1. Introduction
Wind erosion and particulate matter emissions are important and non-negligible reasons that cause landscape degradation and threaten human production and life (Borrelli et al., 2016; Deetz et al., 2016). Wind erosion causes a large amount of soil loss in farmland or forest land. Additionally, the emission of particulate matter caused by wind erosion carries a large amount of pesticides, microorganisms and heavy metal elements for migration (Van Pelt and Zobeck, 2007; Tatarko et al., 2020; Whicker et al., 2006). In recent years, large-scale accidents caused by wind erosion and particulate matter are no longer unusual (Li et al., 2018; Middleton, 2017). Effects of wind erosion and particulate matter emissions range from human health issues to global climate change (Goudie, 2014; Lambert et al., 2008). Influenced by the source of particulate matter emissions, previous studies focused on densely populated urban areas (Currie and Bass, 2008; Jiakai et al., 2016). However, farmland areas are also an important contribution area of particulate matter emissions (Korcz et al., 2009). Therefore, land degradation caused by wind erosion and particulate matter emissions should be analyzed in detail (Schmidt et al., 2017; Tatarko et al., 2020).
The factors influencing the natural wind erosion and particulate matter emission processes can be divided into physical and chemical factors, including: soil stability, soil properties, soil texture, soil density, soil water content, land cover, soil nutrients, etc. (Gillies et al., 2017; Hagen et al., 2010; Kohake et al., 2010; Zou et al., 2018). The soil particle size distribution is highly sensitive to the wind erosion process (Larney and Bullock, 1994). Previous studies have shown that wind erosion is prone to occur when the soil particle size ranges from 0.02 mm to 0.84 mm. Particles with size between 0.05 mm and 0.5 mm are the most easily eroded particles (Chepil, 1955; Skidmore and Powers, 1982; Zou et al., 2018), sometimes even if the surface sand content is low, it may become a high PM10 emitter (Feng et al., 2011; Sweeney et al., 2011). When other factors remain constant, the material with large particle size and low density is more easily transported by wind (Zobeck et al., 2013), and soil density has less variability than soil texture (Kohake et al., 2010; Menut et al., 2013; Shahabinejad et al., 2019).
Previous studies have shown that as soil density increases, soil erodibility under wind conditions will decrease gradually (Campbell et al., 2002). Tillage and watering are main ways that rapidly change soil density and soil properties, affecting wind erosion and dust emission (Larney and Bullock, 1994). The surface aerodynamic parameters of different soil properties are variable, especially in arid and semi-arid areas. This difference has a profound impact on the wind erosion process (Campbell et al., 2002; Cheng et al., 2017; Zou et al., 2018).
The influence of soil nutrients is mainly reflected in the organic matter content. Non-agricultural soils increase soil organic matter through various methods, such as litter decomposition. Thus, as the organic matter increases, soil particles are aggregated and not easily dispersed (Mendez et al., 2006; Panebianco et al., 2016). Although agricultural soils have regular fertilization operations, the continuous cultivation process will inevitably continuously degrade the organic matter content in the soil, resulting in poor aggregation of soil particles and tendency to erosion (Acikgoz et al., 2017).
Irrigation can keep the soil surface moist and minimize wind erosion, but wind can use the evaporation process to continuously take away the water in the soil and reduce the cohesion of soil particles (Larney and Bullock, 1994). Therefore, the influence of soil water content changes is highly correlated with time.
The emission of particulate matter in wind erosion is directly related to the amount of wind erosion itself. The reduction of particulate pollution is a complex and non-linear process (Chang et al., 2019). Therefore, treatment from the source (such as vegetation or soil crust coverage) is a relatively efficient method. Biological soil crusts (BSCs), as one of the means of ground cover, can effectively inhibit wind erosion and impact significantly on the separation and transportation of particles of different sizes (Miralles-Mellado et al., 2011; Neuman et al., 1996). However, BSCs are also one of the sources of particulate matter emissions, which may have a higher level of microbial diversity, thereby endangering human health (Abed et al., 2012). Overall, it is relevant to study wind erosion and particulate matter emission under the BSC cover.
