Using soil minerals to investigate desert expansion in northern Shaanxi Province, China

Aeolian Research 43 (2020) 100577

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Using soil minerals to investigate desert expansion in northern Shaanxi Province, China ⁎

Yanbing Qia, , Tao Chena,1, Manoj K. Shuklab, Qingrui Changa,

T

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a

College of Natural Resources and Environment, Northwest A&F University, Yangling, Shaanxi 712100 PR China Department of Plant and Environmental Science, College of Agricultural, Consumer and Environmental Sciences, New Mexico State University, Las Cruces, NM 88003, USA

b

A R T I C LE I N FO

A B S T R A C T

Keywords: Soil minerals Desert expansion Dune sand migration Loess decline Stabilized dune reactivation

The transformation of fixed dunes into shifting dunes and the diverse feedback loop that drives the transformation process has been observed in various dune systems around the world. However, the precise details of how environmental controls influence the dune transformation and stabilisation remain poorly understood. Comprehensive understanding of processes and mechanisms behind newly formed shifting dunes is essential to identify niche targeting measures for desert expansion control. On the basis of measured soil minerals from expanded desert land in northern Shaanxi province, China, desert expansion mechanisms identified were associated with dune sand migration, loess decline, and stabilized dune reactivation. The dune sand migrationassociated desertification occurred when a horizon of shifting sand dunes was deposited on top of the loess soil of degraded farmland due to the prevailing windy conditions in this region, and it can be identified by the significant difference in soil minerals (SiO2, Al2O3, Fe2O3, CaO, MgO, TiO2, MnO, K2O, and P2O5) between the sand dune and the loess soil. Loess decline-associated desertification mainly occurred in farmlands in the southern part of the desert area by wind erosion, and it can be identified by the nonsignificant difference in soil minerals between the sand dune and the loess soil. Stabilized dune reactivation-associated desertification occurred due to damage to the soil biological crusts by human activities, and it can be identified by the nonsignificant difference in soil minerals between the sand dune and the stabilized dune. The dominant causes of desert expansion identified were heavy winds and intensive human activities.

1. Introduction Land degradation due to desert expansion not only reduces the total soil C pool and transfer of C from soil to the atmosphere (Lal, 2001) but also results in a severe reduction of global food security (Yan and Baas, 2015). Intensive human activities have exacerbated the aeolian activity and desertification, producing more mobile dunes than are currently stabilized by vegetation, and predicted future increases in temperature and drought severity could make the situation even worse (Forman et al., 1992; Muhs and Maat, 1993; Le Houérou, 1996; Lancaster, 1997; Thomas and Leason, 2005; Sun et al., 2006; Ashkenazy et al., 2012). In the past decades, desert expansion and its persistence have aroused great concerns in a number of regions around the world, such as the Great Plains in Canada and America (Wolfe and Hugenholtz, 2009; Barchyn and Hugenholtz, 2012), the Mediterranean Coast of Israel (Tsoar and Blumberg, 2002; Ardon et al., 2009), and the White Sands in

New Mexico (Reitz et al., 2010). In terms of the process and mechanisms of sand dune movement, Baas (2007), Barchyn and Hugenholtz (2012), and Yan and Baas (2017) have proposed the dune activation model of barchan to parabolic dunes through field observation, combined with environmental controls, morphodynamic processes, and ecogeomorphic interactions. Many studies have investigated regional desert delineation (Huang et al., 2015; Jafari and Bakhshandehmehr, 2016), expansion (Varghese and Singh, 2016), factors responsible for expansion, and control measures (Ge et al., 2015) based on interpreting desert expansion using remote sensing images (Zhao et al., 2005; Yang et al., 2007; Liu et al., 2008; Yan et al., 2009; Huang et al., 2009). The consensus is that, although there are various processes and mechanisms of desert expansion in the arid areas of the world (Hou et al., 2012; Spinoni et al., 2015), heavy wind-trigged dune migration is the dominant process of desert expansion. Nevertheless, precise details need to be identified when

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Corresponding authors. E-mail address: [email protected] (Y. Qi). 1 The first two authors contributed equally to this paper. https://doi.org/10.1016/j.aeolia.2020.100577 Received 7 July 2019; Received in revised form 29 January 2020; Accepted 7 February 2020 1875-9637/ © 2020 Elsevier B.V. All rights reserved.

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accounting for 26.82% of the total land area of the study area. About 34.64% of this area was classified as severely degraded land, 35.80% as moderately degraded, and 29.56% as slightly degraded (Fig. 1). Only loess soil areas were under pasture or agricultural land, and were in various stages of degradation.

