1 Introduction

Aeolian processes may cause the losses of nutrients and fine fractions from soil, as well as reduce water retention, which thereby may trigger desertification and reduce biomass and species richness of vegetation (e.g., the loss of herbaceous species) from the community composition (Schlesinger et al. 1990; Breshears et al. 2003; Li et al. 2006; Shao 2008; Field et al. 2012; Wang et al. 2013). The fragile ecosystems of arid and semiarid regions are highly sensitive to aeolian processes (Wang 2013), and soil nutrient loss and desertification can spread to adjacent areas, rendering them unsuitable for cultivation and animal husbandry (UNEP 1992). However, global warming has resulted in increased precipitation and temperature in some arid and semiarid areas (e.g., Ding et al. 2007; Zhong et al. 2010), which indicates that lands unsuitable for grazing previously may now be suitable for reclamation and increased grazing intensity. Consequently, population increase and the desire to generate wealth may increase pressure for these arid and semiarid areas to be reclaimed and used for farmland and increased livestock populations. The impacts of human activity, combined with the effects of aeolian processes, have caused soil nutrient loss and desertification (Gibbens et al. 1983; Okin et al. 2006; Peters et al. 2007).

At present, most tillage practices in arid and semiarid areas of China are applied to converted grasslands, where the freshly turned soil becomes completely disaggregated in preparation for planting. In addition, the increased population density of livestock has caused damage to grassland topsoil due to the trampling by animals. The destruction of soil surface caused by agricultural conversion and trampling combined with aeolian processes results in the loss of soil particles and nutrients such as nitrogen (N), organic matter, and phosphorus (P) (Lyles 1988; Leys and McTainsh 1994; Larney et al. 1998; Yan et al. 2005; Yang et al. 2006; Alfaro 2008; Li et al. 2008, 2009).

The Qinghai–Tibet Plateau in western China covers an area of 2.57 × 106 km2 (Zhang et al. 2002) and has a mean elevation of over 4000 m above sea level (Fig. 1). As the “Third Pole” of the globe, its ecological and environmental evolution plays an important role in regional (Li and Xue 2010) and global (Wu et al. 2007) climate change. At present, the plateau is also threatened by land degradation (e.g., Zou et al. 2002). The Gonghe Basin, located in the northeastern of the Qinghai–Tibet Plateau, is a region with high risks of soil nutrient loss and desertification (Fang et al. 1998; Feng et al. 2005). With increases in both precipitation and temperature occurring since the late 1980s, large areas of grasslands have been converted to agriculture, and grazing intensity has increased. Human activities such as tillage, trampling by farm animals, and wholesale destruction of vegetation have greatly increased over this period (Wang and Cheng 2001). However, the interactions between aeolian processes and soil nutrient loss remain poorly understood.

Fig. 1
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Map of the Gonghe Basin. The inset shows the location of the Qinghai–Tibet Plateau within China. Sampling sites are indicated by black circles. Larger black circles indicate samples used for the wind tunnel experiments

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The occurrence of nutrient loss through aeolian processes is a key mechanism triggering desertification (Schlesinger et al. 1996; Field et al. 2012). Although nutrient loss resulting from land degradation has been documented in other regions of the world using field observations at the landscape scale (Larney et al. 1998; Hobbs and Harris 2001; Okin et al. 2009), few studies have employed field investigations combined with wind tunnel experiments to examine soil nutrient loss on the Qinghai–Tibet Plateau. Therefore, in this study, we employed a wind tunnel to simulate aeolian processes and nutrient loss on land affected by human activities such as tillage and trampling by livestock. The results can be used to appraise the significance of aeolian processes and nutrient loss and their effects on desertification in the Gonghe Basin, Qinghai–Tibet Plateau.

2 Materials and methods

2.1 Study site

Sampling was conducted within the Gonghe Basin at 15 sites (S01 to S15) located at 35.56–36.51° N, 99.30–101.07° E and at altitudes between 2913 and 3305 m above sea level (Fig. 1). Meteorological data from weather stations within the Gonghe Basin record a mean annual precipitation of 246 mm, annual evaporation of 1716 mm, mean wind speed of 2.7 m s−1, and mean annual temperature of 2.4 °C. Vegetation cover during the growing season (i.e., May to September) varies between 5 and 20% but decreases to <5% during the windy seasons (i.e., March to June). The dominant soil at the sampling sites is meadow soil. Most areas within the basin have already been converted to farmland, and nearly all farmland surfaces are completely disaggregated by tilling during the windy season. In addition, trampling by animals has resulted in significant disturbance of grassland soil surfaces as populations of grazing livestock (e.g., sheep, goats, and yaks) have increased.

The primary grass species in the basin include Stipa krylovii, Orinus thoroldii, and Artemisia frigida. Secondary grass species include Cleistogenes squarrosa, Ceratoides latens, Stipa breviflora, Potentilla multifida, Heteropappus altaicus, Astragalus galactites, and Agropyron cristatum. Plant communities in steppe subzones are composed of S. breviflora and the secondary species H. altaicus, Poa sp., Carex stenophylla, Sibbaldia adpressa, Leymus secalinus, Potentilla bifurca, Toruluria humilis, and Allium macrostemon. Alpine grasslands are dominated by Stipa purpurea. Desert plant communities consist of Caragana tibetica and the secondary species Convolvulus ammanii, Stellera chamaejasme, Dracocephalum heterophyllum, Achnatherum splendens, and Xanthopappus subacaulis. In addition, plant communities on anchored sand dunes are characterized by Artemisia arenaria accompanied by Thermopsis lanceolata, Corispermum hyssopifolium, Salsola collina, and Glycyrrhiza uralensis.

