*4.2. Weight Percentage of Soil Particle Size Under Di*ff*erent Treatments*

Previous studies have stated that with decreasing soil particle diameter, the soil cohesiveness gradually increased [29–31]. According to the international classification standard of particles, when a particle of a sample has a diameter of 2–64 mm, it is called "gravel" [32]. In our study, the distribution of the particle size of the samples greatly varied among the different treatments (Figure 4). Site S2 contained the largest amount of gravel among all treatments, accounting for 25.35%. In the other treatments, the proportion of gravel was smaller, or gravel was completely absent. Particles with a diameter of 0.05–1 mm were most abundant in S8, with a significant difference when compared to the other treatments (*p* < 0.05). The weight percentages of d = 0.05–1 mm were 48.86%, 35.18%, 41.35%, 50.28%, and 61.72%, respectively, for the treatments S1, S2, S3, S7, and S8, which were significantly higher than those for the other particle size compositions (*p* < 0.05). The proportions of particles with a diameter of <0.005 mm were 39.52% and 38.65%, respectively, in the treatments S4 and S5, which were significantly higher than those in the other treatments (*p* < 0.05). Lu et al. also found that the size of the soil particles in the soil matrix differed; this was not only the case for the surface soil layers, but also for soil porosity in general [33]. Sandy soil contains coarse gravel and has a high soil porosity. On the contrary, the permeability of loam or clayey soil is lower than that of sandy soil, facilitating surface runoff [34]. Soil particle size and runoff are closely related, and a favorable soil particle size composition can effectively maintain water, nutrients, and organic matter [35]. According to our results, most of the particles with a diameter of more than 2 mm retain the original mineral composition of the parent rock. There are few available mineral nutrients, and the ability to absorb water was also poor. When the content of gravel in the soil exceeds 20% of the total volume of sample, changes in the temperature of the sample will be aggravated, and the water-holding capacity of the soil will be reduced [ *Sustainability* **2019** 36]. , *11*, x FOR PEER REVIEW 8 of 14

**Figure 4.** Variation in weight percentage of particle size of the samples under different treatments along the expressway in Datong City, Shanxi Province, China, in 2013. d represents soil particle **Figure 4.** Variation in weight percentage of particle size of the samples under different treatments along the expressway in Datong City, Shanxi Province, China, in 2013. d represents soil particle diameter.

#### diameter. *4.3. Variations in Volume Weight of the Soil and Total Porosity under Di*ff*erent Treatments*

may not be conducive to water and fertilizer conservation.

*4.3. Variations in Volume Weight of the Soil and Total Porosity under Different Treatments*  The volume weight of the soil (VWS) can be used as an indicator of soil solidity under certain conditions. At the same soil texture, soil with a low volume weight is relatively loose, while soil with a high VWS tends to be firm [36]. Generally, VWS varies greatly with soil texture, structure, and tightness [37]. Total porosity (TP) represents the percentage of soil porosity of the total soil volume. The amount of soil pores is related to the water permeability, air permeability, thermal conductivity, The volume weight of the soil (VWS) can be used as an indicator of soil solidity under certain conditions. At the same soil texture, soil with a low volume weight is relatively loose, while soil with a high VWS tends to be firm [36]. Generally, VWS varies greatly with soil texture, structure, and tightness [37]. Total porosity (TP) represents the percentage of soil porosity of the total soil volume. The amount of soil pores is related to the water permeability, air permeability, thermal conductivity, and compactness of the soil [38]. In our study, VWS and total porosity differed among the different

while those of S3 and S5 were smaller, indicating a low soil compactness of S3 and S5, which is more suitable for plant growth. The TP values of S3, S5, and S8 were relatively high with 59.12%, 58.97%, and 58.97%, respectively, while that of S4 was 53.12% and therefore smaller than the values found for S3, S5, and S8. Generally, the TP ranged between 50% and 60%, and the texture was loose, which

and compactness of the soil [38]. In our study, VWS and total porosity differed among the different treatments (Figure 5). The VWS was highest in S8, reaching 1.60 g/cm3, and lowest in S3, with a value treatments (Figure 5). The VWS was highest in S8, reaching 1.60 g/cm<sup>3</sup> , and lowest in S3, with a value of 1.34 g/cm<sup>3</sup> , with a difference of 19.4%. The VWS of S1 was 1.54 g/cm<sup>3</sup> , with no significant difference between S1 and S7 (*p* > 0.05). The VWS values of S2 and S4 were 1.47 and 1.46 g/cm<sup>3</sup> , respectively, also without a significant difference (*p* > 0.05). The VWS values of S1 and S8 were lager, while those of S3 and S5 were smaller, indicating a low soil compactness of S3 and S5, which is more suitable for plant growth. The TP values of S3, S5, and S8 were relatively high with 59.12%, 58.97%, and 58.97%, respectively, while that of S4 was 53.12% and therefore smaller than the values found for S3, S5, and S8. Generally, the TP ranged between 50% and 60%, and the texture was loose, which may not be conducive to water and fertilizer conservation. *Sustainability* **2019**, *11*, x FOR PEER REVIEW 9 of 14

**Figure 5.** Variations (± SD) in volume weight of the soil and total porosity under different treatments along the expressway in Datong City, Shanxi Province, China, in 2013. VWS and TP represent volume weight of the soil and total porosity, respectively. **Figure 5.** Variations (± SD) in volume weight of the soil and total porosity under different treatments along the expressway in Datong City, Shanxi Province, China, in 2013. VWS and TP represent volume weight of the soil and total porosity, respectively.

