; Gaoet al., 2012) and reduced soil respiration, which is induced bylower soil temperature (Luo and Zhou, 2006;Tesar et al., 2008). Inaddition, since soil properties are always tightly associated withvegetation patterns (Maestre and Cortina, 2002), changes in soil

properties may have signicant inuences on soil respiration,which may indirectly alter carbon ux in desert ecosystems (Luoand Zhou, 2006).

The carbon budget of deserts is largely driven by availability ofwater to vegetation and soil microbes, as well as biological soilcrusts (Noy-Meir, 1973; Smith et al., 1997; Cable and Huxman,2004). For instance, large carbon release is essentially conned toa very limited period immediately after rainfall (Huxman et al.,2004a, b; Veenendaal et al., 2004; Hastings et al., 2005; Pottset al., 2006a, b; Kurc and Small, 2007), and signicant carbonuptake may occur only several days after heavy rainfall events,which increases the rate of photosynthesis in vascular plants (Gaoet al., 2012). Rainfall patterns in deserts also determine the trade-off between carbon uptake (photosynthesis by autotrophs) and

carbon release (respiration by heterotrophs or dark respirationfrom photoautotrophs) (Huxman et al., 2004b), specically, themagnitude and duration of carbon ux are related to rainfallamount (Huxman et al., 2004b) and the interval between consec-utive rainfall events (Sponseller, 2007). For instance, biological soilcrusts are sensitive to all magnitudes of rainfall, whilst vascularplants are only responsive to heavy rainfall, which can penetrateinto deep soil layers and reach the roots. Moreover, biological soilcrusts usually initially exhibit carbon release, then take up carbonwhen moisture is available. Therefore, carbon balance in biologi-cally crusted soil is affected by the duration of rainfall (Huxmanet al., 2004b).

The Gurbantunggute Desert is the biggest xed and semi-xeddesert in China. Its landscape is characterized by mosaic distri-

bution of shrubs, semi-shrubs, biological soil crusts and bareland(Zhang et al., 2007). As in the case of other dry lands, carbondynamics in the Gurbantunggute Desert is highly sensitive to

water availability (Xu et al., 2007). In the present study, twohypotheses were proposed to explain the variances in net carbonux (NCF) in the Gurbantunggute Desert: (1) Photosynthesis invascular plants is usually higher than that of biological soil crusts(Lange, 2003), in addition, though soil organic matter and soil

microbes in biologically crusted soils are higher than those invegetated sites and bareland (Table S1), soil respiration is usuallyconned to very low values due to low soil moisture. Moreover,soil moisture and temperature differ between measurementperiods, and variances in carbon ux might be linked with soilmoisture and temperature. Our rst hypothesis is that carbon uxat different soil cover sites follows as the relationship: biologicallycrusted soil > bareland > vegetated site, carbon ux varies withsoil temperature and soil moisture at different soil cover sites; (2)Biological soil crusts can utilize a wide range of rainfall events toinitiate their physiological activities (Lange, 2003), and theyusually show respiration initially, and then photosynthesis occurs.Vascular plants can only use large rainfall events that penetrateinto deeper soil layers and are accessible to roots, and smallamounts of rainfall only trigger carbon ux in the surface soil layer.Our second hypothesis is then that rainfall translates into dramaticincreases in carbon efux from biologically crusted soil, barelandand vegetated sites irrespective of rainfall magnitude, and thatcarbon inux in biologically crusted soil and vegetated sites mightoccur after relatively intense rainfall, which can trigger photo-synthesis in crust organisms and vascular plants.

2. Materials and methods

2.1. Study site description

The Gurbantunggute Desert is located at the center of theJungger Basin in Xinjiang Uygur Autonomous Region of China. It isthe biggest xed and semi-xed desert in China with an area of

4.88 104 km2. Our experiment was conducted in the southernpart of the Gurbantunggute Desert (44.87N, 87.82E). Precipitationoccurs predominantly during spring, totaling approximately79.5 mm. The mean annual evaporation is 2606 mm. The averageannual temperature is 7.26 C. Wind speeds are strongest duringlate spring, averaging 11.2 m s1 (Zhang et al., 2007). The welldeveloped soil surface cover contributes to the soil stability of theGurbantunggute Desert, which also benets from the nativevegetation and biological soil crusts.Ephedra distachyaLinn. is oneof the dominant species in Jungger Basin, and is mainly distributedon the sand slope and lower land between sand dunes. The averagecanopy volume of an individualE. distachyais 44,602 1,0291 cm3

