Geoderma 132 (2006) 441 – 449
www.elsevier.com/locate/geoderma
The microstructure of microbiotic crust and its influence on wind
erosion for a sandy soil surface in the Gurbantunggut Desert of
Northwestern China
Y.M. Zhang *, H.L. Wang, X.Q. Wang, W.K. Yang, D.Y. Zhang
Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumqi, Xinjiang 830011, China
Received 19 January 2005; received in revised form 13 May 2005; accepted 28 June 2005
Available online 15 August 2005
Abstract
Microscopic examination of microbiotic soil crust shows that the profiles of soil with a dense cyanobacterial cover had two
different layers: a surface thin layer composed of aeolian-born materials and an organic layer formed by filamentous
cyanobacteria associated with sand particles. The results indicate that microbiotic cover is an important determinant of sand
fixation in the Gurbantunggut Desert, northern part of Xinjiang, Northwestern China. Microscopic examination of microbiotic
crusts in this study revealed an intricate network of filamentous cyanobacteria and exopolysaccharides, which binds and entraps
sand grains and conglutinate fine particles with each other. Resistance to wind erosion paralleled the different disturbance levels
on microbiotic soil crust. Sandy soil surface disturbances resulted in greatly decreased soil resistance to wind erosion.
Maximum wind tunnel velocity in this test (25 m s 1) did not lead to any wind erosion on the surface of undisturbed
microbiotic soil crust, i.e. 100% covered by microbiotic soil crust. As for different disturbance levels, the highest threshold
friction velocity was seen in the sand surface with 10% disturbance of microbiotic crust. The surface microbiotic soil crusts
have great effects on wind erosion rates. Wind erosion rates for sandy soil with 0% crust cover was about 46, 21, 17 times the
soil with 90% crust cover at wind velocities of 18, 22, 25 m s 1, respectively.
This study confirms that the planners and managers of nature reserves in this area should understand the important ecological
roles of microbiotic crust in desert ecosystems. The reduction of trampling on the soil will eventually result in the re-establishment
of biological crusts and their associated organisms, and ultimately lead to lower levels of wind erosion. Additionally, strategies
should be developed to manage livestock and oil exploration in order to avoid concentrated zones of impact.
D 2005 Elsevier B.V. All rights reserved.
Keywords: Cyanobacteria; Microbiotic crust; Microstructure; Wind erosion; The Gurbantunggut Desert
1. Introduction
* Corresponding author. Tel.: +86 991 7885450; fax: +86 991
7885320.
E-mail address: [email protected] (Y.M. Zhang).
0016-7061/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.geoderma.2005.06.008
Microbiotic crust is a common and widespread
phenomenon in desert areas all over the world thanks
442
Y.M. Zhang et al. / Geoderma 132 (2006) 441–449
to its extraordinary ability to survive desiccation and
extreme temperatures (up to 70 8C), high pH and
salinity (Friedmann and Galun, 1974; West, 1990).
Despite its unassuming appearance, microbiotic crust
plays a significant role in desert ecosystems, including
the process of formation, stability and fertility of soil,
preventing soil erosion by water or wind, increasing
the possibility of vascular plant colonization, and
being responsible for the stabilization of sand dunes
(more detailed discussion can be found in Eldridge
and Greene, 1994; Belnap and Lange, 2001; Belnap,
2003). Microbiotic crust can be an important source of
fixed carbon on sparsely vegetated areas common
throughout arid lands. While vascular plants contribute organic matter to soils directly beneath them,
large interspaces between plants receive little plant
material input. Where biological soil crusts are present, carbon and nitrogen contributed by these organisms help keep plant interspaces fertile, providing
energy sources from soil microbial populations
(Zaady et al., 1998; Belnap, 2002). In other words,
microbiotic crusts may play an important role in the
structure and function of an ecosystem, especially in
arid areas where water is scarce.
