Wind erosion prevention characteristics
and key influencing factors of bryophytic
soil crusts
Chongfeng Bu, Ying Zhao, Robert Lee
Hill, Chunlei Zhao, Yongsheng Yang,
Peng Zhang & Shufang Wu
Plant and Soil
An International Journal on Plant-Soil
Relationships
ISSN 0032-079X
Plant Soil
DOI 10.1007/s11104-015-2609-z
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DOI 10.1007/s11104-015-2609-z
REGULAR ARTICLE
Wind erosion prevention characteristics and key influencing
factors of bryophytic soil crusts
Chongfeng Bu & Ying Zhao & Robert Lee Hill & Chunlei Zhao &
Yongsheng Yang & Peng Zhang & Shufang Wu
Received: 29 April 2015 / Accepted: 15 July 2015
# Springer International Publishing Switzerland 2015
R. L. Hill
Department of Environmental Science and Technology,
University of Maryland, College Park, MD 20742, USA
Methods We used an orthogonal design and large-scale
wind tunnel simulations to determine the erosionresistance characteristics of BSCs, and the effects of soil
moisture content (SMC), vegetation coverage (VC) and
crust coverage (CC) on its wind erosion reductions.
Results Our results showed that 1) the medium SMC+
low CC+high VC were the most effective contributors
in decreasing the wind erosion modulus; 2) the high
SMC+medium CC+low VC primarily reduced the
sand-transport rate; 3) the medium SMC+high CC+
low VC, with the smallest aeolian sand-flow structure
index, resulted in the lowest level of aeolian sand-flow
saturation; 4) the high SMC+high CC+medium VC
exhibited the highest threshold wind velocity; and 5)
the VC and CC were found to have significant effects on
the surface roughness, but the SMC did not.
Conclusions BSCs should be strongly protected during
dry and windy seasons in the spring and winter because
of their value in reducing wind erosion. The BSC may
be moderately disturbed during wet and weak wind
seasons in the summer or autumn to ameliorate soil
moisture conditions, especially in the sites with high
vegetation coverage, without exacerbating wind erosion. Management that carefully considers the timing
and disturbance of BSCs will help to determine the
sustainable balance between the Bprotection^ and
Bdestruction^ of BSCs and their effective use in reducing wind erosion.
S. Wu (*)
College of Water Resources and Architectural Engineering,
Northwest A & F University, 23 Weihui Road,
Yangling 712100 Shaanxi, China
e-mail: [email protected]
Keywords Bryophytic soil crusts . Erosion modulus .
Sand-transport rate . Structure index of aeolian sandflow . Roughness . Threshold wind velocity
Abstract
Aims Biological soil crusts (BSCs) are generally
considered to reduce wind erosion, however, the
extent of reductions is highly dependent on the
vegetation type, soil moisture status, and their interactions in the field. The interrelationships between these factors or their combined effects are
relatively unknown. The objectives of this study
were to evaluate the contributions of the crust coverage (CC), associated with the soil moisture content (SMC), and the vegetation coverage (VC) to
reduce wind-erosion and to provide a basis for the
effective management of BSCs.
Responsible Editor: Simon Jeffery.
C. Bu (*) : Y. Zhao (*) : C. Zhao : P. Zhang : S. Wu
State Key Laboratory of Soil Erosion and Dryland Farming on
the Loess Plateau, Northwest A & F University, Yangling,
Shaanxi 712100, China
e-mail: [email protected]
e-mail: [email protected]
C. Bu : Y. Yang
Institute of Soil and Water Conservation, Chinese Academy of
Sciences and Ministry of Water Resources, 26 Xinong Road,
Yangling 712100 Shaanxi, China
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Introduction
Biological soil crusts (BSCs) are composite layers of
soils and various organisms, such as mosses, lichens,
algae, fungi, and bacteria. BSCs are important ecosystem components in arid and semi-arid areas (Belnap
2003; Belnap and Lange 2001; Eldridge and Greene
1994), especially when utilized for aeolian erosion control (Hupy 2004; Guo et al. 2008). BSCs have two
binding mechanisms: (1) extracellular excretions, which
act as cementing agents and (2) filamentous growths
that blanket and entangle loose surface particles
(Langston and Neuman 2005). Through a wind tunnel
experiment, Eldridge and Leys (2003) noted that after
moderate disturbance and simulated wind erosion, 90 %
of surface aggregates on the loamy soil and 76 % of
surface aggregates on the sandy soil were dominated by
biological elements (cryptogams). Research in eastern
Australia has also demonstrated the importance of crusts
in binding the soil surface (Eldridge and Leys 1998).
