Impact of clay on wind erosion for an erodible soil crust surface
In this study, we found the crust crushing energy increased binomially with increasing clay amendment (Figure 5a). These results are consistent with previous studies (e.g., Diouf et al., 1990; Hagen, 1995; Pi et al., 2019b). However, the rate of change in crust crushing energy decreased linearly with increasing clay amendment (Figure 5b). The rate of change in crust crushing energy was higher for the two sandy loams than two silt loams. In addition, soil loss significantly decreased with the increasing aggregate GMD. Soil loss was higher for the two sandy loams than two silt loams. Both trends appeared consistent and indicated that decreased soil loss was directly associated with increasing clay amendment, crust crushing energy and aggregate GMD.
Significant differences in soil loss were found between 0 and 2% clay amendment treatments for Warden sandy loam and Farrell sandy loam according to ANOVA (Table 3). The relationship between clay amendment and soil loss is illustrated in Figure 8. Regression analysis revealed similar trends with ANOVA. The relationship between clay amendment and soil loss appeared to be nearly an exponential function for all soil types. Clay particles are important in cementing sand grains together and thereby changing crust strength and soil loss. In fact, we found soil loss decreased with the increasing clay content for the erodible soil crust surface. This trend was consistent with previous studies for a variety of soil types where soil loss was inversely related to clay content (e.g., Zobeck, 1991b; Hagen et al., 1992; Pi et al., 2020b). No statistical differences in soil loss were found with increasing clay amendment for Athena silt loam and Walla Walla silt loam until clay amendment reached 16 and 8% respectively. No significant differences in soil loss were found between 8% and 16% clay amendment for all the soil types except for the Athena silt loam. Regression analysis also suggested that the rate of change in erosion decreased with increasing amounts of clay amendment. For example, soil loss decreased by 3.33, 4.31, 6.32, and 94.1 g m-2 for Athena silt loam, Walla Walla silt loam, Warden sandy loam and Farrell sandy loam when amended clay content increased from 0 to 2%. Nonetheless, soil loss decreased by only 1.66, 1.75, 2.86 and 3.17 g m-2 for the four respective soil types when amended clay content increased from 14 to 16%. Soil loss was thus sensitive to the low clay amendment treatment; this trend was consistent with the effect of clay amendment on crust crushing energy.
Our results indicate that clay amendment protects the soil surface from erosion for the soil types used in this study, at least until the clay amendment exceeded a certain value in which case the protective effect of clay amendment may have reached a peak. For example, strong crust due to greater clay amendment may be more prone to break down into greater aggregate or cloud when undergoing a disruptive tillage, the former of which may be very hard to be eroded. However, we assume soil loss will not decrease to zero because loose and small soil particles are present along the vertical cracks caused by tillage.
Based on the coefficients of linear regression (Figure 8), the effectiveness of clay amendments in decreasing soil loss varied among soil types. The higher regression coefficients suggest the rate of change in soil loss with progressive increase in clay amendment was higher for Farrell sandy loam than Warden sandy loam, followed by Athena silt loam, and Walla Walla silt loam. For example, soil loss decreased by 56 and 66% for Athena silt loam and Walla Walla silt loam and by 82 and 99% for Warden sandy loam and Warden sandy loam in the presence of 16% clay amendment. This was expected because of greater wind soil erodibility and lower clay content for two sandy loams than two silt loams. Clay therefore appeared to influence soil wind erosion with greater sand content in this study. In fact, amending sandy soil using clay has been considered a sustainable and economical reclamation strategy for enhancing plant productivity and soil water retention in previous studies (Zayani et al., 2006; Ismail and Ozawa, 2006).
The impact of clay amendment on soil loss may be primarily due to the changes in aggregate GMD. Aggregates influence soil loss primarily through the amount of available saltation and suspension components which directly causes variations in soil loss. Nonetheless, the flux of abrading particles from aggregates is an additional available source of saltation and suspension. Aggregates with lower crushing energy can result in considerable flux of abrading particles (Hagen, 1991). Mirzamostafa et al. (1998) investigated fraction of the flux of abrading particles in suspension as influenced by clay content for four Kansas soils. They found abrasion flux decreased with clay content ranging from 0 to 17%. In this study, we assumed that aggregate crushing energy and aggregate GMD similarly impacted soil loss. Crust crushing energy has been considered to equal aggregate crushing energy according to the Single-event Wind Erosion Evaluation Program (SWEEP) user guide (USDA, 2016).
Impact of aggregate crushing energy on soil loss through abrasion flux
Soil wind erosion degrades aggregates as a result of abrading (Hagen et al., 1991). Abrasion influences particles potentially available for emissions and aides in the initiation of erosion. In the Jornada Desert of New Mexico, Webb et al. (2016) found that horizontal mass flux was controlled by the supply and abrasion efficiency of saltators.
The abrasion of aggregates is determined by aggregate crushing energy (Hagen et al., 1993). Weak aggregates can be eroded rapidly, especially by wind which carries suspended or blowing soil particles (Zobeck, 1991a), whereas strong aggregates are more resistant to erosion. We found soil loss was sensitive to clay amendment when aggregate crushing energy was low. Indeed, weak aggregates with low aggregate crushing energy may have greater abrading potential. With an increase in clay amendment, the abrading potential decreased until sufficient aggregate strength was achieved to resist an further abrasion. Hagen et al. (1993) found aggregate crushing energy determined aggregate abrasion coefficients and thus abrasion flux. In this study, we used the SWEEP to simulate abrasion flux in order to interpret the effect of clay amendment on aggregate crushing energy. The SWEEP wind erosion submodel was described in detail by USDA (2016) and simulates abrasion flux following:
Qabrasion=\(\sum_{i=1}^{m}Q\)*Fani*Cani(4)
where Qabrasion is simulated vertical abrasion flux (g m-2); Q is saltation discharge (g m-2), which was caused by wind speed at 12 m s-1 for 180 s within abrader environment (Table 3).Fani, Cani are respectively the fraction of saltation abrading surface with ith abrasion coefficient, and abrasion coefficient of ith surface (1/m). Hagen et al. (1993) recommended using a single target surface in wind tunnel tests. TheFani was calculated as:
Fani=[1-4Bf-2SVroc(1- Bf)]*[1-(1- \((\frac{\text{SF}_{84}-\text{SF}_{10}}{\text{SF}_{200}-\text{SF}_{10}})\)) \(e^{(\frac{{-SFA}_{12}}{20})}\)] (5)
where Bf is biomass fraction of flat cover,SVroc is soil volume with rock (m-3 m-3),SF10,SF84,SF200are the aggregate fraction less than 0.1, 0.84, and 2 mm,SFA12 is soil surface fraction with shelter angles greater than 12 degrees. Bf , SVroc, and SFA12 are consistent zero for all the treatments. The Cani was calculated as:
Cani= \(e^{(-2.07-0.077X^{2.5}+Ln(x))}\)