facing expanded mobile dunes in order to implement control measures (Crouvi et al., 2010; Qi et al., 2015; Sterk et al., 2016). In addition, many of the proposed models are not able to explain how and why shifting dunes can be transformed from various landforms (Barchyn and Hugenholtz, 2012). Therefore, comprehensive research and in situ observation of desert expansion mechanisms can be beneficial to identify control measures. In most desert landscapes, the dominant minerals in soils are SiO2, Al2O3, Fe2O3, CaO, MgO, TiO2, MnO, K2O, and P2O5 (Batista et al., 2017). Detecting soil mineral composition can explain weathering intensity of soil, reveal the migration of and changes in soil, and clarify the evolution of chemical properties of soil during development (Yang et al., 2013). Parent material, climate, topography, and biological and human activities significantly affect the composition of soil minerals over time (Han et al., 2011; Lybrand and Rasmussen, 2018). Changes in mineral composition would affect soil properties significantly (Farpoor and Krouse, 2008). Comparing soil mineral contents within the same particle size among soil profiles can determine whether soils are the same or different. Applying this hypothesis to desert areas, we can identify processes and explain expansion mechanisms based on easily measurable soil minerals and field surveys. Abundant sand is generally considered the precondition for aeolian desertification (Hu et al., 2015; Ebeling et al., 2016). However, silt is the dominant aeolian material in China’s loess plateau. Geologically, the northern Shaanxi province was undersea before the Tertiary period; the sea experienced several uplift and subsidence movements during the Tertiary period and changed into a plateau. It became more and more cold and dry when entering the Quaternary period, when deep deposits of loess accumulated throughout the plateau and resulted in the formation of a loess plateau (Wang et al., 2008). These deposits are predominantly silt that has been deposited by windstorms over the ages (Zhao, 2000). This area was enormously important to Chinese history because it formed one of the early cradles of Chinese civilization, and its eroded silt—along with repeated and massively destructive floods of the Yellow River—is responsible for the great fertility of the North China Plain (Wang et al., 2008). But deforestation and overgrazing in recent centuries have caused degradation in the area (Qi et al., 2012). China’s northern Shaanxi province lies in an agro-pastoral transitional zone, which is located at the north of the loess plateau area, with less precipitation, strong evaporation, and lower vegetation coverage, as well as increased human activities in recent years to explore mineral resources (Qi et al., 2012). In this area, observable, newly formed mobile dunes can be transformed from fixed dunes or degraded farmland, or the migration of sand dunes. However, none of the studies we know have attempted to identify the original landforms that the mobile dunes transformed from, and few sutdes have deduced and observed the processes and mechanisms behind desert expansion. The objectives of this research were to identify types, processes, and mechanisms of desert expansion by analyzing soil minerals at different depths but within the same particle size ranges from expanded desert land.

2.2. Sample collection and analysis Based on the interpretation of remote sensing data, Qi et al. (2012) reported that the southwest part of the study area has experienced desert expansion between 1986 and 2008. Therefore, for this study, all sampling sites were located in the expanded desert area delineated in Qi et al. (2012). In addition, a field survey from local residents was also conducted before sampling locations were identified. Prior to the soil sampling, two 2 m × 2 m patches under each sample site were established. A soil pit was dug in each patch, and soil genetic horizons were identified by observing color, structure, and texture (by feel method). Soil samples were collected from two profiles at each sampling site, one located on a bare, shifting dune and the other on a contrast site. The contrast sites were selected based on the environments surrounding the shifting dunes, including loess, fixed dunes, sandy farmland, and sandy grassland. A total of 14 sample sites (28 soil profiles) were identified, and the location of each site was registered in 2012 using a hand-held global positioning system (GPS) (Fig. 1; Table 1). The sampling sites were located in Dingbian (DB), Jingbian (JB), Hengshan (HS), Yuyang (YY), and Shenmu (SM) counties (Fig. 1; Table 1). For all the profiles, two horizons—hereafter referred to as upper soil and lower soil—were identified and sampled separately. Soil samples were collected in July 2012 in the desert expansion area to determine soil mineral and nutrient compositions. About 2 kg (wet weight) of soil sample were collected using a shovel from each horizon at each sampling location. Soil samples were stored in zip-top plastic bags. A total of 168 samples (14 sample sites × 2 patches × 2 horizons × 3 replicates) were collected. The three replicates were 50 cm apart in each patch. The thickness of the horizons was different among sampling sites because of the natural variations in soil horizons. All soil samples were air-dried and sieved through a 2-mm sieve prior to the analysis. Organic matter from the soils was removed by treating a 50-g subsample of air-dried soil with H2O2 (30% v/v) on an electric heating plate at 70 °C. Samples were dispersed with 0.5 mol L−1 (NaPO3)6. In this paper, the particle size is expressed in three size fractions: clay fraction (< 0.002 mm), silt fraction (0.002–0.02 mm), and sand fraction (0.02–2 mm) (Pansu and Gautheyrou, 2003). The silt and sand fractions were each divided into three subfractions: fine silt (0.002–0.005 mm), medium silt (0.005–0.01 mm), and coarse silt (0.01–0.02 mm); and fine sand (0.02–0.05 mm), medium sand (0.05–0.25 mm), and coarse sand (0.25–2 mm). After treatment, soil was passed through a 0.25-mm sieve, and the fraction retained on the sieve was the coarse sand fraction (2–0.25 mm). Pipette method was used to extract silt- and clay-sized particles according to Stokes Law (Gee and Bauder, 1986; Lu, 2000). Soils of different particle sizes were melted using sodium carbonate in a Muffle furnace at 900 °C to determine minerals composition. After melting, minerals were determined following Ulery and Drees (2008) and Lu (2000). The SiO2 was determined by the weighted method, Al2O3 using potassium fluoride substituted EDTA capacity method, and TiO2 using chromotropic acid spectrophotography. CaO, MgO, Fe2O3, and MnO2 were determined by atomic absorption spectrophotometer. Soil K2O was determined by flame photometer and P2O5 was measured by phospho-molybdenum blue-colorimetry method. Soil organic matter (SOM) was determined by the dichromate-wet combustion method (Nelson and Sommers, 1982), total nitrogen (TN) by the Kjeldahl method (Bremner and Mulvaney, 1982), and available nitrogen (Av-N) by the alkali diffusion method. Available N is also known as alkali-hydrolyzable nitrogen. Total phosphorus (TP) was measured colorimetrically with ammonium molybdate

