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