Mixtures of soft rock and sand have been applied extensively in the Mu Us Sandy Land (MUSL, also known as the Mu Us Desert) to limit the loss of top soil by wind erosion. In this study, the efficacy of sand-fixing technology was investigated in a series of experiments. The sand-fixing effect of seven different mixtures (ratios) on soft rock and sand was evaluated in a wind tunnel. The results indicated that the MUSL soils are susceptible to wind erosion, as the textural composition of sand is dominated by coarse particles. MUSL soils dominated by silt and clay particle sizes are more resistant to wind erosion. Each soft rock and sand combination experiences severe wind erosion. However, wind erosion was significantly reduced when soft rock and sand were mixed. An increase in particle size was associated with an increase in the resistance of soft rock and sand mixtures to wind erosion. The ability to resist wind erosion was greatest when the ratio of soft rock and sand was between 1:2 and 1:5. This study provided data to support the approaches to sand-fixing commonly used at present in the MUSL. The results of this study have important practical significance for the improvement of agricultural land potential in dry sandy areas.
The Mu Us Sandy Land (MUSL) is located in the semi-arid and arid climate zone of northern China, and it also known as the Mu Us desert. It lies within an ecotone for agriculture and animal husbandry in northern China. The area is subject to wind erosion and severe desertification. Traditional farming methods exacerbate the situation by overwintering and exposing sandy land in the process. In winter and spring, when it is dry and windy, wind erosion of the surface soil layer can be severe. Based on weather data from 1990 to 2014, the annual average daily high wind velocity (>17 m/s) in MUSL occurs 10–40 days per annum, but not more than 95 days per year. Extended periods with continuous high winds, with a duration > a day account for 60–70% of the high wind days, while periods lasting 2–3 days account for 20–30%. Even longer periods with continuously high winds for 4–6 days account for approximately 5% of the high wind days. Dust storms occurred from 11 to 29 days per year. The frequent occurrence of wind and sand-related environmental hazards has been associated with land degradation and hence had a serious impact on the economic and social development of the region. It seriously restricts the development of local resources and the sustained and stable development of society and local economy. In order to prevent and control wind and sand-related hazards, a large number of agricultural, biological, and engineering related measures have been researched and implemented . Commonly implemented examples include the planting of trees and grass, flood warping land, conservation tillage, soil replacement, and the application of chemical amendments . However, many measures only aim to reduce or prevent wind erosion. Few of these interventions have been implemented in concert with utilization. Aspects of sandy land utilization that have been studied include the mixed utilization of sandy soil and loess , saline-alkali soil , coal gangue , and peat .
Soft rock is widely distributed in the MUSL . It is hard when dry and expands rapidly under wet conditions, but it retains water well and can be used as a natural water retention agent. These properties of the soft rock complement the porosity of sand. The texture of sand is uniform and the structure is loose. The water and fertilizer leakage characteristics of the soft rock and sand have significant differences, although each of the two characteristics can compensate for the other’s inherent defects. A compound soil that could provide plants with a favorable medium for growth was produced by mixing soft rock and sand [8,9,10], thereby mitigating the negative characteristics of each of the components. After years of experiment and practice , the Shaanxi provincial land engineering construction group applied soft rock and sand mixtures to land surfaces in several land renovation projects in MUSL, Yulin. By the end of 2014, the cumulative extent of treated soil was 4,647 ha, of which 4,400 ha was arable land. In addition, the use of soft rock and sand compound soil technology has been extended to more than 13,333 ha in MUSL and surrounding areas. It has effectively increased the area of arable land, supplemented the index of construction land, improved the regional ecological environment, and explored a new way for sandy lands to be managed.