2. Materials and methods 2.1. Study site The study area was located in China’s northern Shaanxi province (longitude range from 107°35′ to 111°29′E and latitude from 37°35′ to 39°02′N), with an approximate area of 3.61 million ha (Fig. 1). Elevation in the region ranges from 800 to 1800 m, sloping generally from the northwest to the southeast. The Mu Us Sands lie to the north and the loess plateau lies to the south. The area has a typical continental semiarid climate, and the annual average temperature varies between 7.0 and 9.0 °C. Mean annual precipitation in the area ranges between 250 and 450 mm, and more than 150 days in a year are windy with wind speeds of > 5 m s−1. The desert area was about 0.96 million ha in 2008 (Qi et al., 2012), 2

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Fig. 1. The location of the agro-pastoral transitional zone in northern Shaanxi province and sample sites.

extraction with 1 N ammonium acetate and using an atomic absorption spectrometer (AAS) (Lu, 2000).

after acid digestion. Soil available phosphorus (Av-P) was extracted with 0.5 mol L−1 NaHCO3 at a pH of 8.5, and P was determined calorimetrically using the molybdate method (Olsen et al., 1954). For total potassium (TK), samples were digested with hydrofluoric acid and perchloric acid. Soil available potassium (Av-K) was determined by Table 1 Sample sites and their land use and desert expansion types. Profile

Sample sites

Land use

Expansion type

Profile

Sample sites

Land use

Expansion type

DB-01 DB-02 JB-03 JB-04 JB-05 JB-06 HS-07 HS-08 HS-09 HS-10 YY-11 YY-12 YY-13 YY-14

Nanyuan in Dingbian

Shifting dune Sandy grassland Shifting dune Sandy grassland Shifting dune Sandy farmland Shifting dune Loess farmland Shifting dune Sandy farmland Shifting dune Loess farmland Shifting dune Loess farmland

Dune sand migration

YY-15 YY-16 YY-17 YY-18 YY-19 YY-20 SM-21 SM-22 SM-23 SM-24 SM-25 SM-26 SM-27 SM-28

Mengjiawan in Yuyang

Shifting dune Stabilized dune Shifting dune Sandy farmland Shifting dune Stabilized dune Shifting dune Loess farmland Shifting dune Sandy farmland Shifting dune Loess farmland Shifting dune Loess farmland

Stabilized dune reactivation

Xiaotanzi in Jingbian Zhenjing in Jingbian Wanggedu in Hengshan Wujiagou in Hengshan Yuhemao in Yuyang Guojiagou in Yuyang

Dune sand migration Dune sand migration Loess decline Dune sand migration Dune sand migration Loess decline

Wubatan in Yuyang Zhangjiamao in Yuyang Lijiaban in Shenmu Dianta in Shenmu Hujialiang in Shenmu Erlintu in Shenmu

Dune sand migration Stabilized dune reactivation Loess decline Dune sand migration Dune sand migration Dune sand migration

Note: For the profile number, DB means the profile was sampled in Dingbian county, JB in Jingbian county, HS in Hengshan county, YY in Yuyang county, and SM in Shenmu county. 3

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2.3. Identification of types of desert expansion

that both are original with similar parent materials.

In this study, identification of types of desert expansion was based on the measured soil mineral content within a particle size range. We focused on the significant differences in the soil minerals among soil horizons, and three types of desert expansion were identified. For the shifting dune adjacent to the loess farmland and sandy grassland, if a significant difference in minerals was detected between the upper and lower soil horizons, the desert expansion process was identified as dune sand migration. This occurred due to the twin wind erosion processes of creep and saltation. Nine sample sites were identified as this type (Table 1). If no significant differences in minerals were detected between the upper and lower soil horizons in the expanded area, the desert expansion process was identified as loess decline. Three sample sites were identified as this type (Table 1). For the shifting dunes adjacent to the stabilized dunes, the desert expansion process of conversion of stabilized dunes into shifting dunes was identified as stabilized dunes reactivation. Two sample sites were identified as this type (Table 1). These results were verified by the field surveys and comparison of minerals in the upper and lower soil horizons. When demonstrating the identification of different desert expansion types in the following analysis, we only selected one sample site as the example and showed the data of soil minerals in the upper and lower soil horizons in each pair of comparable soil profiles.