Wind is a major erosive force in deserts where
there is little organic matter or vegetation cover to
protect the soil surface. The microbiotic crust can
reduce wind erosion (Eldridge and Greene, 1994;
Belnap and Gillette, 1997; Leys and Eldridge,
1998). A number of workers have studied the role
of microbiotic crusts on structural stability of sandy
soils (Bailey et al., 1973; Ancker et al., 1985; Lynch
and Bragg, 1985; Bar-Or and Shilo, 1988; Danin et
al., 1989; Belnap and Gardner, 1993; Verrecchia et al.,
1995; Issa et al., 2001; Li et al., 2002, 2003). The
purpose of this study is to investigate the microstructure of microbiotic crust and to determine the internal
mechanisms on sand surface stability due to the occurrence of cyanobacteria, and to understand how soil
crust surface disturbance affects wind erosion in the
southern part of the Gurbantunggut Desert, the biggest
fixed and semi-fixed desert in China.
Region of China, and also the second largest desert in
China with an area of 48.8 thousands square kilometer.
Because of the bblocking effectQ of the Himalayan
Range, moist air currents from the Indian Ocean fail
to reach the area, resulting in the vast expanse of arid
terrain. Mean annual precipitation is approximately
79.5 mm, falling predominantly during spring. Mean
annual evaporation is 2606.6 mm. Average temperature
is 7.26 8C. Wind speeds are greatest during late spring,
with average 11.17 m s 1, and are predominantly in
WNW, NW and N directions. Natural vegetation in the
area is dominated by Haloxylon ammodendron and H.
persicum etc., with a vegetation cover of less than 30%.
The area is covered by huge and dense semi-fixed sand
dunes with stable moisture content. There is abundant
biological soil crust on the sand surface of the desert,
growing especially during wet, cool periods (fall and
early spring) when dew, fog or temporary rainfalls, as a
moisture source, are available to species relating to the
formation of soil crust (Du, 1990; Zhang et al., 2002;
Kidron et al., 2002). The study was conducted in the
southern part of the Gurbantunggut Desert because it
contains representative biological soil crust found
throughout the desert (Zhang et al., 2002, 2004).
The sand surface of this desert was covered by
biological soil crusts, i.e. microbial communities
dominated by filamentous cyanobacteria, turned into
a hard crust like structure because of the severely
desiccated conditions in which they thrive. Microbiotic crusts in this study area were heavily dominated by
the cyanobacterium Microcoleus vaginatus, with
occasional lichen and moss patches in interdune
areas. The mat’s surface is somewhat rough and
undulating, and is rigid, although the mat is fragile
enough to disintegrate into sand when pressure is
applied. The filamentous algae can bind sand particles
tightly and serve as trap and cohesive agents preventing the sandy surface from wind and water erosion.
3. Materials and methods
3.1. Microscopic observations
2. Study area
The Gurbantunggut desert is situated in the center of
the Jungger Basin, Xinjiang Uygur Autonomous
For surface microscopic observations, pieces of
microbiotic crusts were carefully removed, thanks to
the polygonal cracking occurring naturally at the surface of the soil when microbiotic crusts wet and get
Y.M. Zhang et al. / Geoderma 132 (2006) 441–449
dry. Then a small part of the undisturbed samples
were glued onto aluminum stubs with the exposed
vertical natural section facing upward and coated with
gold. Observations of the microstructures of soil crust
were performed on a LEO1430VP Scanning Electron
Microscope (SEM, LEO Corp, Germany). Determinations of cyanobacteria were performed on an Olympus
System Microscope (DP70, Models BX52, Olympus
Corp, Japan).
3.2. Wind tunnel tests
For wind tunnel tests, the undisturbed surface crust
samples were excavated in open interspaces between
perennial vegetation, which was covered by welldeveloped microbiotic soil crust. Undisturbed crust
samples (20 by 30 cm and 12 cm deep) from the
sand surface were collected using metal boxes, prepared for wind tunnel tests.