Previous research has shown that BSCs can reduce
erosion modulus, sand-transport rate, and wind erosion
by separating the soil from the airflow (Zhang et al.
2006; Langston and Neuman 2005). Zhang et al.
(2006) determined without the protection of the BSC
layer, the wind erosion rates on bare sand were 46, 21,
and 17 times the erosion rates on a 90 % crust cover at
velocities of 18, 22, and 25 m s−1, respectively. Based on
particle kinematics in wind tunnel experiments, O’Brien
and Neuman (2012) demonstrated that BSCs were able
to withstand particle impacts for 4 h before crust rupture
and erosion occurred.
In addition to BSCs, other factors such as the extent
of vegetative cover and the soil moisture content also
affected the amounts of wind erosion in semi-arid and
arid regions (Wang et al. 2006; Ishizuka et al. 2005;
Zhang et al. 2008; Okin et al. 2001). Vegetation significantly affects the structure of aeolian sand-flows, sandtransport rates, and height distributions of sand-grain
sizes in suspension (Li et al. 2007). Zhao et al. (2012)
found that when the soil moisture content was less than
3 %, soils were more susceptible to wind erosion, and
that only weak wind erosion occurred when the soil
moisture content was greater than 7 %.
Numerous studies have shown that BSCs reduced the
infiltration depths and retained the limited rainwater in
shallow soils (Gao et al. 2010) These conditions decreased the supplemental water to deeper soil layers
and contributed to the death and degradation of deep
root shrubs (Li et al. 2010a) which play an important
role in wind erosion reductions for the desert region.
One researcher has suggested that disturbances to BSCs
should be used in these regions as part of a management
strategy (Bu et al. 2013). Li et al. (2006a) demonstrated
that disturbance to BSCs increased the storage of plantavailable water in the herbaceous rooting zone and
improved the environment for germination and subsequent growth of annual herb species. Grazing which is a
common disturbance in desert regions has been shown
to promote the growth and regeneration of Artemisia
ordosica communities in the Mu Us sandland (Guo et al.
2000). In the Tengger Desert, BSCs have been shown to
change the spatiotemporal patterns of soil moisture and
re-allocation of soil moisture (Li et al. 2010a) by decreasing rainfall infiltration, increasing topsoil waterholding capacity, and altering evaporation. As a result,
changes in the soil moisture regime induced the shifting
of sand-binding vegetation from xerophytic shrub communities with higher coverage (35 %) to complex communities dominated by shallow-rooted herbaceous species with low shrub coverage of approximately 9 % (Li
et al. 2010a). Therefore, the moderate disturbance of
BSCs may be beneficial to decrease negative effects
on the soil moisture processes for improved plant
growth. However, BSCs are quite sensitive to disturbances, and improper disturbances may induce the occurrence of desertification (Mario and Navar 2000;
Zhang et al. 2006). Therefore, before disturbance is
implemented, it is necessary to understand the main
factors that affect the impacts of BSCs on wind erosion
prevention.
Limited studies have examined the combined factors
of BSCs and vegetation that may influence wind erosion
under moderate disturbances for different moisture conditions (Rodríguez-Caballeroa et al. 2012; Zhang et al.
2008; Zhao et al. 2012). We hypothesize that BSCs may
weakly contribute to wind erosion control under soil
moisture conditions or vegetation coverage that are lower than a specific threshold value. The present study
comprehensively considers the interactions of BSCs
coverage, vegetation coverage, and soil moisture content on wind control by using a large-scale wind-tunnel
simulation experiment. Our objectives were: 1) to explore the wind erosion characteristics of different combinations of BSCs, soil moisture, and vegetation coverage; and 2) to provide management strategies that balance the impacts of protection and disturbance to rational resources management within the Tengger desert.
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occur mainly on the leeward, shady-side slopes of dunes,
and the coverage of the crusts increases significantly as
the age of the crusts increase (Li et al. 2006b, 2010b).