(Table S1), and its cover can reach 20% at the study site. Biologicalsoil crusts are widely distributed on soils between shrubs, usually

with a cover >60% (Table S1). Two types of biological soil crustsdominate in this region: cyanobacteria/lichen and moss crusts(Zhang et al., 2009). The cyanobacteria/lichen crusts usually exist inevery location in a sand dune, except at dune tops in some highsand dunes. They are mainly composed of Microcoleus vaginatus(Vaucher) Gomont ex Gomont, Microcoleus paludosus (Ktzing)Gomont, Anabaena azotica Ley, Porphyrosiphon martensianus(Meneghini ex Gomont) Anagnostidis & Komrek, Collema tenax(Sw.) Ach., Psora decipiens (Hedw.) Hoffm., Xanthoparmelia deser-torum (Elenkin) Hale., and Diploschistes muscorum (Schreb.) (Wuet al., 2009). Moss crusts are typically located at the bottom ofdunes andform a mosaic with cyanobacteria/lichen crusts. They areusually composed ofSyntrichia caninervis Mitt., Bryum argenteumHedw., andTortula muralisHedw. The soil organic matter and total

nitrogen content, soil bulk density, and soil mechanical composi-tion all differ largely among biologically crusted soil, bareland, andE. distachya-inhabited sites (Table S1).

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2.2. Experimental design

We selected the four most frequent soil cover types in the studyarea to measure NCFfrom April to October in 2010 and 2011: (1) mosscrusted soil (with moss crusts cover>70%), (2) cyanobacteria/lichen

crusted soil (with cyanobacteria/lichen crusts more than 60%), (3)bareland (biological soil crusts cover


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during the day and carbon release at night, with values rangingfrom2.21 0.27 to 0.46 0.03 mmol m2 s1 (Fig.1). Soil moisturewas highest in April and October, and lower values occurred in Juneand August. Soil temperature was highest in August and lowest inOctober. For these reasons, we only show NCF for April, June, Augustand October of 2010 and 2011 in the present study (Fig. 1).

Seasonal NCF in biologically crusted soil and barelandusually showed carbon release, ranging from 0.002 0.001 to0.49 0.07 mmol m2 s1 in biologically crusted soils, andfrom0.0010.001 to 0.44 0.01 mmol m2 s1 in bareland (Fig. 2).Seasonal NCFat the E. distachya-inhabited site usually showed carbonuptake, ranging from 0.58 0.13 to 0.09 0.02 mmol m2 s1

(Fig. 2). Repeated-measure ANOVA showed that NCF differed signif-icantly among soil cover types (Table 1,p


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No difference was observed between the two biologically crustedsoils (Fig. 2, p > 0.05). Total carbon uptake at the E. distachya-inhabited site was 57.0 9.9 gC m2 yr1 in 2011, which wassignicantly higher than that at the other three sites (Fig. 2,p< 0.01).

3.2. Carbonux in the three soil cover types in response to rainfall

amount

NCF and DR increased dramatically after rainfall (Fig. 4), andreturned to their original values after approximately 12 h in cya-nobacteria/lichen crusted soil and bareland and after approxi-mately12 h of 2 mmand 5 mmrainfall at the E. distachya-inhabitedsite. By contrast, their values remained high until 48 h after 15 mmrainfall treatment at the E. distachya-inhabited site (Fig. 4). Rainfallsignicantly enhanced NCF (Table 3, p < 0.01) and DR (Table 3,p0.05). There was no difference inmean NCF and DR between rainfall amounts of 2 mm, 5 mm, and

15 mm in cyanobacteria/lichen crusted soil and bareland ( Table 4,p > 0.05). Mean DR was well-described by logarithmic functionsincorporating rainfall for cyanobacteria/lichen crusted soil, bare-land and the E. distachya-inhabited site (Fig. 4b), and mean NCF wasdescribed by logarithmic functions incorporating rainfall at the

margin of signicance for cyanobacteria/lichen crusted soil and theE. distachya-inhabited site (Fig. 4a).

