Introduction
Wind erosion is a serious problem in arid and semiarid regions where soil is loose and dry and not well protected by vegetation cover. Wind erosion is a process involving the detachment, transport, and deposition of soil particles by wind. Wind erosion not only causes soil loss from the soil surface, but also land degeneration or desertification. Land degradation may result from removing fertile top soil, namely, the clays and organic matter, and leaving the infertile and coarse soil. In addition, fine suspended particulates due to wind erosion is always a concern for air quality for cities located in or around arid and semiarid regions (Sharratt and Lauer, 2006; Chen et al., 2017). Moreover, severe and sustained wind erosion events have resulted in regional dust storms, which threatened aircraft, road transportation, and health of humans and animals (Hudson and Cary, 1999; Nel, 2005).
The inland Pacific Northwest region (iPNW) of the Unites States is surrounded by a series of tall Mountains, which intercept the flow of moist and relatively mild air from the Pacific Ocean, thus resulting in a semi-arid environment. The PM10 concentration caused by wind erosion events has exceeded National Ambient Air Quality Standard (NAAQS) several times per year at Kennewick, WA (Sharratt and Lauer, 2006). An estimated 98 Mg ha-1 soil and 2.7 Mg ha-1 PM10 are eroded each year from the agro-ecological classes of the iPNW (Pi et al., 2019a).
Wind erosion is influenced by a series of plant, soil and wind parameters. For example, plants reduce wind erosion as a result of directly intercepting saltating particle. Soil crusts also reduce wind erosion by binding soil particles together into a cohesive structure, thereby minimizing the availability of loose particles susceptible to transport by wind. Wind speed determines the energy imparted to soil particles and thus the transport capacity of wind erosion. Some wind erosion parameters are nearly static and change very slowly through time. For example, intrinsic soil properties such as clay and organic matter content change slowly with time. In contrast, some wind erosion parameters change very rapidly in response to management or weather. Aggregate and crust properties can be altered by tillage or rainfall events thus are called temporal or external soil properties (Zobeck, 1991a). Temporal soil properties appear to be relevant in controlling daily soil wind erosion whereas intrinsic soil properties largely control long-term soil wind erosion, the latter of which is called soil wind erosion potential (Zobeck, 1991a). Intrinsic or temporal soil properties do not simultaneously influence wind erosion because intrinsic soil properties more or less influence the temporal soil properties. For instance, soil clay content is one of the primary intrinsic soil properties affecting wind erodibility, but it rarely influences wind erosion other than through its impact on crust or aggregate crushing energy (Zobeck, 1991a). Temporal soil properties influencing wind erosion include random roughness, soil water content, as well as aggregate and crust properties (USDA, 2016).
Soil crusting is one of the most important factors influencing wind erosion. Crust crushing energy is highly dependent on clay and organic matter content which are the cohesive forces acting on particles. Crust crushing energy influences wind erosion in terms of its impact on controlling the amount of flaking (Gillette et al., 1982) and abrading (Hagen et al., 1992), Flaking and abrading contribute to the particle load which can be transported by wind, especially for weak crusts with lower crust crushing energy. In addition, crust crushing energy influence wind erosion through its impact on controlling threshold friction velocity (u*t) . Gillette et al. (2001) indicated that hard crusts with greater crust crushing energy tended to show almost no change in u*t with time whereas weak crusts with lower crust crushing energy resulted in a rapid decrease in u*t with time.
Tillage is the primary factor influencing crust degradation on agricultural lands. Other crust properties influencing wind erosion include crust type (Belnap, 2003), crust hardness (Gillette et al., 2001), crust micro-topography (Gillette et al., 2001), crust thickness (Sharratt and Vaddella., 2014), progressive development of a crust cover (Pi and Sharratt, 2019), and loose material on the crust surface.
Aggregation is another essential factor influencing soil wind erosion. The Agricultural Policy /Environmental eXtender (APEX) model considers the aggregate size distribution (ASD) as the sole factor determining soil erodibility (Williams et al., 2015). Aggregate size distribution can influence soil wind erosion because ASD influences (1) the amount of available creep (0.84 to 2.0 mm), saltation (0.10 to 0.84 mm), suspension (<0.1 mm), and PM10 (<0.01 mm) components (USDA, 2016); (2) aggregate stability (Hevia et al., 2007); (3) threshold friction velocity (Gillette et al., 1980); (4) source of abrasion (Mirzamostafa et al., 1998); (5) random roughness (USDA, 2016); (6) the amount of nonerodible components transported by wind (> 2 mm) (Zobeck, 1991b). Kheirabadi et al. (2018) examined the effect of soil bed length, wind velocity and ASD on sediment flux. They found finer aggregates (D2mm) at the surface are more susceptible to transport by wind while soil containing coarser aggregates exhibited less sediment flux. Mirzamostafa et al. (1998) examined the effect of soil aggregate and texture on suspension emission. They found suspension emission was directly related to soil texture (r2= 0.87), whereas abraded emission was directly related to clay content (r2= 0.69). Other aggregate properties influencing wind erosion include aggregate density and stability and minimum and maximum aggregate sizes (USDA, 2016).
The impact of random roughness in controlling wind erosion has been studied by Zobeck and Onstad (1987), Hagen and Armbrust (1992), and Gillette et al. (2001). They found wind erosion increased logarithmically with random roughness. Random roughness is associate with soil surface microrelief which can impact surface aerodynamic roughness and shear stress (Shao, 2008; Okin, 2008). Surface microrelief is greatly influenced by maximum aggregate size and crust rough (Zobeck, 1991a).
Soil clay content is a primary soil component due to its various influences for soil ecological processes. Soil clay amendment can impact soil ecosystems as a result of modifying enzyme activity (Abd-elgwad, 2019), hydraulic conductivity (Frenkel et al., 1978), soil-water retention curves (Gupta and Larson, 1979), soil moisture distribution and water use efficiency (Ismail and Ozawa, 2007; Mi et al., 2017), evaporation (Zayani et al., 1996), soil fertility (Mi et al., 2017), and crop yield (Mojid et al., 2012). Although Diouf et al. (1990) reported soil clay amendment impacted wind erosion as a result of influencing aggregate stability, little information is available that documents the effect of clay amendment on other soil properties such soil wind erosion.
We are not aware of any studies that have measured the change in wind erosion of clay-amended soils, which is a primary soil component that binds individual particles together resulting in changes to aggregate and crust properties. Therefore, the objective of this study was to: (1) identify the impact of clay amendment (Wyoming bentonite) on wind erosion from disturbed crusts of four major loessial soils in the inland Pacific Northwest, USA; (2) test the effect of clay amendment on crust crushing energy; and (3) evaluate the effect of clay amendment on surface characteristics affecting wind erosion such as random roughness, aggregate crushing energy, and ASD.