3.2. Loess decline desertification process The loess decline-associated desertification is caused by wind erosion following intense disturbances on the land surface due to human activities. Loess decline severely degrades the land, and with a loss in fertility, vegetation will be completely lost. Over time, the landscape in this area has changed from a flat surface to heterogeneous sand dunes. Soil texture has changed with the loss of fine particles, and proportions of larger-sized sand particles have increased gradually. This type of expansion mainly occurred in the loess soil and grassland areas where agricultural and grazing activities were intense and resulted in serious degradation of the entire area, with the upper soil texture becoming sandy. As shown in Fig. 4 and Table 3, HS-08 is a loess soil profile and HS07 is an adjacent shifting dune profile. Both profiles were similar prior to degradation of HS-07. The upper soil of HS-07 was composed of sand, while the lower soil was loess soil. Comparing HS-08 and HS-07, we can see that no significant differences were detected in the mineral contents in the upper soil horizon of profile HS-07 in the grain sizes of < 0.002 mm, 0.002–0.005 mm, 0.005–0.02 mm, and 0.02–0.05 mm, and in the upper soil and lower soil horizons in the same grain sizes, respectively. The mineral contents of 0.05–0.25 mm grain size in the upper soil of profile HS-07 were similar to those in the upper soil of profile HS-08. At the same time, the mineral contents of the lower soil horizon in each grain size were similar to those in the upper soil horizon of profile HS-07 and the entire profile of HS-08 in the corresponding grain sizes. These results indicated that the sandy dune soil adjacent to the loess soil was formed in situ. However, most of the fine grains (size < 0.05 mm) were blown away due to wind erosion from the top horizon, and the larger-sized sand grains (> 0.05 mm) were left behind. That is why the 0.05–0.25 mm size accounts for most of the soil texture. Fig. 5 shows that particles with grain size < 0.05 mm were ≥740 g kg−1 in the lower soil horizon of HS-07 and the whole profile of HS-08. The rich fine particles in the loess soil provided a wealth of “source material” for the wind erosion. The dry climate of the study area and intensive anthropogenic activities (such as intensive ploughing on the farmland and overgrazing on the grassland) on the soil surface caused wind erosion. A comparison between the lower and upper soil horizons in HS-07 revealed that the fine particle contents decreased significantly from 723.86 g·kg−1 to 65.95 g·kg−1 by wind erosion. Wind erosion also caused a decline in soil fertility and soil nutrients, with significant decreases in SOM, TN, TP, TK, Av-N, Av-P, and Av-K (Figs. 6 and 7). For example, the SOM content decreased from 8.88 g·kg−1 in the upper soil of HS-08 to 0.86 g·kg−1 in the upper soil of HS-07, and TN decreased from 0.42 g·kg−1 in the upper soil of HS-08 to 0.04 g·kg−1 in the upper soil of HS-07. Loess decline in HS-07 also exacerbated the decline in soil fertility status.

2.4. Data analysis Statistical analysis was carried out using the SPSS software (Raynald, 2007), and included analysis of variance (ANOVA) with leastsignificant-difference (LSD) test. One-way ANOVA was used to examine the difference in soil minerals, soil texture, and soil nutrients among the soil profiles as well as soil horizons. 3. Results 3.1. Dune sand migration desertification process Dune sand migration-associated desertification was caused by the expansion and transition of sand dunes. This type of desert expansion is predominant in the research area. Two factors affecting expansion of the sand dunes are prevailing wind direction and wind speed. In the research area, the prevailing wind directions are west and northwest during spring, and dune sand migration mainly occurs in the spring months. As shown in Fig. 2 and Table 2, YY-12 is a typical loess soil profile, and YY-11 is a sandy shifting dune profile, which is adjacent to YY-12. Profile YY-11 has two distinct horizons: the top 30 cm is sand and below the sand is loess soil. The SiO2, AlO3, TiO2, P2O5, K2O, Fe2O3, MgO, MnO2, and CaO contents among the upper and lower soil of YY-12 and the lower soil of YY-11 have no significant difference, even in different grain sizes. However, mineral contents were significantly different at the upper soil horizons of profile YY-11 and YY-12. This indicated that YY-12 is an undisturbed profile, and the horizons of YY-12 and YY-11 (lower soil) were developed from the same parent material. The sand upper soil horizon of YY-11 was not original, but was composed of wind-deposited material from other places. As shown in Fig. 3, after the dune sand migration and deposition on top of the loess soil, soil texture of the upper soil of YY-11 became significantly coarser. Coarse sand content in the upper soil of YY-11 was significantly higher (p < 0.05) than the lower soil of YY-11 and the upper and lower soil of YY-12, while coarse and fine silt and clay contents were significantly lower (p < 0.05). Coarse sand content (2–0.25 mm) in the upper soil of profile YY-11 was about 50 times higher than that of the lower soil of YY-11. No differences were found in sand, silt, and clay contents among the upper and lower soil horizons of YY-12 and the lower soil horizon of profile YY-11, further confirming

3.3. Stabilized dune reactivation desertification process The stabilized dune reactivation-associated desertification changed the landscape from stabilized dunes to shifting sand dunes. Intense human activities degraded the land, causing a significant decline in vegetation, destruction of biological crusts, and destabilization of the fixed dunes. This type of desert expansion was distributed around the sandy agricultural land; residential, industrial, and mining enterprises; and along traffic lines. As shown in Fig. 8 and Table 4, YY-20 is a stabilized dune profile, while YY-19 is a shifting dune profile near YY-20. However, YY-19 was identified as a stabilized dune in 1986 from remote sensing image analysis (Qi et al., 2012) and was confirmed during field survey by a local resident. Further field investigations revealed that YY-19 transitioned from a stabilized dune to a shifting dune due to coal mining activity. Profile YY-20 has two distinct horizons; the upper soil horizon 4