4. Discussion

Our rst hypothesis was that carbon ux increases from vege-tated site to bareland, is the highest in biologically crusted soil, andvaries with soil temperature and soil moisture availability. Ourndings support this hypothesis. The variances in carbon ux werelarge, and depended on the temporal and spatial scales. Due to theclose relation of NCF with soil organic matter, soil temperature, andsoil moisture, NCF showed both positive (carbon release) andnegative (carbon uptake) values, which is consistent with previousreports (Unland et al., 1996;Emmerich, 2003;Maestre and Cortina,

2003;Hasting et al., 2005;Wohlfahrt et al., 2008;Gao et al., 2012).In the present study, since no vascular plants and biological soilcrusts occurred in bareland, NCF in bareland results from soilrespiration and shows carbon releases for most of the year. Bycontrast, carbon uptake occurred during the day at theE. distachya-inhabited site, possibly because photosynthesis in E. distachyaoffsets soil respiration. This indicates that E. distachya-inhabitedsites are responsible for carbon uptake in this region during most ofthe year.

Moss crusted soil and cyanobacteria/lichen crusted soil showedcarbon release most of the time, which may be explained by thefollowing three mechanisms. First, photosynthesis in biological soilcrusts is rather low, and cannot offset soil respiration. Some labo-ratory studies and few eld experiments have reported photosyn-

thesis in biological soil crusts (Lange, 2003), and biological soilcrusts have been reported to have an estimated median net carbonuptakeof 16 g m2 yr1 in drylands (Elbertet al., 2009). However, in

Table 1

The effects of soil cover type (C), measurement time (T), and their interactions onmean net carbon ux in a day (NCF) (mmol m2 s1) in 2010 and 2011.

Source ofvariance

2010 2011

df F p df F p

C 3 52.9


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a different study, no carbon uptake was found to occur in biologi-cally crusted soil during a 3-month eld observation; even aftera 24 mm rainfall, biologically crusted soil was found to havedecreased carbon release compared with values prior to rainfall(Maestre and Cortina, 2003). Secondly, photosynthesis in biologicalsoil crusts is not as persistent as that in vascular plants and likelyoccurs intermittently and during specic conditions. Some studieshave shown that photosynthesis of biological soil crusts is inti-mately correlated with conditions prior to the availability ofmoisture (Jeffries et al., 1993; Lange, 2003; Lange et al., 2007). Crustorganisms usually take several minutes to days (Lange et al., 1992;

Jeffries et al., 1993; Garcia-Pichel and Belnap, 1996;Harel et al.,2004) to initiate photosynthesis after rainfall. We observedcarbon ux once a month, and did not observe carbon ux inspecial weather conditions, such as periods of snow melting, dewformation and rainfall, all of which may have stimulated photo-synthesisin crusts organisms (Lange, 2003). Third, the frequent andsevere drought at our study site could explain the discrepancybetween our results and some previous studies (Thomas et al.,2008). Precipitation was no more than 80 mm at our researchsite, so biological soil crusts might have been dormant during our

measurements.Soil water availability is a critical factor determining the alter-

ation of plant species, plant physiological activity, and relativeecological processes in deserts (Song et al., 2012). Consistent withthis, our study also shows dependence of carbon ux on soil wateravailability when SVWC is less than 3% in both biologically crusted

soil and bareland, and that NCF does not change with SVWC whenSVWC exceeds 3%, This is closely related to the characteristics ofshallow distributions of soil organic matter, nutrients and soilmicrobes in soil. Higher SVWC usually indicates that the deeper soilis also wet, but deeper soil contributes considerably less carbonrelease compared with shallow soil layer due to limited amount ofsoil nutrients and soil microbes in them (Huxman et al., 2004b;Thomas et al., 2008;Thomas and Hoon, 2010). In addition, highersoil moisture may saturate soil pores, which impedes CO2 release tothe atmosphere (Sponseller, 2007;Thomas and Hoon, 2010). Ourndings are consistent with the reports of a research study con-ducted in a desert grassland system in Arizona (Emmerich, 2003).NCF varies from 22.6 to 5.6 mmol m2 s1 in the wet season, butonly has positive values in the dry season (Emmerich, 2003).Variations in soil temperature alone can explain 22e83% of vari-ances in daily NCF since soil moisture is almost consistentthroughout a day, and daily NCF in our study is comparable tovalues measured at dry periods or in desert ecosystems with lowannual precipitations (Unland et al., 1996; Emmerich, 2003;Hasting et al., 2005;Gao et al., 2012). In a shrub dominant desert,carbonux ranges from 0.75 to 0.5 mmol m2 s1 in a year with

annual precipitation of 147 mm (Hasting et al., 2005). Consistentwith this, daily NCF has been reported to range between 0.4 and0.35 mmol m2 s1 prior to rainfall in the Sonoran desert (Unlandet al., 1996).