The recognition of the mechanism of soil wind erosion and the practice of sand control indicate that there is a close correlation between the occurrence of wind erosion and the grain size distribution of topsoil. The grain size of the topsoil is determined by soil formation . Soil texture is an important determinant of the intensity of soil erosion, especially in the absence of vegetation. The physical and chemical properties of soil also directly affect the wind speed at which soil particles start to move. The essence of the soft rock and sand mixture approach to soil improvement in sandy lands is to improve the original textural characteristics (grain size) of the soil, with regard to susceptibility to erosion and application of fertilizer. This study has investigated the effect of the textural characteristics of soft rock and sand mixtures on sand fixing (i.e., stabilization in relation to wind erosion). Experiments were carried out on different ratios of these mixtures in an indoor wind tunnel. The data produced in this study could be used to guide the development of methods to stabilize sandy lands and enhance the associated agricultural potential.
2 Materials and methods
2.1 Study area
The research area was located in the Yuyang district, Yulin city (Latitude, 38°27′53″N to 38°28′23″N; Longitude, 109°28′58″E to 109°30′10″E), on the southern margin of the Mu Us desert and on the middle reaches of Wuding river. It is a typical semi-arid continental monsoon climate in the temperate zone. The spatial and temporal distribution of precipitation is uneven, with a dry climate and abundant sunshine. The average annual temperature is 8.1°C, the accumulated temperature of ≥10°C is 3307.5 and the duration is 168 days. The annual average frost-free period is 154 days. The average annual rainfall is approximately 413.9 mm, with 60–90% falling from June to September. There were on average 2,879 h of sunshine per annum. According to the meteorological data for the years from 1990 to 2014 (Figure 1), the average daily wind speed was approximately 9 m/s.
The soil types are mainly well-sorted aeolian sandy soils, with coarse sand grains, very few powder grains, a low particle surface activity, a low viscosity, and strong looseness (Table 1). The nutrient content is low, with an average organic matter content of 3.32 g/kg and a total nitrogen content of 0.14 g/kg. In addition, a large percentage of the soft rock in the sandy land has a low level of diagenesis, low structural strength, easily weathered, poorly cemented, and poor in permeability. In contrast, due to the high content of sticky particles, it has a favorable water holding capacity and water retention capacity. The average content of organic matter is 6.20 g/kg, while the total nitrogen content is 0.13 g/kg. As a result, the nutrient content is relatively low.
|Sample||Particle (%)||Texture (USDA)||pH||TN (g/kg)||TP (g/kg)||TK (g/kg)||SOM (g/kg)|
|Sand (0.05–2 mm)||Silt (0.002–0.05)||Clay (<0.002 mm)|
|Soft rock||24.52||64.98||10.50||Silt loam||8.27||0.13||0.59||25.09||6.20|
Note: USDA, United States Department of Agriculture; TN, total nitrogen; TP, total phosphorus; TK, total potassium; SOM, soil organic matter.
The experiment was carried out in the wind tunnel at the Institute of Soil and Water Conservation, Ministry of Water Resources, Chinese Academy of Sciences. The wind tunnel is 19.8 m long, 1.2 m high, and 1.0 m wide . It consists of five main parts, namely a power stage, an air regulation section, a rectifier section, a test section, and a sand collecting section. The size of the sample cell used in the simulation experiment was 1.25 m × 1 m × 0.15 m (length × width × height), with the wind tunnel pushed from the test section during the test (Figure 2).
The wind speed is adjusted continuously by 0–20 m/s using a frequency converter that ranged from 0 to 50 Hz. Before the formal test, the wind speed in the wind tunnel was debugged uniformly, with each blowing simulation test set to blow for 10 min. The wind erosion amount of each blowing test was collected by the tail sand-collecting device in the sand collecting section, as the total wind erosion volume of the secondary erosion simulation test. The eroded sand was then weighed by a balance to an accuracy of 0.01 g. Each blowout test would make the surface of the soil more coarsely grained. The soil sample was therefore reloaded after each blowout. In order to ensure the uniform weight of each sample, each loading was weighed with a platform scale to ensure uniform bulk density.