Materials and methods
Study site
This study was conducted at Shapotou in the southeastern
edge of the Tengger Desert. The Shapotou region (37° 32′
N, 105° 02′ E) is a typical transition zone between the
desert steppe and desert at an elevation of 1339 m with a
semi-arid climate characterized by both a typical
continental-monsoon climate and a desert climate. The
annual average temperature is 10 °C, with the lowest
monthly average temperature in January (−6.9 °C) and
the highest monthly average temperature in July
(24.3 °C). The average annual precipitation is 180 mm,
of which 80 % occurs between May and September. The
average potential evapotranspiration is 3000 mm. Northwestern winds dominate the area, with an annual average
wind velocity of 3.5 m/s and the maximum wind velocity
of approximately 24 m/s. There are two types of azonal
vegetation-psammophilous series and hygrophilous series. The former develops in the dune area and consists of
psammophytes such as Agriophyllum squarrosum Moq.,
Psammochloa cillosa Bor, Artemisia sphaerocephala
Krasch, A. ordosica Krasch and Hedysarum scoparium
Fisch. These plants are typically scattered on the dunes or
aggregated in the interdunal depressions with extreme
variations in cover ranging from less than 1–30 % (Li
et al. 2004a). The development of the vegetation-soil
system results in the gradual formation of BSCs, which
gradually evolve from algal crusts to lichen crusts to
bryophytic crusts (Li et al. 2004b). Mosses and algae,
respectively dominated by Bryum argenteum (Jia et al.
2008) and Microcoleus vaginatus Gom (Hu et al. 1999),
are the main vegetative components of BSCs. BSCs
Experimental design
The experiment employed an orthogonal design that had
three factors: the soil moisture content (SMC), the bryophytic soil crust coverage (CC), and the vegetation
coverage (VC) with three levels for each factor in a
design that is similar to the experimental design used
by Zurovac and Brown (2012). Therefore, this study had
nine treatments in total with 3 replicates for each treatment for evaluation in the wind tunnel simulations. The
three VC levels were 0 % (no vegetation), 25 % (medium coverage), and 50 % (high coverage). Based on the
results of Wang et al. (2004) and Zhang et al. (2006), the
three CC levels were 30 % (severe damage), 60 %
(medium damage), and 90 % (light damage). The crust
coverage was obtained through removing the crust layer
on an entirely crusted sample until our desired percentage cover was obtained. The VC is the ratio of the total
canopy area and the upper surface area of the 30-cm by
20-cm steel box which contained the soil within the
wind tunnel. The CC is the ratio of the BSCs covered
area of the sample and the upper surface of the steel box.
Based on the average moisture-content changes during
the different seasons of plant growth in the study area,
the three SMC levels were 0.37 % (spring), 4.15 %
(autumn), and 7.25 % (summer). The wind-erosion
characteristics of each treatment group were observed
under the experimental conditions of a wind tunnel. The
details of treatments are shown in Table 1.
Table 1 The nine treatment combinations of crust cover (CC), soil moisture content (SMC), and simulated vegetative cover (VC) used the
orthogonal experimental design of the wind tunnel study. (L9 (34))
Treatment groups Levels
Crust coverage Soil moisture content Vegetation coverage Random factor
1
30 % CC+0.37 % SMC+0 % VC
1
1
1
1
2
30 % CC+4.15 % SMC+25 % VC 1
2
2
3
3
30 % CC+7.25 % SMC+50 % VC 1
3
3
2
4
60 % CC+0.37 % SMC+25 % VC 2
1
2
2
5
60 % CC+4.15 % SMC+50 % VC 2
2
3
1
6
60 % CC+7.25 % SMC+0 % VC
3
1
3
7
90 % CC+0.37 % SMC+50 % VC 3
1
3
3
8
90 % CC+4.15 % SMC+0 % VC
3
2
1
2
9
90 % CC+7.25 % SMC+25 % VC 3
3
2
1
2
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Sample collection
Measurement methods
The crust samples were collected in the field. To maintain their integrity, water was evenly sprayed on the
crust surface, and each sample was then collected using
a 30 cm×20 cm×7.5 cm (length×width×height) tin
molded metal frame when the water had penetrated
approximately 10 cm into the soil. The length, width,
and height of the crust sample was the same as the
frame’s dimensions. In order to insure a sufficient
number of useful, non-damaged, representative samples, a larger number of samples were taken than
would be needed during the wind tunnel evaluation.