Carbon uptake in bareland and biologically crusted soil occa-sional occurred at night in August and September of both years in

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0

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-0.5 0 0.25 0.5 1 2 6 12 24 48

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-0.5 0 0.25 0.5 1 2 6 12 24 48

-1

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1

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4NCF(umolm

-2s

-1)

0 mm 2 mm

5 mm 15 mm

DR(umolm

-2s

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Hour(s) after rainfall treatment Hour(s) after rainfall treatment

(a)

(b)

(c)

(d)

(e)

(f)

Fig. 4. Dynamics of net carbon ux (NCF) (a, b, c) (mmol m2 s1) and dark respiration (DR) (d, e, f) (mmol m2 s1) in cyanobacteria/lichen crusted soil (CLCS) (a, c), bareland (BL)

(b, e), and E. distachya-inhabited site (Ed S) (c, f) after four precipitation treatments of 0 mm, 2 mm, 5 mm and 15 mm. Values are the mean S.E.

Table 2

The effects of soil cover type (C), rainfall treatment (R), and their interactionson mean net carbon ux (NCF) (mmol m2 s1) and dark respiration (DR)(mmol m2 s1) in 48 h after rainfall treatments.

Source ofvariance

NCF DR

df F p df F p

C 2 24.0


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our study. In an adjacent study site,Xie et al. (2009)also observedthis carbon uptake at night. Moreover, they found that carbonuptake was higher in drier soil, which was ascribed to alkaline soilabsorption (pH: 9.06 0.1e10.23 0.04). As no CAM plants growinour study area and soil pH in biologically crusted soil was8.20 0.05, we speculate that the observed carbon uptake at nightmight have resulted from alkaline soil absorption. However,substantiating this speculation is difcult. On one hand, carbonuptake is usually very low, and sometimes falls in the range ofinstrument errors, making accurate detection with the analyzerdifcult. On the other hand, as this phenomenon occurred onlyintermittently at our study site, measuring carbon uptake proved tobe difcult.

Partially in line with our second hypothesis, rainfall only trans-lates into carbon release at the E. distachya-inhabited site, in cyano-bacteria/lichen crusted soil and in the bareland. This is easilyexplained in the case of bareland since they do not contain auto-trophic species, and carbonux observed afterrainfall is actually soilrespiration. In biologically crusted soil, the effects of rainfall oncarbon ux are contradictory (Maestre and Cortina, 2003; Cable andHuxman, 2004; Thomas et al., 2008; Thomas and Hoon, 2010).Carbon release has been reported to occur in biologically crustedsoil

irrespective of rainfall amounts (Cable and Huxman, 2004;Thomasand Hoon, 2010). By contrast, Thomas et al. (2008) have shownthat cyanobacterial crusted soil exhibits carbon uptake at a rate of0.14 mmol m2 s1 when the surface and subsurface soil is dry, andcarbon release at a rate of 1.3mmol m2 s1 when the subsurface iswet. Carbonux in biologically crusted soil is the sum of photosyn-thesis or respiration in biological soil crusts and carbon release in thesoil below the biological crust layer. DR was usually higher than NCFin cyanobacteria/lichen crusted soil in our study. This indicates thatrainfall has induced photosynthesis in crust organisms. Photosyn-thesis in cyanobacteria-dominated crusts is usually around or lessthan 1 mmol m2 s1 (Lange et al., 1998;Zaady et al., 2000;Lange,2003;Brostoff et al., 2005), Therefore, lower photosynthesis ratecannotoffset soilrespirationin cyanobacteria/lichencrusted soil,and

cyanobacterial/lichen crusted soil has only shown carbon release.There is no differencein NCF andDR among various rainfall amountsin cyanobacterial/lichen crusted soil and bareland. This is primarilydue to soil organic matter and soil microbes distribution patterns atthose two sites. Soil organic matter and soil microbes mainlyconcentrate at the soil surface (Table S1), and there is not enoughnutrient, substrate, and soil microbes to cause high soil respirationeven moisture is high at deep soil layers. No carbon uptake occurredat theE. distachya-inhabited site after 15 mm rainfall. This could bedue to the short observation period after 15 mm rainfall, Photosyn-thetic rates of deep-rooted plants usually increase signicantly 1e5days after an effective rainfall in arid regions (Hasting et al., 2005;Hao et al., 2010;Gao et al., 2012;Liu et al., 2012).

