Feldspathic sandstone as an emerging soil stabilizer for aeolian sand in the Mu Us Sandy Land: insights into particle size distribution

Study area

In 2012, we initiated the engineering construction and established an experimental field at the sandy land consolidation project site in Dajihan Village, Xiaogihan Township, Yulin District, Yulin City, northern Shaanxi Province. The study area (109°28′58″–109°30′10″E, 38°27′53″–38°28′23″N; Fig. 1) is located on the southern edge of the Mu Us Sandy Land and in the middle reaches of the Wuding River basin, with an elevation of 1206–1215 m. This area belongs to a typical mid-temperate semi-arid continental monsoon climate zone characterized by four distinct seasons. It has a long winter and a short summer, with a windy and dry spring and a cool and humid autumn (Zhang et al., 2024).

The mean annual temperature of the study area is 8.1 °C and its ≥10 °C cumulative temperature is 3307.5 °C. The lasting days of ≥10 °C cumulative temperature and mean annual frost-free period are 168 and 154d, respectively. The annual precipitation varies from 250 to 440 mm (mean: 413.9 mm), and 60.9% of the precipitation is concentrated in July–September with rainy and hot weather. The extreme annual precipitation has a maximum of 695.4 mm and a minimum of 159.6 mm, and the maximum daily precipitation reaches 141.7 mm. The mean annual sunshine duration is 2879 h and the percentage of sunshine is 65% per year. The total annual radiation is 606.94 kJ/cm² and the dryness is between 1.0–2.5. Sand-driving wind with speeds >5 m/s occurs 220–580 times per year (The height at which the wind speed has been recorded was 11 meter), and sand dunes are <10 m in height (Sun, Ma & Cao, 1995).

Experimental design and setup

We adopted an engineering approach (Fig. 2) for soil stabilization (the process flow includes land leveling, roll compaction, paving feldspathic sandstone, soaking feldspathic sandstone, mixing and roll compaction) and plot construction with five different ratios of feldspathic sandstone to aeolian sand (m_f:m_s = 1:0, 1:1, 1:2, 1:5, and 0:1, where m_f is the mass of feldspathic sandstone and m_s is the mass of aeolian sand). Before mixing feldspathic sandstone and aeolian sand, feldspathic sandstone needs to be transported and broken, which means that feldspathic sandstone is broken into pieces ≤4 cm by hand or hammer and kept for mixing. The depths to which feldspathic sandstone was added ranged from 8 to 12 centimetre. Each treatment was applied to one experimental plot (20 m long × 7 m wide) with three replications, and a total of 15 plots were set up.

In the northern Shaanxi region of China, the climate is semi-arid climate, four distinct seasons, long light time, suitable temperature, moderate annual precipitation. This climatic condition makes northern Shaanxi suitable for growing potatoes, because potatoes need sufficient light and temperature to grow, and potatoes also need a certain amount of rainfall. These climatic conditions can be met in northern Shaanxi, so the natural environment for growing potatoes is very superior. Moreover, potatoes are popular in the local market and the market demand is large. So from early April 2013, potato (Solanum tuberosum L. cv. Longshu-7) was grown in the experimental field with a row spacing of 70 cm, a plant spacing of 25 cm, and one cropping season per year. Potato yield of the 10th season was estimated after harvest at the end of September 2023. At the time of harvest, surface soil samples (0–20 cm depth) were collected from five random points in each plot. The soil samples were air-dried, ground, and sieved through a 2-mm mesh before the analysis of particle size and the determination of physicochemical and mechanical properties.

To verify the soil improvement after stabilization, we randomly collected five loessial soil samples (0–20 cm depth) in potato field adjacent to the project site based on the soil type map of China from the second national soil census (2023). The physicochemical and mechanical properties of loessial soil samples were analyzed and compared with the properties of stabilized soil samples.

Sample analysis

Grain size of the experimental soils was analyzed by laser diffraction using the Mastersizer 3000 laser particle size analyzer (Mastersizer 3000; Malvern Instruments Ltd., Worcesteshire, UK). The range of grain-size distribution was determined based on the Chinese system of grain size fractionation (Huang, 2005), and soil mechanical distribution was analyzed based on the USDA soil texture ternary diagram (USDA; Huang, 2005). The sample dosage was 0.5 g each time, and three replicates were performed for each treatment.