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Fig. 2. Soil profiles distribution of dune sand migration desert expansion.

was similar to profile YY-20 that was covered with Artemisia desertorum and Agriophyllum squarrosum, with vegetation coverage of greater than 50% before reactivation of the stabilized dune. After reactivation, the vegetation coverage of profile YY-19 decreased to less than 10%, and soil texture became coarser. The coarse sand particles (> 0.05 mm) were 954.39 g·kg−1 at the upper soil horizon of YY-19, but only 748.97 g·kg−1 and 828.6 g·kg−1 in the upper and lower soil horizons of YY-20, respectively. Clay content (< 0.002 mm) of YY-19 was significantly lower than YY-20 (Fig. 9). Consistent with the change in soil

is the biological crust, which is only 0.5 cm thick, and the lower soil horizon below the biological crust is sand. No significant differences were found in the soil mineral contents among soil in the crust horizon (upper soil) and the lower soil horizon of profile YY-20 and the upper and lower soil horizons of profile YY-19 in different soil particle sizes. Therefore, YY-19 was identified as a stabilized sand dune that was reactivated to become a shifting dune due to removal of vegetation and biological crusts by human activities. Based on our field survey, the upper soil horizon of profile YY-19 Table 2 Soil minerals in the dune sand migration desertification (g·kg−1). Particle size (mm)

Profile

Depth (cm)

SiO2

Al2O3

TiO2

P2O5

K2O

Fe2O3

MgO

MnO2

CaO

< 0.002

YY-12

0–20 20–60 0–30 > 30

415.24a 425.85a 361.43b 419.27a

172.75a 178.62a 127.22b 174.57a

4.93b 5.26b 8.03a 4.64b

6.38b 7.35b 8.36a 6.29b

15.26a 15.41a 10.62b 14.29a

30.79a 31.63a 24.58b 31.59a

43.19a 43.08a 36.23b 42.37a

0.68a 0.64a 0.46b 0.67a

10.12a 9.09a 7.08b 11.32a

0–20 20–60 0–30 > 30

449.57a 407.71a 329.48b 445.36a

120.75b 118.97b 150.94a 119.47b

5.54b 5.37b 7.18a 5.69b

5.36a 5.84a 3.09b 5.63a

9.49a 10.44a 6.76b 10.13a

28.30a 28.35a 23.97b 29.14a

39.80a 40.25a 28.86b 39.86a

0.99b 1.34b 2.42a 1.19b

204.83b 207.63b 281.36a 207.71b

0–20 20–60 0–30 > 30

611.78a 559.64a 505.23b 615.43a

111.23a 111.07a 102.22b 112.37a

5.09b 4.62b 6.05a 4.85b

3.44a 3.16a 1.39b 3.26a

10.23a 9.63a 7.11b 10.57a

31.65a 32.85a 22.38b 31.71a

35.49a 37.92a 26.37b 36.18a

0.61b 0.73b 1.32a 0.67b

115.62b 122.79b 246.56a 117.82b

0–20 20–60 0–30 > 30

656.31b 659.35b 751.53a 661.24b

100.69a 108.20a 86.25b 103.56a

4.31a 4.23a 1.65b 4.27a

1.81a 1.85a 0.91b 1.84a

9.72b 9.49b 11.37a 9.53b

24.43a 24.82a 17.42b 24.72a

28.18a 28.73a 18.23b 28.48a

0.61a 0.65a 0.37b 0.63a

77.27a 76.77a 49.45b 76.86a

0–20 20–60 0–30 > 30

719.68b 713.64b 789.46a 715.43b

95.14a 99.03a 87.58b 97.81a

3.74a 3.67a 1.99b 3.72a

1.03a 1.44a 0.43b 1.25a

8.54a 7.74a 3.06b 8.87a

21.81a 21.24a 18.07b 21.65a

18.91a 19.03a 14.16b 18.98a

0.37a 0.36a 0.27b 0.38a

45.18a 46.76a 27.16b 45.62a

0–20 20–60 0–30 > 30

– – 775.45 –

– – 88.24 –

– – 2.25 –

– – 0.75 –

– – 10.86 –

– – 15.46 –

– – 11.13 –

– – 0.22 –

– – 23.33 –

YY-11 0.002–0.005

YY-12 YY-11

0.005–0.02

YY-12 YY-11

0.02–0.05

YY-12 YY-11

0.05–0.25

YY-12 YY-11

> 0.25

YY-12 YY-11

Note: “–” means no data because this particle size was not extracted. Significant interactions were at α = 5%. Different letters indicate significant differences among same particle size. 5

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Fig. 3. Soil particle composition change in the dune sand migration desertification. (Significant interactions were at α = 5%. Different letters on the bars indicate significant differences.)