All samples were collected under similar conditions
with the 27 best crust samples selected for the imposition of the nine treatments with 3 replications that
were evaluated during the wind tunnel simulations.
Soil in and around plots was composed of loose and
impoverished shifting sands. The crust layer typically
develops on fine sand and contains silt or fine sand
which was captured by the crust (Zhang et al. 2007).
The landscape and sampling process are shown in
Fig. 1.
The experiment was conducted in the field wind-tunnel
laboratory at the Shapotou Desert Experimental Research Station of the Cold and Arid Regions Environmental and Engineering Research Institute (CAREERI),
Chinese Academy of Sciences (CAS). This blow-type
non-circulating wind tunnel has a total length of 37 m
and was composed of a power section (2.60 m long), an
expansion section (6.40 m long), a stabilization section
(1.50 m long, where drag screens and honeycombs were
used to reduce large-scale eddying), a compression section (2.50 m long), a work section (21 m long), and a
diffusion section (3 m long). The cross-sectional area
used for evaluating samples was 1.2 m×1.2 m (Fig. 3).
Wind speed could be changed continuously from 2 to
30 m s−1. The stability coefficient of the air flow, ŋ<
3.0 %, the uniformity coefficient of the wind speed δu <
2.0 %, and the depth of the boundary layer can reach 40
–50 cm in the work section (Dong et al. 2003).
The previously described sample box was placed in
the elevation section so that the crust surface was level
with the bottom of the wind tunnel. A 5 cm×15 cm
board was wrapped in white paper with double-sided
tape stuck on the paper was placed at the rear of the
sample (Fig. 3). A high resolution camera was linked to
a computer and was positioned to observed wind-borne
particles near the rear of the sample box. Under these
conditions, researchers used the computer to analyze
and determine the exact number of sand particles that
moved from the sample through the camera’s view path.
The threshold wind velocity was the lowest wind speed
at which soil movement was initiated (Sout 1998), and it
was determined by gradually increasing the wind speed
in the tunnel until consistent forward soil particle movement was observed across the soil surface (Belnap and
Gillette 1998).
Preparation of samples
A sketch map showing the different treatments is given
in Fig. 2. Triangular brushes (8 cm diameter and 3 cm
height) with the top removed were used to simulate
vegetation. The height from the bottom of the brush to
the soil surface was 0.5 cm. Three VC levels were
achieved by controlling the number of brushes. The
different CC levels were achieved by removing the
appropriate corresponding area of crust. After the crust
was removed, the damaged region was filled with soil
similar to the underlying soil.
Fig. 1 The landscape at the sampling site, samples being taken at the site, and overall of the sampling area after sample were obtained
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Fig. 2 Photograph of the wind
tunnel apparatus with the
experiment section identified and
showing the placement of the
sample in the wind tunnel
After the measurement of threshold wind velocity,
the wind velocity was rapidly decreased to 3 m/s, and
then the sand sampler and the wind profiler were
installed in the wind tunnel. To simulate the maximum
wind speed in the Tengger Desert (Zhang et al. 2008),
the wind velocity in the tunnel was set to 24 m/s which
is equivalent to a wind speed of force nine. After the
sand sampler and wind profiler were installed, the wind
velocity was rapidly increased to 24 m/s and continued
for 5 min at this velocity. During this period, the computer repeatedly recorded the wind velocity at different
heights (4, 7, 12, 16, 32, 62, 200, 350, and 500 mm)
through the wind profiler and the wind-profile was then
calculated by a least-squares regression function as described by Dong et al. (2001). The roughness was calculated using the wind velocity at different heights that
was measured by the wind profiler. The calculations are
described in Dong et al. (2000). The wind erosion
modulus was calculated as M • (1-P), where M was
the mass difference before and after the wind test and
P was the soil moisture content. A segmented sand
sampler (Dong et al. 2003) measured the blown sand
flux where the bottom of the lowest opening of the
sampler was set flush with the sample surface and
downwind tunnel. The height and sampling interval of
the sand sampler were 20 and 2 cm, respectively. After
the wind test, the sand sampler was removed with the
sand inlets tilted slightly upward and the sands
transported at different heights were weighed (Dong
et al. 2004). The sand-transport rate in this study is the
sum of the transport rate from 0 to 20 cm. The aeolian
sand-flow structure index, which reflects the degree of
saturation and trends of erosion and deposition (Dong
et al. 2003), is the ratio between the sand transported at
heights of 0–2 cm and the sand transported at heights of
0–20 cm.