Water-stable aggregates were determined using the wet-sieving technique (Liu et al., 2016). Capillary porosity was calculated from soil water content (measured by oven drying at 105 °C for 12 h) and bulk density (measured using a 100-cm³ cutting ring) (Huang, 2005). The determination of organic matter content and cation exchange capacity was based on oil bath heating–K₂Cr₂O₇ volumetric method and ammonium acetate extraction–Kjeldahl distillation method, respectively (Huang, 2005). Field capacity measurements were conducted using a 100-cm³ cutting ring (Wilcox, 1962). A four-point pattern hydraulic conductivity meter (Japan, DIK-4012) was used to measure saturated hydraulic conductivity, the oven-drying method (the SOP number is HJ 613-2011)was employed to determine effective water content, and a triaxial apparatus was used to measure the cohesion of particles and the angle of internal friction (Huang, 2005). The aluminum box with the lid open (the lid is placed on the side of the aluminum box or the lid is placed flat under the box), and the oven is heated at 105 °C ± 2 °C for 8–12 h. When the oven temperature drops to about 50 °C, cover the lid, place the aluminum box in the dryer for about 30 min, cool to room temperature, and weigh. Then, open the lid of the box, and bake for 4 h, cool and weigh until the difference between the two weighing is not more than 1%.

Data analysis

To evaluate soil gradation, the coefficient of uniformity (C_u) and the coefficient of curvature (C_c) were calculated (Gong, Wang & Zhou, 2002). C_u measures the uniformity of soil particles ( C_u = d ₆₀/d ₁₀, where d_i are the particle size corresponding to the cumulative mass fraction of i%). In principle, C_u is greater than 1 and a value closer to 1 indicates that the soil sample is more uniform; a soil with C_u <5 is considered uniform and poorly graded. The greater the C_u value, the broader the particle size distribution; a soil with C_u >10 is considered well-graded. However, if the C_u value is too large (generally >100, with difference in the order of magnitude), it means intermediate particle sizes may be absent, and the soil is gap graded, which means that whether the soil contains particles in various particle size ranges, if all, the soil gradation is continuous; if the soil particles in some particle size ranges are missing, the soil gradation is discontinuous and the soil is gap graded. This necessitates the use of C_c, which describes slope continuity in the cumulative particle gradation curve (C_c = d ₃₀ × d ₃₀/(d ₆₀ ×d ₁₀)), to evaluate soil gradation characteristics.

The experimental data are presented as means ±standard deviation (n = 3). SigmaPlot v13 (Systat Software Inc., San Jose, CA, USA) was used for graph construction. SPSS Statistics v18.0 (SPSS Inc., Chicago, IL, USA) was used for one-way analysis of variance followed by the Duncan’s new multiple range test. A P-value of less than 0.05 was considered to indicate statistical significance.

Soil textural characteristics

The distribution of particles across different size ranges in soil samples with various ratios of feldspathic sandstone to aeolian sand is shown in Fig. 3. In the feldspathic sandstone (m_f:m_s = 1:0), coarse silt (0.01–0.05 mm) accounted for the highest proportion (by mass), followed by fine silt (0.002–0.005 mm) and medium silt (0.01–0.005 mm) in comparable proportions. The feldspathic sandstone had relatively low contents of coarse clay (0.001–0.002 mm) and fine clay (<0.001 mm), with almost no particles >0.25 mm. In the aeolian sand ( m_f:m_s = 0:1), coarse sand (0.25–1 mm) was the most abundant fraction, followed by fine sand (0.05–0.25 mm). The aeolian sand had low silt content (0.05–0.002 mm, 4.05%) and almost no clay content (<0.002 mm, <1%).

In the stabilized soils containing both feldspathic sandstone and aeolian sand ( m_f:m_s = 1:1, 1:2, and 1:5), coarse sand (0.25–1 mm) comprised the largest proportion, followed by fine sand (0.05–0.25 mm) and coarse silt (0.0–0.05 mm). Taking the particle size of 0.05 mm as a boundary (which divides sand and silt fractions in the USDA soil texture), the content of particles with a size <0.05 mm in the stabilized soils exhibited a upward trend with increasing addition ratio of feldspathic sandstone (m_f:m_s = 1:0 >1:1 >1:2 >1:5 >0:1), whereas the opposite pattern was observed for the content of particles with a size >0.05 mm (m_f:m_s = 1:0 <1:1 <1:2 <1:5 <0:1). This result was attributable to the notably high proportions of >0.05 mm particles in the aeolian sand and <0.05 mm particles in the feldspathic sandstone (Fig. 3).

Table 1 provides the granulometric composition and texture of various soil samples according to the USDA textural soil classification system. With increasing addition mass ratio of feldspathic sandstone, the content of silt and clayincreased in a linear pattern. The relation between feldspathic sandstone and silt contents can be expressed as: y = 69.04x + 10.41, R ² = 0.9630; the relation between feldspathic sandstone and clay contents can be expressed as: y = 13.37x + 2.42, R ² = 0.8873 (where y is the mass fraction of soil particles, %; and x is the mass fraction of feldspathic sandstone, %). The soil texture also transitioned with increasing addition ratio of feldspathic sandstone (sand–sandy loam–loam–silty loam).