Five replicate crust samples were collected for each
disturbance treatment. We adopted the method
designed by Leys and Eldridge (1998) to simulate
the disturbance by stock. Seven disturbance levels,
i.e. no disturbance (control), 10%, 20%, 30%, 50%,
80% and 100% destruction of soil crust surface, were
applied to simulate a range of disturbance created by
domestic stock and wild animals. These treatments
and the justification for using them are described in
more detail in Leys and Eldridge (1998). Artificial
dhoovesT were constructed using small pieces of rectangular shaped steel bar with a surface area of 15
cm2. The undisturbed crust surface, with a surface
area of 600 cm2 (20 by 30 cm), was then randomly
trampled by artificial dhoovesT for four times. This
treatment represents 10% of the crust disturbed. The
rest of the treatments may be deduced by analogy
representing 20%, 30%, 50%, 80% and 100% of the
crust were disturbed respectively. The outer periphery
(i.e., about 15 mm) of the crust was then treated with a
light, aerosol lacquer to provide additional stability,
since earlier tests had shown that the boundary effects
may occur here (Neuman and Maxwell, 1999). Crust
samples were air dried to a moisture content of about
2.5%.
Each run began with inserting one of the prepared
metal trays of sand into an opening in the wind tunnel
floor so that the surface of the crust was level with that
of the fixed tunnel floor. The experiments were carried
443
out in a wind tunnel in the Key Laboratory of Desert
and Desertification, the Chinese Academy of Sciences.
The blow type non-circulating wind tunnel has a total
length of 37.78 m of which 16.23 m is the working
section. The cross sectional area of the working section
is 0.81 m. The wind speed can be changed continuously from 1 to 40 m s 1.
Threshold friction velocities (TFVs) were determined by gradually increasing wind speed in the
tunnel until consistent forward soil particle movement
was observed across the soil surface. Wind velocity
was then recorded at the soil surface and 6, 12, 24,
40, 80, 120, 200 mm above the soil surface with a
pitot tube installed parallel to the central axis of the
wind tunnel and 20 cm above the sample surface. In
brief, wind velocity profiles were taken in the wind
tunnel for each plot. Because the wind velocity close
to the surface, such as in the wind tunnel, varies
according to the surface roughness, it is necessary
to select a wind away from the surface. Data for the
mean horizontal wind velocity U versus height z
(mm; wind profile data) were fitted to the function
for aerodynamically rough flow, using a linear least
squares routine:
UTt ¼ kz ðdUt =dzÞ
ð1Þ
where U *t is the friction velocity (measured in m
s 1), kz is roughness characteristics of the surface,
U t is wind speed at the particle movement threshold,
and k is von Karman’s constant (taken as 0.4).
Threshold velocities and aerodynamic roughness
heights are reported in terms of friction velocity
and roughness height. All wind velocity measurements within the wind tunnel are converted to friction velocity using Eq. (1) and form the wind data set
used in this paper.
The eroded sediments were captured in a passive
vertical sediment trap located at the downwind edge
of the crust surface. Weight loss was converted to an
erosion modulus (g m 1 min 1) to quantify the wind
erosion rate. An electronic balance (10 Ag precision)
weighed these captured particles, and the cumulative
amount was downloaded every 5 s to a microcomputer. The wind erosion rate was determined by dividing
the difference in weight between successive readings
and the length of the time interval (5 s).
Comparisons across treatments and controls were
done using two-way ANOVA and multiple range
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Y.M. Zhang et al. / Geoderma 132 (2006) 441–449
B
A
50µm
C
D
1µm
E
F
Fig. 1. SEM micrographs of the microbiotic soil crust. (A) The surface of the crust, a thin coating of aeolian dust matrix and the sand and coarse
silt particles were tightly packed. (B) The open structure of the crust, showing the network of cyanobacterial filaments. (C) The profiles of soil
crust: a surface thin layer composed of aeolian-born materials and an organic layer formed by filamentous cyanobacteria associated with sand
particles. (D) Sand grains entrapped by cyanobacterial filaments. (E) Abandoned sheath material adhering to the sand grains. (F) Sand grains
linked with each other by glutinous polysaccharides secreted by cyanobacteria.
Y.M. Zhang et al. / Geoderma 132 (2006) 441–449
Fig. 2. The microbiotic soil crust in the Gurbantunggut Desert,
showing the filamentous algae associated with sand particles.
tests. A t-test was used to distinguish between disturbance treatments and controls.