2.3 Experimental design
In order to make test samples more representative, the samples used in this experiment were collected from Dajihan village, Xiaojihan town, Yuyang district, Yulin city. In total seven sets of ratios of soft rock to sand were considered, namely 1:0, 5:1, 2:1, 1:1, 1:2, 1:5, and 0:1. Physical and chemical properties of soil, such as water content, organic matter content, bulk density, and particle size composition, significantly affect wind erosion. In order to simply consider the effect of the proportions on wind erosion, two kinds of samples were first treated by air drying to ensure that their respective water content was less than 2% and that the organic matter content of the soft rock (0.78 g/kg) and sand (3.32 g/kg) was very low. The effects of water content and organic matter on wind erosion were therefore not considered any further in the study. Consequently, it was assumed that only the effect of particle size distribution on wind erosion was studied in the wind tunnel. The aeolian sandy soil was relatively homogeneous, with a particle size below 2 mm. In contrast, the particle size of the soft rock had a significant influence on the wind tunnel test. As a result, 2, 8, and 20 mm were selected for assessment in the experiments. The soft rock with different diameters was mixed evenly with the sand for each treatment and subsequently replicated three times in the experiments. The sand grains in the MUSL are exposed to wind speeds of generally around 6 m/s, with a maximum daily mean wind speed of 9 m/s. Therefore, three wind speeds (viz. 7, 9, and 11 m/s) were selected for investigation in this study.
3.1 Grain size composition characteristics of different proportions of soft rock and soil
The particle size distributions of different proportions of soft rock and sand are shown in Figure 3. The grain size distributions of different proportions of soft rock and sand mixtures were different from the grain size distributions for aeolian sand and soft rock. The aeolian sand was well sorted with grain sizes concentrated in the sand grain segments (0.05–1 mm). In contrast, the grain size distributions of the soft rock were concentrated in the powder particle size and clay fraction (0.01–0.05 mm), with a size range larger than the size range of each soft rock and sand combination. The mixing of the two components thus changed the limitations of their grain size composition, with regard to susceptibility to wind erosion. The frequency distribution of their grain sizes can be divided into two distinct parts. In the finer fraction, with an increase in the proportion of soft rock in the mixture, the grain size of the smaller particle sizes increases gradually. In the coarser grain size fraction, the particle size gradually decreased, as the proportion of soft rock in the mixture increased. In general, as the proportion of soft rock in the mixture increased, the grain size distribution of the mixture changed in a finer direction. In terms of texture, the key grain size component (powder and clay) of the structure in the mixture is gradually improved, as is the coarse sand content. The soil texture changes from sandy loam to sandy soil and eventually to a loam soil. The mixture can therefore meet the need for crop root ventilation, improve water retention, provide a fertilizer protection effect, and gradually improve the ability to resist both wind and water erosion. The addition of soft rock to sand is of great significance in strengthening the sand-fixing ability of soil in MUSL.
3.2 Wind erosion of different proportions of soft rock and soil
3.2.1 Influence of wind force on wind erosion of soft rock and sand compound soil
Figure 4 shows the intensity of wind erosion in relation to wind velocity for seven ratios of soft rock to sand mixtures and three different soft rock grain size categories (viz. 2, 8, and 20 mm). In general, the higher the wind velocity, the greater the intensity of wind erosion on a soil surface. One departure from this relationship was when the soft rock (2 mm) and sand mixture has a ratio of 1:0 (i.e., erosion at 7 m/s > 9 m/s > 11 m/s). When soft rock size categories of 2 and 20 mm were used, there was a notable drop in the sediment loss, when the soft rock was added to sand (i.e., ratio of 1:5). This effect was largely lost when ratio of soft rock (2 mm) to sand mixture was 5:1. Perhaps counter to expectations the soft rock (2 mm) on its own was more susceptible to erosion at low wind velocities than at higher ones. The 8 mm soft rock category also reduced wind erosion when used in a mixture, but the benefits were generally less if the other two categories (i.e., 2 and 20 mm) were used. The reasons for the abnormal wind erosion of the soil mixture with a large particle size of 2 mm have been discussed further below. In terms of wind and sand dynamics, the finer particles are more easily entrained by the wind. Gillette  found that the particle size distribution of a land surface plays an important role in the onset velocity of wind erosion (Table 2).