Soil gradation characteristics

The cumulative and frequency distributions of particle size distribution in various soil samples are shown in Fig. 4. The feldspathic sandstone ( m_f:m_s = 1:0) had a broad particle size distribution, with no distinct peak in the frequency distribution curve. Its cumulative particle size distribution curve also showed no steep slope, representing a polydisperse curve. These results are indicative of low particle size uniformity in the feldspathic sandstone with no dominant particle size fractions. The aeolian sand was overall coarse-grained, with particle sizes mainly ranging from 0.05 to one mm and showing a narrow peak in the frequency distribution curve. Its cumulative particle size distribution curve also showed a steep slope, representing a monodisperse curve. These results reflect that the aeolian sand had high particle size uniformity and good sorting property.

As for the stabilized soils with feldspathic sandstone and aeolian sand mixed at three different ratios (m_f:m_s = 1:1, 1:2, and 1:5), their frequency particle size distribution curves were divided into two portions by the particle size of 0.05 mm (Fig. 4). The content of relatively fine particles <0.05 mm increased with increasing addition ratio of feldspathic sandstone, where as the content of relatively coarse particles >0.05 mm exhibited the opposite trend. The cumulative particle size distribution curves all showed clear trend turning, which typifies polydisperse curves. However, with increasing addition ratio of feldspathic sandstone, the cumulative particle size distribution curves showed greater difference compared with the monodisperse curve of aeolian sand and shifted towards the polydisperse curve of feldspathic sandstone. This means that the uniform particle size distribution of aeolian sand was improved upon addition of feldspathic sandstone. The particle size distribution of stabilized soils showed the mixing of coarse and fine particles with broadened distribution of particle sizes.

The particle gradation parameters of soil samples with various ratios of feldspathic sandstone to aeolian sand (Table 2) were obtained based on data from laser diffraction analysis and Fig. 4. With increasing addition ratio of feldspathic sandstone, the volume average particle sizes of soils, as well as the particle sizes corresponding to various cumulative mass fractions, exhibited a downward trend. This means that the coarse-grained condition of aeolian sand was improved by adding feldspathic sandstone, and the soil particle size distribution was changed in a finer direction. The coarse grain which means 0.05–2 mm is decreasing, fine grain which means <0.05 mm is increasing. Based on the d_i values of feldspathic sandstone (Table 2), the particle sizes were relatively fine and mainly concentrated in the silt and clay fractions. The feldspathic sandstone had C_u value of 12.07 and C_c value of 1.01. Evidence suggests that when the two conditions C_u >10 and C_c = 1–3 are satisfied simultaneously, the samples are well graded soils (Xu et al., 2022). In summary, the feldspathic sandstone had a broad particle size distribution and continuous gradation, making it possible to serve as a soil stabilizer for aeolian sand.

Based on the d_i values of aeolian sand (Table 2), the particle sizes were relatively coarse and mainly concentrated in the sand fraction. The aeolian sand had C_u value of 3.32 and C_c value of 1.21. Accordingly, the aeolian sand had continuous gradation with a narrow particle size distribution, and the soil was uniformly graded or poorly graded, consistent with the results presented in Fig. 4. Therefore, despite its continuous particle gradation, the aeolian sand had a too low C_u value, a narrow particle size distribution, and lack of particles in the small size fractions, so it was classified as poorly graded soil.

Based on calculations, the C_u and C_c values of stabilized soils were respectively 76.21 and 1.12 at m_f:m_s = 1:2, and 54.71 and 2.54 at m_f:m_s = 1:5. In both cases, the C_u values were greater than 10 and the C_c values ranged between 1–3. Therefore, the two stabilized soils showed good particle gradation at m_f:m_s = 1:2 or 1:5. Such soil samples had mixed composition of coarse and fine particles (Li, Rao & Xu, 2022; Li, Zhang & Yu, 2022). Furthermore, the stabilized soils had much larger C_u values than feldspathic sandstone and aeolian sand (Table 2). This provides evidence that the addition of feldspathic sandstone resolved the textural defect of aeolian sand in terms of particle size uniformity, leading to superior particle size distribution and soil gradation.

Soil physicochemical and mechanical properties

Considering the changes in soil particle size distribution after adding feldspathic sandstone to aeolian sand, we selected m_f:m_s = 1:5 to verify the improvements in soil physicochemical and mechanical properties and crop yield (Table 3). The addition of feldspathic sandstone changed the soil texture from sand to sandy loam after 10 seasons of potato cropping. Compared with the sandy soil, the stabilized soil showed superior properties (e.g., texture, water-stable aggregates, organic matter content, cation exchange capacity) close to those of loessial soil. The potato yield of stabilized soil more than doubled that of sandy soil and reached a similar level to that of loessial soil. This indicates that following the addition of feldspathic sandstone to aeolian sand, the major soil properties and crop yield were comparable to those of loessial soil with a light loamy texture.