3.2 Influence of wind speed and coverage on particulate
emissions of biological soil crusts
This study further analyzed the emission of particles of different sizes
and their relationship with wind speed and coverage under different BSC
conditions. Figure 3 shows the influence process of wind speed and
coverage on total suspended particulate matter, PM10, PM2.5, and PM1
emissions for algae crust. All particulate matter emissions increase
with increasing wind speed and decrease with increasing cover degree.
From the axial color change in the figure, the horizontal axis
(coverage) has a larger change span, and the vertical axis (wind speed)
has a smaller change span.
Figure 3. Relationship between particulate emissions size, wind speed
and coverage of algae crusts. (The color from dark to light indicates
the emission of particulate matter from small to large. There are
differences in the emission of particles of different sizes, which is
reflected in the legend of this figure.)
Similarly, for moss crust, the influence of wind speed and coverage on
total suspended particulate matter, PM10, PM2.5, and PM1 emissions, is
shown in Figure 4. All particulate
matter emissions increase with increasing wind speed and decrease with
increasing coverage. From the axial color change in Figure 3, the
horizontal axis (coverage) has a larger change span, and the vertical
axis (wind speed) has a smaller change span. The same behavior exhibited
by the two BSCs proves that the degree of influence of coverage is
greater than wind speed for particulate emissions. In order to quantify
this result, we also established linear regression equations for the
particulate emission of different particle sizes, wind speed and
coverage under different BSC conditions (Table 3). The fitting effect of
the particulate emission, wind speed and coverage of the two types of
BSCs is good. Through the method of coefficient standardization, it can
be seen that the influence of coverage on the dust emission is always
higher than the wind speed.
Figure 4. Relationship between particulate emissions, wind speed and
coverage of moss crusts. (The color from dark to light indicates the
emission of particulate matter from small to large, respectively. There
are differences in the emission of particles of different sizes, which
is reflected in the legend of this figure.)
Table 3. Regression equations of particulate emissions of biological
soil crust, where R represents goodness of fit, and P represents
significance.
3.3 The proportion of particulate emissions from
biological soil crusts in the amount of wind erosion
Particulate emissions are an important part of wind erosion materials,
and clarifying the threshold of the proportion of particulate emissions
in the amount of wind erosion will be beneficial to the deployment of
targeted measures in the control of regional soil erosion. First, it is
clear that the emissions of particulate matter of various sizes are
directly related to the amount of wind erosion: the greater the amount
of wind erosion, the greater the emission of particulate matter. The
change of the amount of wind erosion is closely related to the wind
speed and coverage (see Figure 5). Although the overall particulate
emissions increase with increasing wind speed and decrease with
increasing coverage, the amount of wind erosion also changes
accordingly. Thus, proportion of particulate emissions in the amount of
wind erosion has its own characteristics.
Figure 5. Distribution of particulate emissions in the corresponding
wind erosion. The masses of wind erosion, PM10, PM2.5 and PM1 are
overlapped under the same coverage (1, 2, 3, and 4) and wind speed (8,
10, 12, 14, and 16 m/s), to show the distribution and changes of
particles of different sizes in the wind erosion.
The proportion of different particle size emitted by different BSCs
under different coverage and wind speed in the corresponding wind
erosion amount is not only inversely proportional to wind speed, but
also inversely proportional to coverage (see Table 4). This phenomenon
is inconsistent with the above-mentioned wind erosion and particulate
emission behaviors. For moss crusts, for coverage <25%, the
proportion varies with wind speed between 4.91% and 55.91%; for
coverage between 25% and 50%, the variation range of the proportion
with wind speed is 4.08% to 30.25%; for coverage between 50% and
75%, the variation range of the proportion with wind speed is 1.19% to
21.65%; for coverage>75%, the variation range of the
proportion with wind speed is 0.60% to 9.85%.
For algae crust, for coverage <25%, the variation range of
the proportion with wind speed is 8.80% to 29.16%; for coverage
between 25% and 50%, the variation range of the proportion with wind
speed is 3.26% to 28.01%; for coverage between 50% and 75%, the
variation range of the proportion with wind speed is 7.15% to 45.97%;
for coverage >75%, the variation range of the proportion
with wind speed is 0.72% to 11.70%. In wind-eroded areas, the wind
speed is changeable, and the ground cover will not change significantly
without man-made influence. Therefore, this article only gives the
threshold range of the change in the proportion of particulate emissions
in wind erosion with wind speed under the same coverage conditions.
Table 4. The proportion of particulate emissions in the corresponding
wind erosion, where C represents coverage type; WS represents wind
speed; WE represents wind erosion; and SD represents standard deviation
3.4 Particulate emission capacity of different
biological soil crusts
According to the principle of aerodynamics, the magnitude of wind speed
will always drive changes in wind erosion and particulate matter
emissions. From the above results, it can be seen that as the wind speed
increases, the gap between the increase in wind erosion and the increase
in particulate matter emissions increases, resulting in an inverse
relationship between proportion and wind speed. When increasing wind
speed, if the proportion of particulate emissions in the amount of wind
erosion decreases slowly, it can be determined that this BSC has
relatively strong particulate emissions under the corresponding coverage
conditions. Conversely, if this proportion decreases rapidly with the
wind speed, it can be considered that the BSC has a relatively weak
particulate emission capacity under the corresponding coverage
conditions. This study performed a linear fitting of the proportion of
the emission of particles of different sizes in the wind erosion with
the change of wind speed under the same coverage condition. The slope
quantifies the particulate emission capacity of BSCs, as shown in Figure
6. For moss crusts, the emission capacity of particulate matter is
directly proportional to particle size and inversely proportional to
coverage. For algae crusts, although the emission capacity is
proportional to particle size of the particulate matter, the
relationship with coverage is not regular. It is embodied that PM10 has
the strongest emission capacity for coverage between 50% and 75%.
PM2.5 and PM1 have the strongest emission capacity for coverage between
25% and 50%. The effective quantification of the emission capacity of
particulate matter of different sizes can make control measures more
targeted. The abnormal results of algae crust may be related to the
physical properties of the soil. In general, the conditions of low soil
bulk density, high porosity, and high water storage capacity lead to the
development of biological soil crust. Otherwise, it is easy for the soil
hardening phenomenon to occur. According to the morphological
characteristics of the two biological soil crusts, algae crusts are flat
and have less water conservation ability than moss crusts (Belnap et
al., 2013). Therefore, under the condition of low coverage, the open
space around the algal crust develops into a smooth and hard physical
crust after precipitation (Sanders et al., 1986). The structure of the
physical crust is compact and stable, and it will not cause a lot of
wind erosion and particulate matter emission without external force
damage. However, the over developed physical crust does not lead to
vegetation succession and ecosystem restoration.
Figure 6. Particulate emission capacity of different biological soil
crusts