In order to prevent and control wind erosion and particulate matter emissions, physical, chemical and biological methods have been adopted. For instance, Schmidt et al. (2017) used the Index of Land Susceptibility to Wind Erosion (ILSWE) to quantify the elements of wind erosion, and assessed the wind erosion potential in Europe. In addition, Tian et al. (2018) used chemical solutions to consolidate the surface of the soil, which significantly increased the anti-erosion strength of the surface. Chang et al. (2021) suggested that windbreak forest belts effectively reduced wind erosion by 20% and significantly weakened PM10 emission concentration. Among all the prevention and control measures, biological measures are effective and nearly natural. However, most biological methods require a long growth period and have inherent limitations (Diouf et al., 1990; Maleki et al., 2016; McClure, 1998). Therefore, the use of BSCs for rapid growth and mulching is not only a more efficient wind erosion and particulate emission prevention and control method, but also provides a better growth basis for future vegetation succession. BSCs can effectively combine fine particles with each other and increase their threshold wind speed (Belnap and Gillette, 1998; Reynolds et al., 2001; Zhang et al., 2014). Previous studies have shown that mycorrhiza can significantly increase the degree of root colonization of plants during sowing, thereby decreasing the erodibility (Burri et al., 2013). Therefore, the use of BSCs to prevent wind erosion and particulate matter emission has good long-term benefits. Zhang et al. (2014) used dust fall and soil crust distribution to describe the stability of the wind and sand environment on a typical desert railway protection system, proving that it is beneficial to use soil crust to artificially improve the wind and sand environments.
In order to understand the process of wind erosion and particulate matter emission, researchers more often use wind tunnel methods. Under natural conditions, changes in wind erosion and particulate emissions are more sensitive to height. Zobeck and Van Pelt (2006) suggested that the emission concentration of particulate matter in the height range of 2-5 m is 2-5 times that in the height range of 5-10 m. Generally speaking, bryophytes have a pseudo-leaf and pseudo-stem structure, and the formed moss crust is also relatively thick. Algae plants do not differentiate into leaves and stems. Thus, the algae crust is relatively flat (Belnap et al., 2013). Although the aerodynamic roughness length provided by the two types of biological soil crusts only measures millimeters or micrometers, their ability to prevent and control wind erosion and particulate matter emissions is still significant. The wind erosion material in the wind tunnel mainly moves horizontally, so the amount of wind erosion and the emission of particulate matter can be expressed by the horizontal flux (Panebianco et al., 2016), which has higher research efficiency. Bu et al. (2015) optimized the combination of soil water content, crust coverage, and vegetation coverage through experiments, and gave the most effective combination to reduce wind erosion. However, the soil water content is a constantly changing instantaneous value under the action of wind. Therefore, this combination method cannot be effectively used in practice. Copeland et al. (2009) used a wood-based long-strand material and agricultural straw to test wind erosion and particulate matter emissions. Their results show that wood materials have higher wind speed adaptability than agricultural straws, and wood materials have reduced wind erosion and particulate matter emissions by 90%, under a wind speed of 11 m/s. Although straw cannot significantly reduce the total wind erosion, PM10 emissions have dropped by 75%. In addition, Tatarko et al. (2020) conducted wind tunnel tests on 15 typical soils in the United States and concluded that abrasion coefficients and particulate matter emissions are more sensitive to soil types, soil textures and farming methods.
For the prevention and control of suspended dust generated by wind erosion, it is not only necessary to understand its source (Mirzamostafa et al., 1998; Hui et al., 2019), but also to clarify the content and proportion of particles of different sizes. Currently, studies have focused on the ratio of particles with different particle sizes (Cowherd, 2006; Feng et al., 2011; Li et al., 2015; Tatarko et al., 2020). Scholars have developed formulas for predicting particulate matter emissions (Tatarko et al., 2020), although some researchers believe that this empirical formula may lack universality (Sullivan and Ajwa, 2011).
Traditionally, in the research of farmland wind erosion and particulate matter emission control, BSCs have been studied as a prevention and control method. However, studies that can clarify the quantitative relationship between particulate matter emissions and wind erosion processes are not present in the literature (Li et al., 2015; Tatarko et al., 2020). This study will take two common BSCs and explore the thresholds of the proportion of particulate matter emissions during wind erosion, in order to provide an effective reference for the targeted management of wind erosion and particulate matter emissions.