|Ratio||Soft rock: 2 mm||Soft rock: 8 mm||Soft rock: 20 mm|
|7 m/s||9 m/s||11 m/s||7 m/s||9 m/s||11 m/s||7 m/s||9 m/s||11 m/s|
|0:1||13.54 ± 3.26||43.34 ± 5.59||54.06 ± 0.53||13.54 ± 3.26||43.34 ± 5.59||54.06 ± 0.53||13.54 ± 3.26||43.34 ± 5.59||54.06 ± 0.53|
|1:5||0.13 ± 0.02||0.94 ± 0.10||3.17 ± 0.93||17.61 ± 6.01||41.19 ± 7.59||46.24 ± 8.85||0.18 ± 0.05||0.83 ± 0.16||1.13 ± 0.08|
|1:2||0.49 ± 0.12||0.82 ± 0.14||1.38 ± 0.16||11.93 ± 0.10||19.31 ± 0.66||32.22 ± 1.66||0.11 ± 0.02||0.62 ± 0.19||0.75 ± 0.23|
|1:1||0.34 ± 0.05||0.38 ± 0.08||3.80 ± 0.80||4.11 ± 1.24||12.48 ± 5.61||25.15 ± 5.84||0.00 ± 0.00||0.35 ± 0.09||1.10 ± 0.21|
|2:1||0.73 ± 0.26||2.76 ± 0.55||7.85 ± 0.92||0.83 ± 0.07||4.19 ± 1.29||7.57 ± 2.92||0.00 ± 0.00||0.12 ± 0.03||0.29 ± 0.11|
|5:1||0.94 ± 0.08||3.74 ± 0.57||13.11 ± 0.49||2.51 ± 1.02||11.38 ± 4.96||10.93 ± 2.60||0.00 ± 0.00||0.09 ± 0.01||0.30 ± 0.06|
|1:0||37.25 ± 2.33||28.79 ± 1.37||22.30 ± 0.78||4.85 ± 0.36||12.50 ± 5.93||22.26 ± 0.88||0.00 ± 0.00||0.00 ± 0.00||0.00 ± 0.00|
From the analysis above, we know that the grain size of the soft rock is mainly concentrated in the fine powder and clay segments. Fine granular soft rock is therefore easy to move with the wind. In the wind tunnel test, there is a distinct transition for the sandy soil at a wind speed of 6 m/s, as most of the particles of the sifted 2 mm sandstone are very fine. It has a distinct grain transition at a wind speed of approximately 4 m/s, with notable wind erosion evident. As the wind speed increased, the surface of the sample trough became rougher, as the fine particles are removed by the wind (Figure 5). Even with notable erosion in the sample trough, there was no increase in the collection of tail sand in the collection plate. The main reason is that the wind speed at the outlet of the sand collection section is high, while the distance to the sand plate is small. High wind velocities at the outlet of the sand collection section were associated with a notable loss of fine sediment at the sand collection plate, which caused the wind erosion measurements to deviate greatly. Li and Shen  also pointed out that a sand sampler in the sand collecting section would lead to large deviation and low reliability of sand flow measurements.
3.2.2 Effect of the particle size of soft rock on wind erosion of a mixed sediment sample
It can be seen in Figure 4 that the particle sizes of the samples of the soft rock and the sandy soil have a significant effect on the amount of wind erosion. From the view of soil mineral particles, the particles larger than 2 mm are called gravel. Gravel cover is an effective sand fixing measure [16,17,18]. Therefore, the sample size of 8 and 20 mm is equivalent to the stabilization of gravel-covered sand. The amount of wind erosion at different soft rock and sand ratios with a grain size of 20 mm, under three kinds of wind speed, is almost zero. This indicated that the particle size of the soft rock increases the surface roughness, absorbs and decomposes the surface wind movement, and reduces the shear force [19,20] on the eroded bed surface. Moreover, the soft rock covered the surface layer of aeolian sand, which reduced the area of sand available for entrainment [16,21,22,23] on the surface. Therefore, the soft rock is not blown away by the wind, and at the same time, it can protect the aeolian sand from wind erosion. In a land management context, the larger particle size soft rock (20 mm) and sand compound soil would be particularly useful for managing wind erosion in MUSL in winter and spring, if applied prior to the onset of these seasons. The use of a rotary tiller during crop planting would ensure the soils are evenly mixed, thereby improving water conservation and reducing the demand for fertilization of the tilled layer.