In the Gurbantunggute Desert, rainfall is usually sporadic and

small, with the period between consecutive rainfalls being at leastseveral days or even weeks long (Zheng et al., 2012). Biological soilcrusts are dormant when they are dry, and they initially activate

respiration prior to photosynthesis when moisture becomes avail-able. The longer the drought is before moisture availability, thelonger the respirationlasts in biological soil crusts (Lange et al.,1992;

Jeffrieset al.,1993; Garcia-Picheland Belnap,1996; Hareletal.,2004).In contrast,E. distachya-inhabited site usually shows carbon uptakewhen the surface soil is dry or after a large rainfall, and exhibitscarbon release after a small rainfall. Combining the physiologicalactivities of both biological soil crusts and E. distachya with therainfall pattern in this desert ecosystem, we can infer thatE. distachya-inhabited sites show carbon uptake and biologicallycrusted soil acts as a carbon source for most of any given year.However, carbon uptake in biologically crusted soil may be inter-mittent and is usuallyconned to specialweather conditions,such asmelting of snow, consecutive rainfall, and heavy rainfall in deserts.

5. Conclusion

Arid and semiarid areas are characterized by a mosaic distri-bution of vascular plants and biological soil crusts. A betterunderstanding of carbon ux in fundamentally different soil coversis necessary for identifying and studying the relationships betweencarbon sinks and sources of desert ecosystems. Our study has

revealed that carbon ux differs considerably among sites withdifferent soil covers and among measurement times. Biologicallycrusted soil typically showed carbon release during regular dailymeasurement times from April to October in 2010 and 2011. Incontrast, the E. distachya-inhabited site showed carbon uptake formost of the year. Rainfall elicited only carbon release in bareland,cyanobacterial/lichen crusted soil and the E. distachya-inhabitedsite. The magnitude of carbon release in cyanobacterial/lichencrusted soil and bareland were the same irrespective of rainfallamount. These ndings suggest that E. distachya-inhabited sitescontribute to carbon absorption while biologicallycrusted soil playsan important role in carbon release for most of the year in theGurbantunggute desert. Moreover, since our experiments were notconducted continuously throughout the year, and carbon ux

under special weather conditions when moisture became availableto biological soil crusts was ignored, biologically crusted soil mighthave shown carbon uptake under these conditions. The novelty ofour study lies in the revelation that it is the vegetated sites, notbiologically crusted soil, that contributes to carbon uptake duringmost of the year in the Gurbantunggute desert. This is in contrast toprevious studies implicating that biologically crusted soil, owing totheir extensive distribution and high photosynthetic capacity(Austin et al., 2004; Belnap et al., 2004; de Soyza et al., 2005;Sponseller, 2007), is responsible for carbon uptake in desertecosystems that show net annual carbon uptake (Jasoni et al., 2005;Wohlfahrt et al., 2008;Xie et al., 2009).

Acknowledgments

The authors would like to thank Dr. Gang Huang for hisconstructive suggestions during the realization of the manuscript,

Table 4

Mean net carbon ux (NCF) (mmol m2 s1) and dark respiration (DR) (mmol m2 s1) in 48 h after rainfall amounts of 0 mm, 2 mm, 5 mm and 15 mm in cyanobacteria/lichencrusted soil (CLCS), bareland (BL), and E. distachya-inhabited site (Ed S). Values are expressed as mean S.E. Different small letters indicate signicant differences betweenrainfall treatments, different capital letters indicate signicant differences between sites atp 0.05 level.

Rainfall(mm)

NCF DR

CLCS BL Ed S CLCS BL Ed S0 0.23 0.07Aa 0.15 0.02Aa 0.26 0.02Ba 0.22 0.04Aa 0.15 0.02Aa 0.23 0.04Aa

2 0.68 0.19Ab 0.86 0.01Ab 0.02 0.04Bb 0.67 0.18Ab 0.84 0.02Ab 0.57 0.05Ab

5 0.88 0.20Ab 0.88 0.04Ab 0.27 0.12Bb 0.96 0.22Ab 0.88 0.04Ab 0.89 0.06Ac

15 0.87 0.09Ab 0.89 0.22Ab 0.67 0.08Ac 1.09 0.10Ab 0.89 0.22Ab 0.99 0.05Ac

Y.G. Su et al. / Soil Biology & Biochemistry 58 (2013) 332e340338


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Junhui Chen for his eld work, as well as the two anonymousreviewers for valuable comments on the manuscript.This work wassponsored by the Chinese National Natural Scientic Foundation(41001067, 31000217, U1203301, 2010FA92720-06), and West LightFoundation of the Chinese Academy of Sciences (RCPY201101,

XBBS201007).

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.soilbio.2012.12.006.

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