Fig. 3 Representative samples from the different treatment combinations of crust coverage (CC), soil moisture content (SMC),
and simulated vegetative cover (CC) where (a) 60 % CC+7.25 %
SMC+0 % VC, (b) 30 % CC+4.15 % SMC+25 % VC and (c)
30 % CC+7.25 % SMC+50 % VC
Data analysis
Data were expressed as the means±the standard error.
The variance analysis and multiple means comparison
tests among the different treatments were tested using
SPSS 17.0 and LSD, respectively.
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Results
Effects of different treatments on the wind-erosion
modulus
The mean wind-erosion modulus among treatments 2, 3,
5, 6, 8, and 9 was 48.6 g·m−2 ·min−1 and only accounted
for 9.6 % of the mean wind-erosion modulus between
treatments 1 and 4 (Fig. 4a). The results of the variance
analysis showed that the CC, SMC, and VC all had
highly significant effects on the erosion modulus
(Table 2; P <0.01). The effect of the random factor on
the wind-erosion modulus was also statistically significant and indicated that unknown factors or interactions
between factors influenced this parameter. The results of
multiple comparisons showed that the differences between treatment 1 and treatments 2, 3, 5, 6, 7, 8, and 9
were highly significant and treatment 4 and treatments
2, 3, 5, 6, 7, 8, and 9 were also highly significant. The
difference between treatment 7 and treatments 2, 3, 5, 6,
8 and 9 was significant. There were no significant
differences between treatments 1 and 4 (P>0.05). The
wind-erosion modulus of treatment 5 (4.15 % SMC+
30 % CC+50 % VC) consistently had the smallest
erosion loss values among the nine treatments. Although
there were no significant means differences between
treatment 5 and treatments 2, 3, 6, 8 and 9, the results
indicated that the medium SMC, the low CC and the
high VC were the most effective treatment combination
in reducing wind erosion modulus. Another justification
for recommending treatment 5 for reducing the wind
erosion modulus is that this combination of factors
would likely be easier to maintain in the harshness of a
desert environment.
Effects of different treatments on the sand-transport rate
The statistical analysis indicated that the sand-transport
rates of treatments 1, 4, and 7 were much larger and
statistically different than the values for treatments 2, 3,
5, 6, 8, and 9 (Fig. 4b). The effects of the three factors on
the sand-transport rate all were highly significant
(p<0.01) as well as the effects of the random factor on
the sand-transport rate (Table 2). These results indicated
that unknown factors or interactions between factors
affected the sand-transport rate. Multiple comparisons
indicated that the differences among treatments 1 and 4
and treatments 2, 3, 5, 6, 7, 8, and 9 were significant.
The differences between treatments 1 and 4 were
significant (p <0.05) while the differences among treatments 2, 3, 5, 6, 7, 8, and 9 were not significant. The
sand-transport rate of treatment 8 (7.25 % SMC+60 %
CC+0 % VC) had the lowest value among the nine
treatment means with an average value of 0.2 g ·
min−1 ·cm−1. Therefore, it was indicated that the combination of high SMC, the medium CC and the low VC
were the most effective treatment combination in reducing the sand-transport rate.
Effects of different treatments on the structure index
of aeolian sand-flow
Treatment 8 had the smallest structure index of aeolian
sand-flow with an average value of 20.3 % while treatment 1 had the largest average value of this index at
65.0 % (Fig. 4c). The variance analysis showed that the
SMC and the VC treatments affected the structure index
of aeolian sand-flow and were highly significant (P
<0.01). The effect of the random factor was significant
(P =0.048) and the CC treatment had no significant
effect (P =0.077) on aeolian sand-flow (Table 2). Multiple comparisons indicated that treatment 8 was highly
significantly different than treatments 1 and 6 while
treatment 1 differed significantly from treatments 2, 3,
4, 5, 7, and 9. The smallest aeolian sand-flow structure
index treatment was 4.15 % SMC+90 % CC+0 % VC
(medium SMC+high CC+low VC) and indicated that
treatment combination produced the lowest level of
aeolian sand-flow saturation which indicated a stronger
resistance to surface erosion.