There are two explanations for the differences in the patterns of wind erosion recorded for soft rock with a particle size of 8 mm and soft rock with a particle size of 2 mm at three wind speeds. First, the error in the collection of 2 mm tail sand is larger than the size of the particle size mentioned above, while the amount of wind erosion at the three wind speeds is underestimated at different soft rock (2 mm) and sand ratios. Second, although the sandstone with a particle size of 8 mm could be assigned to the gravel particle size class, it does not provide the same protective effect as a gravel surface cover. A possible reason for this could be the presence of grain sizes >8 mm in the soft rock (8 mm) sample that have a low density and that are of poor quality. At the same wind speed, compared to the grain size, the sand particles may be more easily blown by wind erosion. Therefore, in practice, the crushed soil particle size should not be too fine, otherwise it may lead to more severe wind erosion than the original aeolian sand. When the soft rock and sand mixture is applied in agriculture, it will add the cumulative effect of other factors that increase resistance to wind erosion. These additional parameters include the organic matter content of the soil, the cementation of the clay particles, and the cementation of the sand particles in the soft rock.
3.2.3 Effect of texture on wind erosion of soft rock and sand compound soil
The textural characteristics of soil are highly variable in nature. The texture and specific gravity of soil are important determinants of the susceptibility of a soil to wind erosion . Different compound soils of soft rock and sand have different textural characteristics and hence variable moderating effects on wind erosion . In order to illustrate the effect of particle size distribution on wind erosion, we have considered wind erosion at different wind speeds, when the soft rock particle size is 2 mm. Dong and Li  found that the main resistance to wind erosion in loose aeolian sand is the inertia of a grain and the strength of cohesion between grains. When particle size is <0.09 mm, the inter particle strength of cohesion predominates, which means the wind speed required for entrainment increases with the decrease in particle size. In contrast, when particle size is >0.09 mm, inertia is the main determinant of the wind speed required for entrainment. It can be seen from Figure 5 that the wind erosion of pure aeolian sandy soil is relatively serious under different wind speeds. More than 90% of the aeolian sandy soil has a grain size >0.09 mm. In this size class, the water holding capacity of particles is poor and agglomeration is weak. As inertia is the dominant factor for the larger grain sizes (viz. >0.09 mm), the resistance to wind erosion is relatively low. Although the content of soft clay particles is high, the cohesive strength they provide in this experiment is very low . Although the wind erosion is less than that of the aeolian sandy soil, it still has a great influence on the results. After mixing the soft rock and sand, the grain size of the two components was modified and the resistance to wind erosion was enhanced.
3.2.4 The particle size distribution and erosion resistance of the tail sand
From the analysis of the particle size distribution of the tail sand in Figure 6, it can be seen that the eroded particles are mainly concentrated in two grain size classes, namely 0.01–0.05 and 0.25–1 mm. The two size classes are the same size as the soft rock and aeolian soil, so the measures to mitigate wind erosion should focus on these two particle sizes. Neglecting the error associated with the collection of tail sand by the sand collector mentioned above, the erosion of the mixtures with ratios from 1:5 to 1:2 was significantly reduced, due to the single particle structure (i.e., limited adhesion) of the sandy soil. After adding soft rock, the clay particles in soft rock filled the gaps between the sand particles. The increase in the number of clay particles changed the wind erosion resistance force, which had been dominated by inertia up to that point, into a wind erosion resistance force progressively dominated by the forces of cohesion. The main factor determining resistance to wind erosion would then gradually change to dependence on the cohesive strength obtained from the fine-grained sediment weathered from the soft rock. As there is a high content of sand in this mixture, a small amount of sticky particles can efficiently fill the gaps between the sand particles, with indirect contact between the particles tighter.