#### *4.4. Variations in Soil and Water Content and Soil Organic Matter under Different Treatments 4.4. Variations in Soil and Water Content and Soil Organic Matter under Di*ff*erent Treatments*

The soil water content (SWC) is mainly affected by both precipitation and evaporation [39]. In our study area, SWC and SOM levels differed greatly among the different treatments, most likely because of the inherent soil characteristics. The SWC levels increased with increasing soil depth, which might be explained by the high evaporation of the surface soil, resulting in low water content. However, SOM levels decreased with increasing soil depth (Figure 6). This, owing to these surface soil layers, can easily be supplemented with organic matter. The changes in SWC among treatments ranged from 9.6% of the average SWC of S2 to 18.8% of the average SWC of S4. The average SWC of S4 was twice the average SWC of S2. The average SWC levels of S1, S3, S5, S6, S7, and S8 were 10.3%, 14.5%, 17.9%, 14.8%, 12.7%, and 16.3%, respectively, following the order S4 > S5 > S8 > S6 > S3 > S7 > S1 > S2. The average SOM level of S8 was 0.40%, which was highest than in the other treatments, but still relatively low when compared with the lowest Grade 6 of the second National Soil Census and related standards (<0.6%) [40]. The average SOM content of S6 was 0.17%, which was 57.5% lower than that of S8. The average SOM levels of S1, S2, S3, S4, S5, and S7 were 0.23%, 0.25%, 0.23%, 0.20%, 0.19%, and 0.22%, respectively, following the order S8 > S2 > S1 > S3 > S7 > S4 > S5 > S6. The nutrient contents of the treatments were relatively low, without SOM, and the soils are therefore not suitable for the growth of newly transplanted. Most shrub species, when transplanted into a new environment, require adequate SOM levels to adapt and grow, in contrast to naturally growing shrubs, which have adapted to these environments through natural selection. The soil water content (SWC) is mainly affected by both precipitation and evaporation [39]. In our study area, SWC and SOM levels differed greatly among the different treatments, most likely because of the inherent soil characteristics. The SWC levels increased with increasing soil depth, which might be explained by the high evaporation of the surface soil, resulting in low water content. However, SOM levels decreased with increasing soil depth (Figure 6). This, owing to these surface soil layers, can easily be supplemented with organic matter. The changes in SWC among treatments ranged from 9.6% of the average SWC of S2 to 18.8% of the average SWC of S4. The average SWC of S4 was twice the average SWC of S2. The average SWC levels of S1, S3, S5, S6, S7, and S8 were 10.3%, 14.5%, 17.9%, 14.8%, 12.7%, and 16.3%, respectively, following the order S4 > S5 > S8 > S6 > S3 > S7 > S1 > S2. The average SOM level of S8 was 0.40%, which was highest than in the other treatments, but still relatively low when compared with the lowest Grade 6 of the second National Soil Census and related standards (<0.6%) [40]. The average SOM content of S6 was 0.17%, which was 57.5% lower than that of S8. The average SOM levels of S1, S2, S3, S4, S5, and S7 were 0.23%, 0.25%, 0.23%, 0.20%, 0.19%, and 0.22%, respectively, following the order S8 > S2 > S1 > S3 > S7 > S4 > S5 > S6. The nutrient contents of the treatments were relatively low, without SOM, and the soils are therefore not suitable for the growth of newly transplanted. Most shrub species, when transplanted into a new environment, require adequate SOM levels to adapt and grow, in contrast to naturally growing shrubs, which have adapted to these environments through natural selection.

85

*Sustainability* **2019**, *11*, x FOR PEER REVIEW 10 of 14

*Sustainability* **2019**, *11*, x FOR PEER REVIEW 10 of 14

**Figure 6.** Variations (± SD) in soil and water content and soil organic matter under different treatments along the expressway in Datong City, Shanxi Province, China, in 2013. **a** and **b** represent the variations in soil and water content and soil organic matter, respectively. **Figure 6.** Variations (± SD) in soil and water content and soil organic matter under different treatments along the expressway in Datong City, Shanxi Province, China, in 2013. **a** and **b** represent the variations in soil and water content and soil organic matter, respectively. 80 **Figure 6.** Variations (± SD) in soil and water content and soil organic matter under different treatments along the expressway in Datong City, Shanxi Province, China, in 2013. **a** and **b** represent

the variations in soil and water content and soil organic matter, respectively.