Effects of different treatments on the threshold wind
velocity
Treatments 1–3 had low threshold wind velocities (7.2–
8.2 m/s), which increased with increasing moisture content (Fig. 4d). The variance analysis indicated that SMC
had a highly significant effect on the threshold wind
velocity (P <0.01). The effect of VC on the threshold
wind velocity was greater than the effect of CC, but was
not significant with P equal to 0.62 and 0.71, respectively (Table 2). Multiple means comparisons showed
that treatment 9 was significantly different from treatments 1–8 and that treatments 2, 3, 5, 6, and 8 were
significantly different from treatments 1, 4, and 7 (P
<0.05). Treatment 9 had the highest threshold wind
velocity (17.5 m/s) which indicated that the 7.25 %
SMC+90 % CC+25 % VC (high SMC+high CC+
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Fig. 4 Effects of the nine different treatments on the wind-erosion
modulus (a), sand-transport rate (b), structure index of aeolian
sand-flow (c), threshold wind velocity (d), and surface roughness
(e). Levels of the three factors in each treatment group: 1–30 %
CC+0.37 % SMC+0 % VC; 2–30 % CC+4.15 % SMC+25 %
VC; 3–30 % CC+7.25 % SMC+50 % VC; 4–60 % CC+0.37 %
SMC+25 % VC; 5–60 % CC+4.15 % SMC+50 % VC; 6–60 %
CC+7.25 % SMC+0 % VC; 7–90 % CC+0.37 % SMC+50 %
VC; 8–90 % CC+4.15 % SMC+0 % VC; 9–90 % CC+7.25 %
SMC+25 % VC. Capital and minuscule letters represent highly
significant difference at P <0.01 and significant difference at P
<0.05 between different treatments respectively
medium VC) was the most effective treatment in increasing the threshold wind velocity.
was only 0.055 mm with an overall average roughness
of 0.029 mm. The multiple means comparisons indicated that roughness values from treatment 2 differed significantly from the mean roughness values from treatments 1, 3, 6, and 8 and was highly significant from the
roughness value from treatment 1.
Effects of different treatments on surface roughness
Treatment 1 was characterized with the highest roughness value of 0.055 mm, while treatment 4 possessed the
lowest roughness value (Fig. 4e). The variance analysis
showed that VC and CC had significant effects on
roughness (P <0.05), while SMC did not have significant effects on roughness (P >0.05) (Table 2). Under a
wind force of scale 9, the highest roughness observed
Discussion
The SMC affected the wind-erosion modulus, sandtransport rate, threshold wind velocity and the structure
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Table 2 Variance analysis of the five indices
Effect
Intercept
Erosion modulus
Sum of squares
Degrees of freedom
Mean square
Significance
0.000
720995.641
1
720995.641
Sand-transport rate
10081.949
1
10081.949
0.000
Structure index of Aeolian sand-flow
27843.994
1
27843.994
0.000
3780.750
1
3780.750
0.000
0.017
1
0.017
0.000
712829.937
2
356414.968
0.000
18814.821
2
9407.410
0.000
2159.850
2
1079.925
0.001
287.207
2
143.603
0.000
0.000
2
0.000
0.369
77560.571
2
38780.285
0.001
3202.288
2
1601.144
0.000
626.121
2
313.060
0.077
Threshold wind velocity
2.240
2
1.120
0.709
Roughness
0.004
2
0.002
0.000
Threshold wind velocity
Roughness
SMC (%)
Erosion modulus
Sand-transport rate
Structure index of Aeolian sand-flow
Threshold wind velocity
Roughness
CC (%)
Erosion modulus
Sand-transport rate
Structure index of Aeolian sand-flow
VC (%)
Random factor
79251.028
2
39625.514
0.001
Sand-transport rate
Erosion modulus
3131.394
2
1565.697
0.000
Structure index of Aeolian sand-flow
2117.334
2
1058.667
0.001
Threshold wind velocity
3.127
2
1.563
0.621
Roughness
0.004
2
0.002
0.000
83835.218
2
41917.609
0.001
3295.234
2
1647.617
0.000
761.164
2
380.582
0.048
3780.750
1
3780.750
0.000
0.004
2
0.002
0.000
Erosion modulus
Sand-transport rate
Structure index of Aeolian sand-flow
Threshold wind velocity
Roughness
index of aeolian sand-flow at highly significant levels
and resulted in significantly reduced wind erosion
primarily attributable to wet bonding effects on the
threshold velocity. However, the effects are different
for different size of particles. Ravi et al. (2006) reported
that the saltation flux for wet conditions was smaller
than the flux for dry conditions for the particles that
were 69–203 μm in diameter. However, for 38 and
54 μm diameter particles, the saltation flux did not
change under either dry or wet conditions. Coincidentally, the particle size distribution analysis indicated that
clay (<2 μm in diameter) and silt (2–50 μm in diameter)