4. Results
4.1. Microstructure of microbiotic soil crust in the
Gurbantunggut Desert
SEM micrographs of the microbiotic crust are presented in Fig. 1. The profiles of soil with a dense
cyanobacterial cover showed two different layers: a
surface thin layer composed of aeolian-born materials
445
and an organic layer formed by filamentous cyanobacteria associated with sand particles.
The distribution of silt and clay within the crust is
apparent in thin section of the surface layer, in which a
thin coating of aeolian dust matrix and the sand and
coarse silt particles were tightly packed (Fig. 1A, C).
The extracelluar sheath material of cyanobacteria
coheres the aeolian dust and deposition together, providing soil surface protection. In the upper layer of the
crust, biological elements are rare and can hardly be
seen. The sand grains are embedded in a dense and
compacted smooth matrix of fine-grained particles.
The number and size of voids are very limited.
In the lower part of the same sample, i.e. the
organic layer, some filaments of cyanobacteria can
be seen and grew in the grains interspace, and surround and bind sand particles with each other into a
coherent network (Fig. 1B). This layer of the crust is
predominantly composed of sand grains and almost
completely devoid of fine-grained particles in comparison with those observed in the upper layer. The
open structure of the lower part of the crust is composed only by sand grains trapped in a well-developed
cyanobacterial filaments network. Additionally, the
sand grains also linked with each other by glutinous
polysaccharides secreted by cyanobacteria (Fig. 1 F).
M. vaginatus can be seen with a 10 hand lens on
the edge of a broken clump of soil (Fig. 2). Under
higher magnification, the filamentous cyanobacteria
are surrounded by gelatinous sheaths that bind the
Fig. 3. (A) Filamentous algae are surrounded by gelatinous sheaths that bind the sand particles. (B) The living filaments can migrate through the
soil, leaving abandoned sheath material and a stabilized soil matrix behind. Bar is 5 Am.
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Y.M. Zhang et al. / Geoderma 132 (2006) 441–449
30
paludosus, Xenococcus lyngbye, Chroococcus turgidus var. solitarius, Anabaena azotica, Lyngbya martensiana, Stigonema ocellatum, Amphora ovalis,
Chlamydomonas mutabilis and Calothrix stagnalis.
a
25
Max. wind velocity in the experiments
TFV(m/s)
20
b
15
c
c
4.2. Threshold friction velocities
c
d
10
d
TFVs for the different levels of disturbance treatments and control are shown in Fig. 4. Resistance to
wind erosion paralleled the different disturbance
levels on microbiotic soil crust. Sandy soil surface
disturbances resulted in greatly decreased soil resistance to wind erosion. Maximum wind tunnel velocity
in this test (25 m s 1) did not lead to any wind erosion
on the surface of undisturbed microbiotic soil crust,
i.e. 100% covered by microbiotic soil crust. As for
different disturbance levels, the highest TFVs were
seen in the sand surface with 10% disturbance of
microbiotic crust. The crust with 10% disturbance
had TFVs of 16.04 m s 1 compared to 8.42 m s 1
for bare sand and 8.74 m s 1 for 100% disturbance of
microbiotic crust; consequently, 10% destruction of
crust had about two times the wind resistance of bare
sand. When 20% of the crust surface was disturbed,
the TFVs decreased sharply to 12.27 m s 1 and then
were gradually close to the TFVs of bare sand along
d
5
0
0%
10%
20%
30%
50%
80%
100%
sand
Different disturbance levels
Fig. 4. Threshold friction velocities (TFVs) for microbiotic crusts at
different disturbance levels. Categories with different lower case
letters differ significantly from each other ( p b 0.05). Error bars
indicate standard error.
sand particles (Fig. 3A). After a spell of dry weather
the cyanobacteria die, but the sheath on drying or on
wetting still adheres to the sand grains, acting as a
cementing agent (Fig. 1E).
Also, the living filaments can migrate through the
soil, leaving abandoned sheath material and a stabilized soil matrix behind (Fig. 3B). Apart from M.
vaginatus, other common species include Microcoleus
90.000
0%
10%
20%
30%
50%
80%
100%
80.000
Wind erosion (g/min)
70.000
60.000
50.000
40.000
30.000
20.000
10.000
0.000
6
10
14
18
22
25
Wind velocity (m/s)
Fig. 5. Variation of wind erosion rates for microbiotic crust at different disturbance levels. All values were significantly different ( p b 0.01).