#### *4.5. Variation in Soil Erosion under Different Treatments 4.5. Variation in Soil Erosion under Di*ff*erent Treatments*

During the observation period, the water erosion (WrE) of the different treatments varied because of the different soil properties. Throughout the study area, water erosion was greater than wind erosion (WdE), ranging between 36 and 80 t/ha, while WdE was between 7 and 24 t/ha (Figure 7). According to the classification of soil erosion intensity in China (SL190–2007), this area is moderately affected by soil erosion. The average WrE of S4 was the largest (69.70 t/ha), while that of S3 was the smallest (39.70 t/ha). The WrE of S3 was 43.03% lower than that of S4. The WE values of S2, S6, and S7 were relatively similar and reached about 50 t/ha. In S3 and S4, wind erosion was lower (8.10 and 8.63 t/ha, respectively), while at S7 and S8, it was higher (21.91 and 21.75 t/ha, respectively). The WdE values of S7 and S8 were 2.70 and 2.52 times higher than those of S3 and S4, respectively. We found no significant differences in WdE among S1, S2, S5, and S6 (*p* > 0.05). With increasing soil porosity, WrE gradually decreased; generally, these two factors are not correlated [41]. The larger porosity of the surface soil facilitated the infiltration of runoff on the slope, thereby reducing runoff on the slope and the scouring force of runoff on the surface soil [42]. At the same time, the infiltration water also increased the erosion resistance of the surface soil [43]. Although there were numerous factors affecting slope surface erosion, and the interaction among them is more complex, in general, the pore size of the surface soil plays a role in inhibiting surface erosion. During the observation period, the water erosion (WrE) of the different treatments varied because of the different soil properties. Throughout the study area, water erosion was greater than wind erosion (WdE), ranging between 36 and 80 t/ha, while WdE was between 7 and 24 t/ha (Figure 7). According to the classification of soil erosion intensity in China (SL190–2007), this area is moderately affected by soil erosion. The average WrE of S4 was the largest (69.70 t/ha), while that of S3 was the smallest (39.70 t/ha). The WrE of S3 was 43.03% lower than that of S4. The WE values of S2, S6, and S7 were relatively similar and reached about 50 t/ha. In S3 and S4, wind erosion was lower (8.10 and 8.63 t/ha, respectively), while at S7 and S8, it was higher (21.91 and 21.75 t/ha, respectively). The WdE values of S7 and S8 were 2.70 and 2.52 times higher than those of S3 and S4, respectively. We found no significant differences in WdE among S1, S2, S5, and S6 (*p* > 0.05). With increasing soil porosity, WrE gradually decreased; generally, these two factors are not correlated [41]. The larger porosity of the surface soil facilitated the infiltration of runoff on the slope, thereby reducing runoff on the slope and the scouring force of runoff on the surface soil [42]. At the same time, the infiltration water also increased the erosion resistance of the surface soil [43]. Although there were numerous factors affecting slope surface erosion, and the interaction among them is more complex, in general, the pore size of the surface soil plays a role in inhibiting surface erosion. *4.5. Variation in Soil Erosion under Different Treatments*  During the observation period, the water erosion (WrE) of the different treatments varied because of the different soil properties. Throughout the study area, water erosion was greater than wind erosion (WdE), ranging between 36 and 80 t/ha, while WdE was between 7 and 24 t/ha (Figure 7). According to the classification of soil erosion intensity in China (SL190–2007), this area is moderately affected by soil erosion. The average WrE of S4 was the largest (69.70 t/ha), while that of S3 was the smallest (39.70 t/ha). The WrE of S3 was 43.03% lower than that of S4. The WE values of S2, S6, and S7 were relatively similar and reached about 50 t/ha. In S3 and S4, wind erosion was lower (8.10 and 8.63 t/ha, respectively), while at S7 and S8, it was higher (21.91 and 21.75 t/ha, respectively). The WdE values of S7 and S8 were 2.70 and 2.52 times higher than those of S3 and S4, respectively. We found no significant differences in WdE among S1, S2, S5, and S6 (*p* > 0.05). With increasing soil porosity, WrE gradually decreased; generally, these two factors are not correlated [41]. The larger porosity of the surface soil facilitated the infiltration of runoff on the slope, thereby reducing runoff on the slope and the scouring force of runoff on the surface soil [42]. At the same time, the infiltration water also increased the erosion resistance of the surface soil [43]. Although there were numerous factors affecting slope surface erosion, and the interaction among them is more complex, in general, the pore size of the surface soil plays a role in inhibiting surface erosion.

25

Datong City, Shanxi Province, China, in 2013. **a** and **b** represent the variations in water erosion and wind erosion during the experimental period. **Figure 7.** Variation (± SD) in soil erosion amount under different treatments along the expressway in Datong City, Shanxi Province, China, in 2013. **a** and **b** represent the variations in water erosion and wind erosion during the experimental period. **Figure 7.** Variation (± SD) in soil erosion amount under different treatments along the expressway in Datong City, Shanxi Province, China, in 2013. **a** and **b** represent the variations in water erosion and wind erosion during the experimental period.

**Figure 7.** Variation (± SD) in soil erosion amount under different treatments along the expressway in