After wind erosion of the loose particles between the surface particles, the specific gravity of the surface is increased, the roughness of the surface of the soil is increased, and the fine particles are protected from further erosion. The greater the use of the mixture, the lower the volume of material that will be eroded . From the observation of the wind erosion trough, it can be seen that the surface soil is only coarse grained at a wind speed of 7 m/s, the surface soil is coarse grained at a wind speed of 9 m/s, and the lower layer soil is still stronger in resisting wind erosion even if the particles are fine. Only at a wind speed of 11 m/s will both the top surface and the lower soil be severely eroded. The differences in wind erosion characteristics of mixtures in two ranges (viz. soft rock: sand, 1:1–5:1 vs 1:5–1:2) could be attributed to the infilling of interstitial spaces by sticky particles. The strength of cohesion between particles is less and relatively weak. This shows that in the case of dry soil, the high content of clay particles in the grain size of the soil may not enhance the cohesive strength and resistance to wind erosion, as the “pulling effect” of water molecules among the soil particles will be absent. Therefore, only when the particle size distribution is reasonable at all levels can the ability to resist wind erosion be enhanced.
As the main particle size of the soft rock was silt and clay, erosion of the rock by raindrop splash and irrigation leads to the loss of clay and silt size particles which plug the gaps between the sand grains [29,30]. As a result, a soil crust 2–6 mm thick can form on the surface of the compound soil. This crust can significantly increase the wind speed required for entrainment of grains from the land surface, and hence effectively prevent wind erosion. For uncultivated soil, this crust will gradually form soil biological crust with the improvement of ecological environment, which has a significant effect on wind erosion resistance . For cultivated soil, the surface coverings during crop growth significantly improved the ability to resist wind erosion. After crop harvest, soil crusts can easily form on the surface of the compound soils. Therefore, sand fixation can be achieved in both the growing and fallow periods of crops . The field tests showed that high wind velocities (Category 6, speed = 12 m/s) would not lead to the loss of particles from the surface of the compound soil. In addition, there is a strong water-retaining capacity of soft rock [29,30]. In winter, the compound soil of soft rock and sand will freeze and fix the wind-erosion particles from the surface layer, which can significantly reduce the wind erosion in windy weather in winter . Therefore, from the perspective of the grain size of soft rock and sand compound soil compared to the sand soil, it had the ability to resist wind erosion with soil crusting, frozen cover. Soft rock and sand compound soil technology can guarantee the ability to resist wind erosion, the technology in the local promotion does not lead to more serious wind erosion and deterioration of the local ecological environment.
This study investigated the resistance of soft rock and sand mixtures to wind erosion. Seven mixtures (ratios) of soft rock and sand were used in the wind tunnel experiments. The results showed that in the absence of organic matter and water, particle size could determine the resistance of a soft rock and soil mixture to wind erosion. The grain size composition of the aeolian sandy soil in the MUSL was dominated by the 0.05 and 1 mm size fractions. The texture was loose, with weak interaction between particles. The soils were therefore susceptible to wind erosion. In the range of 0.01–0.05 mm, the particle size of the soft rock is high, and the cohesive force between particles is dominant. The soft rock is therefore much less susceptible to erosion than the sandy soils. The amount of wind erosion of single soft rock and sand is larger, while the wind erosion amount of these two is reduced significantly. Composite soils had a higher resistance wind erosion than sandy soils on their own. The ratios of soft rock to sand that provided the greatest level of protection from wind erosion ranged from 1:5 to 1:2. The results of our research can therefore be used to identify low cost, practical management measures, to protect sandy soils in ecologically vulnerable areas.
This work was supported by the Natural Science Basic Research Program of Shaanxi Province (2021JZ-57), the National Natural Science Foundation of China (No. 51679188), and Xi’an University of Technology Doctoral Dissertation Innovation Fund (310-252072018).
Author contributions: Huanyuan Wang and Wei Tong: writing – original draft, methods, formal analysis; Jinbao Liu and Jichang Han: formal analysis, visualization, project administration; Siqi Liu: resources.
Conflict of interest: The authors state that they have no conflict of interest.
Data availability statement: The datasets produced for this study are available from the corresponding author on reasonable request.
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