Fig. 4. Soil profiles distribution of loess decline desert expansion.

contents, researchers have explained the impact of different land uses on soil health (Kodnik et al., 2017), as well as determined the degree and types of soil degradation (Yu et al., 2014). When referring to desert expansion, researchers generally attribute the mechanisms to intensive disturbance of the soil surface in the desert area (Hu et al., 2015; Saowanee, 2016). Desert expansion types, such as displacement of sand dunes (Collado et al., 2002) and reactivation of dormant sand dunes (Yang et al., 2007), have been identified based on remote sensing imagery interpretation but not using measured soil properties and in situ observation. In this paper, we identify three types of desert expansion, i.e., dune sand migration, loess decline, and stabilized dune reactivation, by contrasting minerals between expanded desert and nearby areas. The results of this research provide useful exploration on mineral changes as an indicator of desert expansion processes and mechanisms, and could be helpful for creating desert control measures.

texture, soil nutrients were depleted significantly in YY-19. Soil organic matter content, Av-N, Av-P, and Av-K were 1.27 g·kg−1, 23.56 mg·kg−1, 4.88 mg·kg−1, and 14.10 mg·kg−1 in YY-19, while they were 23.23 g·kg−1, 36.28 mg·kg−1, 9.11 mg·kg−1, and 47.01 mg·kg−1 in the upper soil horizon of YY-20, respectively (Fig. 10). Declines in SOC, N, P, and K clearly demonstrate that the reactivation of the stabilized dune caused desert expansion. 4. Discussions 4.1. Application of mineral material on desertification Mineral materials are the essential part of the soil, and usually account for more than 95% of the soil solid parts. The chemical composition of soil minerals is closely related to the soil-forming factors and the process of soil formation (Yu et al., 2014). Using soil mineral 6

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Table 3 Soil minerals in the loess decline desertification (g·kg−1). Particle size (mm)

Profile

Depth (cm)

SiO2

Al2O3

TiO2

P2O5

K2 O

Fe2O3

MgO

MnO2

CaO

< 0.002

HS-08

0–20 > 20 0–45 > 45

407.37a 438.12a 439.24a 426.53a

187.24a 176.45a 172.47a 181.48a

4.98a 4.54a 4.73a 4.65a

7.67a 7.90a 8.14a 7.98a

14.75a 14.73a 14.75a 14.69a

35.54a 34.07a 35.13a 34.72a

42.70a 42.82a 42.20a 42.74a

0.65a 0.62a 0.61a 0.64a

26.93a 14.10b 12.69b 15.18b

0–20 > 20 0–45 > 45

406.19a 445.89a – 432.54a

107.70a 130.70a – 127.38a

4.64a 5.31a – 4.87a

2.77a 2.97a – 2.85a

9.66a 10.08a – 9.86a

32.33a 33.07a – 32.64a

38.03a 39.07a – 38.92a

0.44b 1.01a – 1.21a

245.35a 197.56b – 213.47b

0–20 > 20 0–45 > 45

546.81a 571.57a 577.00a 565.43a

111.45a 103.50a 109.80a 107.47a

4.38a 4.57a 4.30a 4.41a

2.26a 2.47a 2.38a 2.36a

8.70a 9.63a 9.51a 9.14a

26.40a 29.43a 29.36a 29.28a

34.65a 34.19a 34.17a 34.35a

0.75a 0.81a 0.83a 0.79a

140.94a 130.95a 131.65a 133.26a

0–20 > 20 0–45 > 45

633.00a 670.34a 680.28a 673.56a

98.58b 131.15a 88.25b 104.35a

4.30a 4.61a 4.63a 4.53a

1.67a 2.22a 2.24a 2.16a

9.11a 10.27a 10.21a 10.13a

36.76a 28.75a 27.85a 28.37a

27.92a 27.78a 27.24a 27.63a

0.70a 0.68a 0.61a 0.65a

87.76a 82.69a 82.65a 81.53a

0–20 > 20 0–45 > 45

705.00a 595.81b 719.52a 612.43b

116.54a 149.13a 116.54a 143.52a

4.50a 5.49a 4.74a 5.24a

1.38a 1.78a 1.44a 1.56a

10.25a 13.83a 10.80a 13.25a

23.68a 31.60a 22.46a 29.65a

21.18a 22.54a 21.61a 21.86a

0.61a 0.26b 0.58a 0.31b

52.86a 20.37b 53.43a 22.42b

HS-07 0.002–0.005

HS-08 HS-07

0.005–0.02

HS-08 HS-07

0.02–0.05

HS-08 HS-07

0.05–0.25

HS-08 HS-07

Note: “–” means no data because this particle size was not extracted. Significant interactions were at α = 5%. Different letters indicate significant differences among same particle size.

the soil minerals of accumulated sand dunes were quite different compared to the stacked loess soil. As shown in Table 1, compared with the loess soil, the contents of SiO2 in the sand dunes were decreased significantly in the grain size ranges of < 0.002 mm, 0.002–0.005 mm, and 0.005–0.02 mm, while they increased significantly in the grain size ranges of 0.02–0.05 mm and 0.05–0.25 mm. The other soil minerals showed significant increase or decrease between sand dunes and loess soil in the correspong grain size ranges. Therefore, significant differences in soil minerals can be used to identify dune sand migration-associated desert expansion. Aeolian loess with rich silt content removal due to dry and windy weather in spring caused loess decline-associated desertification. Loess

4.2. Mechanisms and processes of three types of desert expansion Wind speed and wind frequency are the driving forces of dune sand migration-associated desertification (Bo et al., 2013). When heavy wind occurs, fine sand in the bottom of the windward slope gets blown, while the coarser sand moves with saltation or creep to the top of the dune along the slope, especially on the surface, with the sand wriggling forward in a striated shape (Okin et al., 2001; Webb and Strong, 2011) and getting deposited after crossing sand ridges due to the inertia effect (Zhou et al., 2015). All the moving sand was accumulated in the leeward slope along the direction perpendicular to sand ridges, and consequently vegetation was completely destroyed. Besides the soil texture,

Fig. 5. Soil particle composition change in the loess decline desertification. (Significant interactions were at α = 5%. Different letters on the bars indicate significant differences.) 7

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Fig. 6. Soil organic matter and total nutrients in the loess decline desertification. (Significant interactions were at α = 5%. Different letters on the bars indicate significant differences.)