contents together only composed 22.02 % of the soil
separates. These results helped explain why the relative
high SMC of>4.15 % had a large impact on erosion
prevention and are consistent with the results of Ishizuka
et al. (2005) and Fécan et al. (1998). It should be noted
that when the SMC is greater than 4.15 %, neither the
erosion amount (in this case the wind-erosion modulus
and sand-transport rate) (Fig. 4a and b) nor erodibility
(in this case the threshold wind velocity) (Fig. 4d) is
smaller or larger, and the effects of VC and CC were
relatively small. These results are similar with the
findings reported by Chen et al. (1996) and helped
emphasize that a relatively high SMC may have partially obscured the contributions of VC and CC on erosion
prevention. When the SMC is relatively low, there was a
relatively larger difference for different levels of the VC
and CC. It is also for this reason that the sand-transport
rate and the wind-erosion modulus showed a decreasing
trend with the increased values of VC and CC as may be
noted in Fig. 4a and b. Since the soil moisture had no
effects on the microrelief of the soil or crust surface, it
had no significant effects on the roughness.
BSCs can help separate the soil from the airflow and
avoid a direct contact with the soil (Zhang et al. 2006),
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Plant Soil
and thus it can reduce the amounts of erosion. In this
study, the CC had highly significant effects on the sandtransport rate and the wind-erosion modulus. However,
the threshold wind velocities were primarily influenced
by the exposed surface of the sample, namely where the
BSCs were removed. All of the samples had some
exposed soils, so there were no obvious differences
between the BSCs coverage levels when the SMC was
held constant (Fig. 4d). Thus, under these conditions the
effects of an increased CC obviously played an important role on wind-erosion inhibition.
The VC had significant effects on the sand-transport
rate and the wind-erosion modulus. The sand-transport
rate and the wind-erosion modulus decreased significantly with increased VC by protecting the covered soil
from wind. The wind momentum was partially absorbed
by the VC such that less shear was exerted on the
erodible surface, and by also trapping eroded material
from further movement (Dong et al. 2001). This observation has been supported by previous researchers who
have reported that the direct effects of vegetation is the
increased roughness that results in more absorption of
the wind momentum from the air stream (Dong et al.
2001; Okin et al. 2006). However, the aeolian sand-flow
structure and the threshold wind velocity revealed different increasing trends in this study. For example, the
threshold wind velocity initially increased and subsequently decreased, while the aeolian sand-flow structure
index initially decreased and subsequently increased.
These phenomena may be related to the layout of the
simulated vegetation (Qu et al. 2008). The sand sampler,
wind profiler, and baffle for observing the threshold
wind velocity were all placed near the central axis of
the samples. In the treatments with 50 % VC, the simulated vegetation was arrayed in two columns, forming
a narrow channel along the central axis. When the air
flowed through this region, the flow rate would have
increased, reducing the erosion resistance of the highVC treatments. The configuration of the VC should be
considered in further studies.
Although the VC had significant effects on the surface roughness (Table 2), it should be noted that the
surface roughness showed no obvious pattern with increased VC. The surface roughness of 0 % VC was
higher than the surface roughness values for 25 % and
50 % VC. It is also felt the most probable reason for
these differences was the layout of the simulated vegetation. When the VC was 25 or 50 %, the layout of false
vegetation may have caused narrow channel air flow
which would be expected to accelerate wind speed and,
thus, lead to a smaller surface roughness than the plot
which had no vegetation (Okin 2008). These results
indicated that improper vegetation layout or revegetation may have unexpected effects on erosion reduction. There may be several other possible explanations for the observed behavior. Firstly, a previous study
had demonstrated that the magnitude of roughness is
closely related to the wind velocity and it decreased as
the flow velocity gradually increased (Dong et al. 2000).
