沙漠研究 19-1, 237- 240 (2009 )Journal of Arid Land Studies“Desert Technology IX ”
Refereed Paper

Comparison of Erodibility on Four Types Biological Crusts inGurbantunggut Desert from Wind Tunnel Experiments
Wang XUEQIN1, Zhang YUANMING1, Zhang WEIMIN2, Xu XINWEN1, Fu CHUNLI1
Abstract: Experiments of wind erodibility on four types biological crusts (moss crust, lichencrust, algal crust and algal-lichen crust) obtained from Gurbantunggut Desert were conducted inwind tunnel. The results showed that biological crusts and its destructive percentage had
considerable influences on surface erosion. There was no sand grain movement and groundsurface erosion occurred on the four types biological crusts even wind speeds reached to 25 m
s-1 while the relative wind erosion rate (WER) on bare sand reached 1214.82 g m-2 sec-1.
Under the same destructive percentage, the threshold wind velocity (TWV) over the moss
crusts was the highest, followed by lichen crusts, algal crust and algal-lichen crusts was thelowest. The WERs increased exponentially with increasing of destruction percentageof crust cover and wind velocity.
Keywords: Biological crusts, Dune surface stability; Gurbantunggut Desert, Threshold windvelocity, Wind erosion rate

1. Introduction
Biological soil crust, formed by different combinations of mosses, lichens, liverworts, algae, fungi,
cyanobacteria and bacteria, is a common and widespread phenomenon in desert areas all over the world(West, 1990; Evans and Johansen, 1999). Despite its unassuming appearance, biological soil crust plays asignificant role in the desert ecosystem and has received increasing attention in recent years for theiroutstanding contribution in control land desertification in the world (e.g. Anderson et al., 1982; Li et al.,2001). Most studies agreed that destruction of biological crusts can significantly decrease the threshold
wind velocities for sand moving, increase soil erosion (e.g. Mackenzie and Pearson, 1979; Belnap andGillette, 1997; Williiams et al., 1999) and make the organic matter losses (West, 1990). More than 80%
area of Gurbantunggut Desert, the second largest desert in China, is covered by biological crusts and thesand surface as a whole is relative stable, while mobile area confined to the crest and the upper part of dune
slopes. However, the frequency of wind erosion events in the desert tends to increase with frequent
human activities in resent years. A detailed study on soil erodibility of different crust type and their
disturbance is required. The aims of the research reported here were to determine typical TWVs andWERs on four biological soil crusts at different destruction percentage by wind tunnel experiments.
2. Study area and methods
Gurbantunggut Desert, with an area of 4.88×104 km2, is located in the hinterland of the Junggar Basinin northwest China (44˚11′-46˚20′ N, 84˚31′-90˚00′ E) as shown in Figure 1. Main morphological dune
type is longitudinal dune with a few hundred meters to more than 10 km in length, 10-50 m in height andorientated north-south. Interdune and middle to lower part of slope are stabilized but the crest often has
10-40 m wide mobile zone. Interdune and middle to lower slope are covered by Ephedra distachya
communities and beneath them are ephemeral plants and biological crusts. Biological crusts in the study
area contain moss crusts, lichen crust, algae crusts and algal-lichen crusts (Zhang et al., 2004). Thesefour types biological crusts and pure sand samples were collected from different position on a typicallongitudinal dune surface in the southern part of Gurbantunggut Desert as shown in Figure 1. Each typecrusts contain 10 samples. In order to keep completeness of crust samples in fields, the soil surface weaimed at was uniformly sprayed with water prior to sampling. Till water seeped down to 15 cm or so, we1

National Engineering Technology Research Center for Desert-oasis Ecological Construction, Urumqi, 830011, China(Fax: 0086-991-7885320; Tel: 0086-991-7885411; E-mail: xqwang@ms.xjb.ac.cn )2
Key Laboratory of Desert and Desertification, Cold and Arid Regions Environmental and Engineering ResearchInstitute, CAS, Lanzhou, 730000, China