Error bars indicate standard error.
Y.M. Zhang et al. / Geoderma 132 (2006) 441–449
with further disturbances. This indicated that the
microbiotic soil crust gradually lost its ability to resist
wind erosion.
Fig. 4 also indicated that three types of disturbance
in this desert can be identified as: (i) slight disturbance
— 0–10% destruction of soil crust surface; (ii) moderate disturbance — 20–60% destruction of soil crust
surface; and (iii) severe disturbance — 80–100%
destruction of soil crust surface.
4.3. Wind erosion rates
Soil erodibility by wind reflects the fragility of
soils suffering from wind deflation and abrasion
(Liu et al., 1998). The natural wind erodibility of
the fixed sandy soil is affected appreciably by surface
vegetation and crust. Fig. 5 shows that wind erosion
rate increases with decreasing soil crust cover, suggesting that soil crust is one of the factors responsible
for reducing wind erosion for the fixed sandy soil.
These results demonstrate that soil crust has a high
effectiveness in controlling wind erosion and soil
crust breakage can reduce soil resistance to wind
erosion.
In the case of all the disturbed microbiotic soil
crust, there was virtually no wind erosion at the
wind speeds less than 6 m s 1, while wind erosion
began at velocity 10 m s 1. The results showed that
wind erosion rate increases with increasing crust disturbance levels, suggesting that microbiotic soil crust
is one of the factors responsible for reducing wind
erosion for the biologically fixed sandy soil in the
desert ecosystem. The surface microbiotic soil crusts
have great effects on wind erosion rates. Wind erosion
rates for sandy soil with 0% crust cover was about 46,
21, 17 times the soil with 90% crust cover at wind
velocities of 18, 22, 25 m s 1, respectively. These
results demonstrate that soil crust has a high effectiveness in controlling wind erosion and soil crust
disturbance can reduce soil resistance to wind erosion.
5. Discussion and conclusion
Microbiotic soil crusts are present on surface soils
throughout the world. A key feature of these crusts
in arid zones is the abundance of filamentous sheathforming and polysaccharide-excreting cyanobacteria
447
(Mazor et al., 1996). Contribution of microorganisms
to soil aggregate stability is a well-known phenomenon. Two major mechanisms are suggested by
Lynch and Bragg (1985): the ability of some microorganisms, mostly filamentous, to mechanically bind
soil particles, and the production of binding agents
by some others (bacteria). Our studies indicate that
microbiotic cover is an important determinant of
sand fixation in the Gurbantunggutt Desert, northern
part of Xinjiang, Northwestern China. Microscopic
examination of microbiotic crusts in this study
revealed an intricate network of filamentous cyanobacteria and exopolysaccharides, which binds and
entraps sand grains and conglutinate fine particles
with each other. This will result in sand particles
stabilization and reduce water and wind erosion
(Williams et al., 1995; Belnap and Gillette, 1998).
The results of this study confirm the role of cyanobacterial material (filaments and extracellular secretion) as binding and gluing agents in the formation
of stable soil crust, which had the strong and significant relationship between crust cover and wind
erosion.
Microbiotic cover is an important determinant of
sand fixation in the Gurbantunggut Desert, northern
part of Xinjiang, Northwestern China. The network
of filamentous cyanobacteria served as main agent
for surface stability. Additionally, the living filaments
can migrate through the soil, leaving abandoned
sheath material and a stabilized soil matrix behind.
These results demonstrate that soil crust has a high
effectiveness in controlling wind erosion and soil
crust breakage can reduce soil resistance to wind
erosion. The results suggested that by far the largest
increase in the vulnerability of the surface to wind
erosion is with any disturbance to the crust. When
undisturbed, the crusts are quite bwind proofQ and
that with any disturbance, this resistance declines
radically. After that initial large drop, things flatten
out a bit. The crust surface disturbance can substantially accelerate the erodibility of the fixed sandy soil
by wind. When the disturbance levels ranges from
20% to 50%, i.e. the moderate disturbance, the wind
erosion rates are quite similar except for wind velocity of 25 m s 1. But the wind velocity of 25 m s 1
will be seldom encountered in this desert. Soil erodibility by wind reflects the fragility of soils suffering
from wind deflation and abrasion (Liu et al., 1998).