Western Great Plains in the United States that grain sizes > 0.84 mm can be called non-moving size. Wang et al. (2007) indicated that when wind speed is higher than 6.4 m s−1, sandy particles with diameter > 0.25 mm move by saltation or creep. In our research area, heavy wind occured usually during spring. Higher soil moisture reduces the dislodging and migration of the sand. Wind tunnel tests by Sharratt et al. (2013) showed that the sand particles migrate only when moisture content is below 1%. This is because water film, adhesion, and electrostatic attraction prevent sand movement. With blowing wind, soil at the surface dries gradually, and with cohesion and adhesion gone, loose sand starts to migrate by creep or saltation based on the particle size and wind speed. This means that the mobile dune covering the sandy

formation in general is a highly researched but poorly understood process (Crouvi et al., 2010), and the question of silt production, especially in deserts, is an ongoing debate in sedimentology and in quaternary paleoclimatology (Wright, 2001; Muhs and Bettis, 2003). Unlike in subtropical deserts, loess deposits in the loess plateau of China are mainly aeolian silt deposited by windstorms after the glacier retreat (Zhao, 2000; Frechen, 2011). Soil particle size is a key factor for explaining the process of loess decline-associated desertification, and rich silt content has been observed in the selected loess farmland profiles (HS-08, YY-14, and SM-22 in Table 5). Whether or not a soil particle can become airborne with wind is a function of size, wind speed, and soil moisture. Chepil (1952) concluded from experiments in the

Fig. 7. Soil available nutrients in the loess decline desertification. (Significant interactions were at α = 5%. Different letters on the bars indicate significant differences.) 8

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Fig. 8. Soil profiles distribution of stabilized dune reactivation desert expansion.

blown into the windward side by heavy wind from the “break point,” and then the sand developed parabolic barkhan dune chains. Hagemann et al. (2017) reported that disturbance of biological soil crusts in stabilized dunes would decrease water-holding capacity and increase water evaporation rate from the desert surface. Because of isogenesis of the shifting sand and the stabilized dunes, a nonsignificant difference in the soil minerals (SiO2, AlO3, TiO2, etc.) was detected between them (Table 3). Therefore, nonsignificant difference in soil minerals between shifting sand and stabilized dunes can be used to identify stabilized dune reactivation-associated desert expansion.

loess soil was formed in situ when the fine silt and clay were blown away. Significant differences in soil texture were detected between the mobile dune and the sandy loess soil; their mineral (SiO2, AlO3, TiO2, etc.) contents were similar between them within a particle size range (Table 2). Therefore, nonsignificant differences in soil minerals can be used to identify loess decline-associated desert expansion. Biological soil crusts are the community of organisms living at the surface of desert soils (Wang et al., 2017). The appearance of crusts on the desert surface is indicative of improvement of the ecological environment (Belnap and Gillette, 1998; Housman et al., 2006; Qi et al., 2006) and stabilization of shifting dunes. During the past several years, many mining companies were established, and development of roads, buildings, and transportation has put severe pressure on the surface of the stabilized sand (Xue et al., 2013), destroyed the biological crusts and vegetation, exposed the underlying sand, and provided a “break point” for wind erosion. For the stabilized dunes, the shifting sand was

4.3. Desert expansion types: distribution in northern Shaanxi province In our research area, the most dominant desert expansion type was identified as dune sand migration, followed by loess decline and stabilized dune reactivation (Table 1). This is in contrast to the popular

Table 4 Soil minerals in the stabilized dune reactivation desertification (g·kg−1). Particle size (mm)

Profile

Depth (cm)