238

dug a groove by its side and drove a
30cm×20cm×10cm (length×width×height)
dustpan-like iron box into the soil. Then
used a sharp knife to cut it apart and take out
the undisturbed soil and closed the side door
as shown in Figure 1. To avoid theinfluence of soil moisture on the experiment,each sample was air-dried under the natural
condition for three weeks and kept theirmoisture content to no more than 0.6%.
The tests were carried out in thestraight-line, blow-type wind tunnel at theLaboratory of Blown Sand Physics andDesert Environment, Cold and Arid Regions
of Environmental and Engineering ResearchInstitute, Chinese Academy of Sciences.
The wind tunnel has a total length of 37.8 m.
Its working section is 16.2 m long, 1.0 mwide and 0.6 m high. The experimentpoint was located at 12 m downwind fromthe entrance of the working section. Wind
velocity can be changed continuously from
2-40 m s-1. Detailed descriptions of thisfacility and related instrumentation weregiven by Dong et al. (2001). For the Fig. 1. The location of sampling site in Gurbantunggut Desert
and four type biological crusts and loose sand samples.
experiments, the samples were placed in the(a): moss crusts; (b): lichen crusts; (c): bare sand sample;
lift trough at the end part of working section(d): algal crusts; (e): algal-lichen crusts
and the crust surface was kept even with thefloor of the wind tunnel. The impact area of each sample was 30 cm×20 cm. In designing theexperiment, six levels of destruction rates of biological crusts cover (10%, 20%, 30%, 50%, 80% and 100%)
were conducted, which are expressed by the percentage of damaged area out of the whole impact area. To
obtain TWVs for the disturbed crusts and loose sand surface, wind speed in the tunnel was gradually
increased until consistent forward sand particle movement was observable across the soil surface. The
corresponding wind velocity, which was measured by standard Pitot tube at 20 cm above the floor, wasdefined as the threshold velocity in wind tunnel. Since >25 m s-1 wind velocities are rare in the desert, thetests for soil erodibility were conducted at six levels of wind speed (6, 10, 14, 18, 22 and 25 m s-1). Eachexperiment was run for 10 min. The quantity of soil erosion at different conditions was calculated fromchanges in weight of samples (with a 0.01 electronic balance) before and after each experiment. The rateof erosion (WER) was calculated in g m-2 sec-1 by using quantity of wind erosion materials per unite area in
per unite time.
3. Results and Discussion
3.1. Impact of biological crusts on threshold wind velocity (TWV)The experiment demonstrated that no sand grain movement was observed on all undisturbed crusts
surface at 25 m s-1 wind velocities, while we could identify the initiation of grain movement on loose sandsurface only at wind velocity of 8.42 m s-1. Under the same destructive percentage, the TWV over moss
crusts was the highest, followed by lichen, algal and algal-lichen crusts, showing that moss and lichen
crusts have stronger wind erosion resistance. Under 10% of destructive percentage, the TWVs over moss
crusts were higher than 18.52 m s-1. Under 30% of destructive percentage, the TWV reduced to 12.62 m
s-1. When destructive percentage reached 100%, sand grain could be observed in motion at a windvelocity of 9.90 m s-1. TWVs of all crusts decreased rapidly from initial disturbance to 30% of239

TWVs /m sec-1

destructive percentage. There were no obvious changes
25
Moss crust
were observed when destructive percentage was over 50%.
Lichen crust
20Comparatively, disturbed algal and algal-lichen crusts had
Algal crust-1
poor erosion resistance. Their TWVs were 16.04 m s andAlgal-lichen crust
15
Loose sand surface
11.78 m s-1 respectively under the destruction rate of 10% and-1-1
they reduced to 12.27 m s and 10.72 m s as destructive
10percentage increased to 20%. The TWVs reduced to 8.6 ms-1 when the crusts were entirely destroyed, almost
5approximated to the bare sand surface. The curve of algal
0crusts in Figure 2 had a great change at a destructive
020406080
100
percentage of about 20%, which suggesting that smaller
Destructive percentage of crust cover / %disturbance to algal crusts can cause sand moving. Algallichen crusts had a less significant variation and had the Fig. 2. TWVs for four type crusts at
different destruction percentage oflowest TWVs among the four type crusts.
crust cover.

WERs / g m -2 sec -1

3.2 Impact of biological crusts onwind erosion rate (WER)
As shown in Figures 3 and 4, theresults of WER showed that thecrust-covered surface has much stronger
wind erosion resistance than loose sandsurface, among them moss and lichen
crusts are the strongest, followed byalgal-lichen crust and the weakest isalgal crust. Under 8.42 m s-1 windvelocity, the WER on bare sand surface
was 0.09 g m-2 sec-1, at 18 m s-1 thefigure was 298.88 g m-2 sec-1 and at 25m s-1 it could reached 1214.82 g m-2
sec-1. For moss and lichen crusts, evenbeing entirely destroyed (not removed),
their WERs only accounted for 1.70%
and 1.95% of that on loose sand surface
respectively at wind velocity of 25 m s-1, Fig. 3. Variation of average WERs for four type crusts with windspeed at different destructive percentage of crust cover.
while algal and algal-lichen crusts hadlarge values, which were 5.68% and 4.13% respectively.
1.40
The WERs for four type crusts did not exceed 8.95% ofMoss crust
1.20
that on the loose sand surface at the same wind velocity.
Lichen crust
1.00
Algal crustWith increase in destructive percentage of crust cover
Algal-lichen crust
0.80
and wind velocities, the WERs of all type crusts
0.60
increased exponentially. WERs could reach 0.18-0.25
g m-2 sec-1 only under the destruction percentage of0.40<20%. At the destruction percentage of 50-80%, the0.20
WERs corresponding to the TWVs decreased to0.00
0.07-0.15 g m-2 sec-1 and when the crusts were entirely
0102030405060708090 100-2-1
destroyed, they increased to 0.41-0.58 g m sec . For
Destructive Percentage of crust cover / %algal crusts, the WERs varied between 0.11-0.19 g m-2
sec-1 at the destructive percentage of <50%, and it Fig. 4. WERs for four type biological crusts atincreased to 1.26 g m-2 sec-1 when entirely destroyed.
different destructive percentage atcorresponding TWVs.
The changes of WER with destructive percentage on
algal-lichen crusts was not obvious, the maximum value