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Y.M. Zhang et al. / Geoderma 132 (2006) 441–449
The natural wind erodibility of the biologically fixed
sandy soil is affected appreciably by surface vegetation and crust.
Some other researches indicated that management
strategies which reduce either cryptogam cover
(West, 1990; Eldridge and Greene, 1994) or aggregate stability are likely to be more influential as soil
texture becomes coarser, i.e. as the soils become
more sandy (Eldridge and Leys, 2003). In extensive
(rangeland) grazing systems, reductions in cryptogam
cover occur predominantly through trampling and
burning (Eldridge and Greene, 1994). This study
indicated that the degree of microbiotic soil crust
coverage could be significant in determining the
threshold friction velocities of sand surface. Resistance to wind erosion paralleled microbiotic soil crust
disturbance. Decreasing TFVs are directly associated
with increased wind erosion rates. It is no doubt that
the cyanobacterial cover at the surface of sand dunes
in the Gurbantunggut Desert improves soil crust
structure and surface stability.
Under drought conditions, soil crusts are brittle,
and crush easily when subjected to compressional or
shear forces (Belnap and Gardner, 1993; Gillette et al.,
1980; Webb and Wilshire, 1983). When the surface
crust is broken, the unconsolidated loose sand grains
below the crust are exposed to wind, resulting in
severe soil erosion. Belnap and Gillette (1998)
reported that soil crust disturbance reduced the resistance to wind erosion from 69% to more than 5200%.
Hoof print disturbance increased wind erosion susceptibility by 69–5247%, while disturbance with a
vehicle increased susceptibility by 1372–3399%.
Once disturbed, the soil-forming process of the fixed
aeolian sandy soils is reversed under unfavorable
conditions such as drought and windstorm; namely
from the fixed aeolian sandy soil to the mobile aeolian
sandy soil. Moreover, soil surface disturbance could
reduce nitrogenase activity (Belnap, 2002).
The recovery of the disturbed biological soil crust
is very slow. On newly disturbed surfaces, mosses and
lichens often have extremely slow colonization and
growth rates. Assuming adjoining soils are stable and
rainfall is average, recovery rates for lichen cover in
southern Utah have been most recently estimated at a
minimum of 45 years, while recovery of moss cover
was estimated at 250 years (Belnap and Gardner,
1993). Therefore, effective measures should be taken
to prevent disturbing soil crust in the management of
the fixed sandy soils. Inappropriate human activities
such as grazing, off-road vehicle trample, everincreasing recreational and commercial activities are
resulting in unprecedented levels of local and regional
disturbance, accelerating desertification processes
(Belnap, 1995). The human activities in this desert,
such as livestock and oil exploration have now
became the main menaces to stabilities of sand dune
in this desert, which had disturbed the fragile desert
ecosystem a lot, especially on the microbiotic soil
crust. Therefore, specific actions including reduction
in types and intensities of the use as well as adjustments in timing of use should be taken to ensure the
sustainable management of the aeolian sandy soils.
This study confirms that the planners and managers of
nature reserves in this area should understand the
important ecological roles of microbiotic crust in
desert ecosystems. The reduction of trampling on
the soil will eventually result in the re-establishment
of biological crusts and their associated organisms,
and ultimately lead to lower levels of wind erosion.
Additionally, strategies should be developed to manage livestock and oil exploration in order to avoid
concentrated zones of impact.
Acknowledgments
The authors gratefully acknowledge the assistance
and advice of Prof. Pan Huixia and two anonymous
reviewers. This work was supported by grants from
the Key Knowledge Innovation Project of Chinese
Academy of Sciences (No. KZCX3-SW-343), the
National Natural Science Foundation of China (No.
90202019), and the Key National Basic Research and
Developmental Project (No. G1999043509).
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