SiO2

Al2O3

TiO2

P2O5

K2O

Fe2O3

MgO

MnO2

CaO

0.005–0.02

YY-20

0–0.5 0.5–60 0–20 20–40

485.78a 523.11a – –

124.57a 116.46a – –

10.66a 12.61a – –

2.48a 2.20a – –

13.50a 12.76a – –

39.09a 37.31a – –

43.38a 41.77a – –

0.36a 0.45a – –

104.60a 112.16a – –

0–0.5 0.5–60 0–20 20–40

633.00ab 693.61a 715.00a 704.53a

110.40ab 106.78ab 131.56a 117.42a

6.75a 6.09a 5.55ab 7.14ab

1.61a 1.37a 0.90ab 0.83ab

10.63ab 10.80ab 12.34a 11.25a

22.59a 20.98a 16.53ab 17.61ab

27.32a 25.57a 17.03b 18.14b

0.64a 0.59a 0.38ab 0.35ab

55.48a 56.07a 44.95ab 46.86ab

0–0.5 0.5–60 0–20 20–40

727.54a 675.00ab 761.95a 759.62a

112.50a 123.68a 109.68a 107.51a

6.09a 6.90a 4.20ab 4.19ab

0.75a 0.80a 0.63ab 0.59ab

10.61a 11.20a 13.43a 12.18a

18.98a 19.89a 15.01ab 14.69ab

12.62a 10.99a 8.39ab 8.86ab

0.51a 0.53a 0.39ab 0.41ab

27.51a 27.39a 19.20ab 18.37ab

0–0.5 0.5–60 0–20 20–40

743.51a 789.42a 794.35a 769.68a

102.36a 123.60a 116.28a 114.51a

1.05ab 3.64a 2.37a 2.48a

0.27a 0.33a 0.31a 0.33a

13.83a 14.02a 13.59a 12.95a

6.28ab 11.94a 9.72a 10.78a

2.55ab 5.05a 3.56a 4.63a

0.24a 0.29a 0.28a 0.27a

16.02ab 26.71a 22.37a 23.57a

YY-19 0.02–0.05

YY-20 YY-19

0.05–0.25

YY-20 YY-19

> 0.25

YY-20 YY-19

Note: “–” means no data because this particle size was not extracted. Significant interactions were at α = 5%. Different letters indicate significant differences among same particle size. 9

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Fig. 9. Soil particle composition changes in the stabilized dune reactivation desertification. (Significant interactions were at α = 5%. Different letters on the bars indicate significant differences.)

Fig. 10. Soil nutrient changes in the stabilized dune reactivation desertification. (The unit is g·kg−1 for soil organic matter and mg·kg−1 for available nitrogen, phosphorus, and potassium. Significant interactions were at α = 5%. Different letters on the bars indicate significant differences.) Table 5 Soil texture in the loess farmland (mm, g·kg−1). Profile

Depth (cm)

Sand

Silt

Clay

1–0.25

0.05–0.25

0.05–0.02

0.02–0.01

0.01–0.005

0.005–0.002

< 0.002

HS-08

0–20 20–60

100.86 128.73

252.02 406.90

146.43 53.96

178.78 139.17

141.41 143.64

26.67 4.86

153.83 122.74

YY-14

0–25 25–65

2.37 345.75

107.03 150.01

383.92 92.20

194.89 139.60

192.26 132.49

6.26 27.42

113.27 112.53

SM-22

0–20 20–60

4.42 2.95

154.32 149.94

323.10 336.69

230.84 277.31

132.60 105.07

31.60 9.73

123.11 118.32

10

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Program of China (2017YFC0504501). Authors thank Mr. Frank Sholedice, New Mexico State University, for editing the manuscript.

belief that the dominant type of desert expansion in the area is the reactivation of fixed dunes (Wang et al., 2010). The agro-pastoral transitional area is located at the junction of the arid and semi-arid climate regions. In the winter and spring months, strong winds blow due to the powerful cold air masses moving southeast from Siberia (Qi et al., 2012). Controlled by the wind regime, vegetation cover, and sediment supply of this area, the migration of active parabolic dunes occurred with high frequency, especially along the valley area. Therefore, most of the desert expansion was caused by dune sand migration. This is a common phenomenon in desert areas around the world, such as in northern Australia with a migration rate of 0.05 m yr−1(Story, 1982), as well as in Manawatu with a migration rate as fast as 80 m yr−1 (Hesp, 2001). Along with abundant sandy materials, intensive cultivation of loess soils and overgrazing of the grasslands adjacent to the desert were the main factors causing loess decline-associated desertification. Intensive cultivation over several years loosened the surface of the loess soil, and constant treading by livestock exposed the soil of the grasslands. Both of these activities provided opportunities for heavy winds to blow away the fine clay and silt from the loosened soil, while leaving the sand in place. Stabilized dune reactivation-associated desertification gradually occurred with the coal, petroleum, and gas resources exploration located around the coal mining company. Exploration of energy resources needs intensive labor, mechanical equipment to excavate the ground, and heavy machinery to transport the mineral products. The increased human activities have threatened the stabilized dunes around the mining company, and several stabilized dunes have been reactivated. This type of desert expansion only occurred in a small area around the mine, near the road, and near the buildings. Collado et al. (2002) indicated that displacement of sand dunes, equivalent to dune sand migration in this research, was the most important process of desert expansion in the agricultural and rangeland boundary in Argentina, which is consistent with our results because of the similar land use pattern. Avni et al. (2006) reported that natural desertification, equivalent to loess decline in this research, was the dominant type of desert expansion in the Negev highlands in southern Israel. Yang et al. (2007) indicated that reactivation of dormant sand dunes, equivalent to stabilized dune reactivation in this research, was the main type of desert expansion in the Hunshandake sandy lands in China. These results corroborate with this study because farmland was the dominant land use in the Negev highlands and grassland was the dominant land use in the Hunshandake sandy lands.

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5. Conclusions Desertification is a complicated process. Three desert expansion types, i.e., dune sand migration, loess decline, and stabilized dune reactivation, were identified in this study based on the mineral contents within a particle size range by in situ observations and field surveys. All three types of desert expansion were not always present, but at least one of them was found. Both heavy winds and intensive human activities were the dominant causes for desert expansion in the study area. Therefore, there is a need to enhance efforts toward reducing human activities in the area to prevent further desert expansion. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the National Natural Foundation of China (41877007) and the National Key Research and Development 11

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