240

did not exceed 0.11 g m-2 sec-1. The reason for this is unclear now, which may be related to the biologicalcharacteristics of the crust type (Zhang, 2005).
4. Conclusions
Biological soil crusts extensively occur in Gurbantunggut Desert and their distribution exhibits
obvious selectivity to landform. The wind tunnel experiments confirmed that the biological crusts greatly
increase the TWVs. There is no sand grain movement and wind erosion was observed on the all
undisturbed crusts under 25m s-1 wind, while the TWV on the bare sand surface was only 8.42 m s-1.
Under the same destructive percentage, moss crust had the highest TWV, followed by lichen and algal
crusts, and algal-lichen crusts had the smallest one.
The WER on bare sand surface was 0.09 g m-2·sec-1 at wind velocity of 8.42 m s-1 and it reached1214.82 g m-2·sec-1 at a high speed of 25 m s-1. At the same wind velocity the WERs on four type crusts
did not exceed 8.95% of that over the bare sand surface. The WERs increased exponentially withincreasing of destruction percentage of crust cover and wind velocity. From its magnitude order andvariation tendency, Moss and lichen crusts were similar, varying between 0-23.67 g m-2·sec-1; algal andalgal-lichen crusts were approximate, ranging from 0-69.00 g m-2·sec-1. Moss and lichen crusts showedstronger wind erosion resistance. Their erosion rates were no more than 1.95% of that on bare sandsurface even at wind velocity of 25 m s-1. However, once wind velocity approached the threshold for sandmoving, they reached 0.18-0.23 g m-2·sec-1 and thereby caused a significant loss of organic matter evenunder low destruction percentage. When the destruction percentage was larger than 80%, the WERs on
algal and algal-lichen crusts was close to that on bare sand surface and thus the sand stream may cause
sandblast damages in the disturbed area.
Acknowledgements
This research was funded by the National Natural Science Foundation of China (Grant No. 40771032),
the State Key Development Program for Basic Research of China (Grant No. 2009CB421303), theNational Science Supporting Program (Grant No. 2007BAC17B03). The wind tunnel experiment wasfinished at the Laboratory of Blown Sand Physics and Desert Environment, Cold and Arid Regions ofEnvironmental and Engineering Research Institute, Chinese Academy of Sciences. We sincerely thank
Liu Xuecan from Xinjiang Institute of Ecology and Geography, CAS, for his helping with collecting crust
samples in fields. Sincere thanks are due to Dr. Dong Zhibao, Dr. Yao Zhengyi, Dr. Yang Zuotao, Dr. Li
hong and Dr. Li Fang from Cold and Arid Regions Environmental and Engineering Research Institute,CAS, for their valuable assistance in wind tunnel experiments.

ReferencesAnderson D.C. Harper K.T., Rushforth S.R. (1982): Recovery of cryptogamic soil crust from grazing in Utah deserts. Journal ofRange Management, 35: 180-185.
Belnap J., Gillette D.A. (1997): Disturbance of biological soil crusts: impacts on potential wind erodibility of sandy desert soils
in southeastern Utah. Land Degradation and Development, 8: 355-362.
Dong Z.B., Wang X.M., Zhao A.G. (2001): Aerodynamic roughness of fixed sandy beds. Journal of Geophysical Research, 106:
11001-11011.
Evans R.D., Johansen J.R. (1999): Microbiotic crusts and ecosystem processes. Critical Reviews in Plant Sciences, 18: 183-225.
Li X.R., Jia Y.K., Long L.Q. (2001): Advances in microbiotic soil crust research and its ecological significance in arid andsemi-arid regions. Journal of Desert Research, 21(1): 4-11.
Mackenzie H.J., Pearson H.W. (1979): Preliminary studies on the potential use of algae in the stabilization of sand wastes andwind blow situations. British Journal of Phycology, 14:126.
West N.E. (1990): Structure and function of microphytic soil crusts in wildland ecosystem of arid and semi-arid regions.
Advances in Ecological Research, 20: 179-223.
Williiams J.D. Dobrowolski J.P., West N.E. (1999): Microphytic crust influences on unsaturated hydraulic conductivity. AridSoil Research and Rehabilitation, 13: 145-154.
Zhang Y.M., Pan H.X., Pan B.R. (2004): The Distribution Characteristics of Biological Crust on Sand Dune Surface inGurbantunggut Desert, China. Journal of Water and Soil Conservation, 18(4): 61-66.