The wind velocity in the present study was relatively
high, thus, the roughness of each treatment was generally low with an average value of 0.029 mm. Since a
high wind velocity was used, any small changes in the
experimental conditions might be expected to have a
large impact on the roughness. Secondly, the height of
the false vegetation was relatively small, but typical for
the plants in the studies environment. Similar studies
have shown that when the plant height is less than 0.2 m,
the aerodynamic roughness basically fluctuated around
a small value and when the plant height gradually increased above 0.2 m, the aerodynamic roughness increased linearly (Zhang et al. 2010).
Therefore, the effects of vegetation layout, coverage,
and types on wind-erosion characteristics require further
research. An orthogonal experiment was needed to test
the effects of different vegetation layout, vegetation
coverage, and plant types (which had different sizes of
canopies and trunks) on erosion amounts and erodibilities. There are also some other factors that should be
included in further research studies. The random factors
in the variance analysis had highly significant effects on
the sand-transport rate and significant effects on the
erosion modulus and the aeolian sand-flow structure.
These findings of significance indicated that unknown
factors or interactions between these factors affected the
experimental results. These unknown factors may include the air temperature and humidity since these two
factors will partially determine the speed of water loss in
the soil. Since the samples were tested at different times
of the day and the temperature differences between day
and night was approximately 15 °C during the experimental period (Wu et al. 2013). Water has been shown to
evaporate at a faster rate on BSC under relatively high
temperature and low relative humidity (Wang et al.
2014; Neuman 2003). In further studies, all the forementioned factors should be tested by analysis of variance to determine if the factors have significant effects
on wind erosion parameters.
Author's personal copy
Plant Soil
The results of this study suggested that the SMC
played a key role on the wind erosion reduction in the
Tengger desert and its contribution was greater than the
effects of the VC and the CC when the SMC was greater
than 4.15 %. Although rainwater at the study site is
limited, approximately 80 % of that rainfall occurred
from May to September such that the prevailing characteristics of this wetter portion of the year are low wind
speeds, relative high soil moisture, and vegetation that
flourishes. Thus, this study indicated that appropriate
disturbance of the BSCs from July to September would
not be expected to increase the wind erosion. A previous
study found that the BSCs development discouraged the
growth of deep-rooted shrubs which have strong wind
erosion reduction capabilities and the shrubs were replaced by annual herbs whose growth was attributable
to decreased soil moisture in deeper soil layers that was
characteristic of soil with BSCs. (Li et al. 2002). Xiong
et al. (2011) determined that the disturbance on welldeveloped BSCs could lead to greater infiltration depth
after rainfall which would be very important for the
growth and regeneration of deep-rooted shrubs. Moreover, our earlier research has demonstrated that moderate disturbances to well-developed BSCs didn’t result in
rapidly increased wind erosion when the shrub’s coverage reached approximately 50 % (Yang et al. 2014). A
factor offsetting the disturbance of BSCs is that the
fragmentation of the BSCs caused by disturbance would
be partially compensated by an increase of newly
formed BSCs (Hiernaux et al. 1999).
Conclusions
Therefore, this study demonstrated that BSCs should be
strongly protected during dry and windy seasons in the
spring and winter to avoid exacerbating wind erosion.
The BSC may be moderately disturbed without wind
erosion exacerbation during wet and weak wind seasons
in the summer or autumn to ameliorate soil moisture
conditions, especially in the sites with high vegetation
coverage. This type of management will help to determine the balance between the Bprotection^ and
Bdestruction^ of BSCs, which are beneficial to the sustainable utilization of BSC to help control wind erosion.
Furthermore, this study demonstrated that well developed BSCs may be safely and beneficially disturbed
in the Tengger desert in situations where there are a
>50 % vegetative coverage during the rainy season. This
disturbance of the BSCs will not cause the degradation
of the ecosystem in the Tengger desert as long as the
occasion and sites of disturbance are appropriate.
Acknowledgments This research was supported by the NSFC
(National Natural Science Foundation of China, 41071192), the
Chinese Universities Scientific Fund (Grant no. 2014YQ006), and
the Foundation of State Key Laboratory of Soil Erosion and
Dryland Farming on the Loess Plateau (K318009902-1405). The
preparation of this paper was supported by the 111 Project
(B12007). Professor Zhibao Dong and engineer Aiguo Zhao of
the CAREERI, CAS, provided substantial assistance during this
study. Professor Xinrong Li of CAREERI, CAS facilitated our
field sampling and living conditions.
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