2.2 Methodology

2.2.1 Field sampling and wind tunnel experiments

In August 2014, five surface soil samples were collected from each of 15 sites within the Gonghe Basin (Fig. 1). At each site separated by 100~200 m, surface samples were collected with criteria that had a smooth and intact surface (sealed, with no cracks). Soil samples were collected from the upper 20 cm since this is the maximum depth affected by human activities such as tillage and trampling by grazing livestock. One sample from each of three sites (S06, S07, and S08) was randomly selected for wind tunnel experiments, which were conducted at the Key Laboratory of Desert and Desertification of the Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou, China. Details of the wind tunnels and related experimental processes have been described by Dong et al. (2004) and Wang et al. (2013). The soil samples were disaggregated with a hammer to simulate tillage and trampling by animals. During the wind tunnel experiments, the relative humidity was between 40 and 41%, which was similar to that in the sampling sites in August. Samples were distributed over an area of 0.3 × 1.5 m to a depth of about 2 cm thick in the wind tunnel. Windblown materials were collected by a sand trap. The trap was 30-cm wide (the same width as the surface sample) and 30-cm high and was placed 10 cm downwind of the sample. This placement allowed for collection of over 95% of the sediment transported during the wind tunnel experiment (Wang et al. 2012).

During the experiment, free-stream wind velocities of 8–22 m s−1 were used, increasing at intervals of 2 m s−1, and the wind velocity of each run was measured using a Pitot tube (Fig. 2). Each experimental run was continued until all erodible sample particles at a given wind velocity were removed. After each run, the sand trap was emptied and the total mass of the transported sediments was determined (balance precision, 0.001 g). All sediment transport values at a high wind speed are expressed as cumulative values that include the materials collected at all previous lower wind speeds. In addition, the wind erosion depth was measured using a three-dimensional laser scanner (EXAscan, Creaform, Canada). The scanning speed of the laser is 25,000 times per second with approximately 50,000 points per second. Resolution in the X, Y, and Z directions is 0.05 mm, and the maximum accuracy is 40 μm. After each experimental run, the three-dimensional laser scanner was used to produce a digital elevation model (DEM) of the sample surface, and the wind erosion depth was evaluated by calculating the difference between the DEMs.

Fig. 2
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Schematic diagram of the setup for the wind tunnel experiments

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2.2.2 Sample treatment and analyses

Following the wind tunnel experiments, the sediments collected in the sand trap were used for particle size analysis using a Mastersizer 2000 (Malvern, UK; sample diameter range 0.02–2000 μm) and nutrient analyses. Nutrient analyses included measurement of soil organic matter (SOM), total nitrogen (TN), total phosphorus (TP), and total potassium (TK). The SOM was calculated from the soil organic carbon content, which was determined by dichromate oxidation using the Walkley Black procedure (Nelson and Sommers 1982). Total N was measured using the micro-Kjeldahl procedure, total P was measured colorimetrically using a spectrophotometer following H2SO4–HClO4 digestion. and TK was measured using flame photometry following HF–HClO4 digestion. Since higher wind velocities transport progressively larger particle sizes, all particle size fractions and nutrient contents are expressed as weighted average values for the sediments collected at a given wind velocity and all lower velocities.

3 Results

3.1 Aeolian transport intensity

A summary of wind erosion depth and intensity is showed in Table 1. The results obtained from the three-dimensional laser scanner indicate that although both erosion and deposition occurred on the sample surfaces during the wind tunnel experiments (Fig. 3), there is a close positive correlation between erosion depth and erosion intensity (Fig. 4). At low wind velocities (8–12 m s−1), the mean wind erosion depth of the three samples is 0.83 mm with a mean erosion intensity of 30.6 g m−2, whereas at high wind velocities (14–16 m s−1), these values increase to 2.10 mm and 170.2 g m−2, respectively. At very high wind velocities (18–22 m s−1), the erosion depth and intensity are 5.10 mm and 420.5 g m−2, respectively.

Table 1 Summary of data produced by wind tunnel experiments conducted on sediments collected from three sites in the Gonghe Basin
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Fig. 3
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Spatial distribution of wind erosion depth caused by different wind velocities. Negative values of wind erosion depth indicate deposition

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Fig. 4
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Wind erosion intensity versus wind erosion depth. A strong linear relationship exists between the amount of material removed and the depth to which erosion takes place

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3.2 Particle sizes of the transported materials

The fine sediment fraction (<100 μm in diameter) accounted for between 50 and 90% of the original samples used in the wind tunnel experiments. At wind velocities between 8 and 22 m s−1, the fine-grained fraction of the transported sediments accounted for between 30 and 90% of the total sample, and this was dependent mainly on the composition of the parent materials. Furthermore, fine fraction transport displays little variation with increasing wind velocities and in some cases even exhibits a minor increase (Fig. 5). The particle size fractions of the eroded materials show that aeolian processes not only transport the fine fraction from the crushed surface soils but also transport large quantities of the coarse fraction (>100 μm in diameter), which may decrease the permeability of surface soils. In addition, after surface soils are eroded, the coarse fraction may accumulate downwind to form sand sheets or dunes, while the fine fraction may be involved in dust storms (Wang et al. 2004) that have the potential to affect ecosystems well outside the Gonghe Basin (Reynolds et al. 2001; Soderberg and Compton 2007; Okin et al. 2008).

Fig. 5
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Comparison of the particle size distribution of the initial surface soil samples and material transported at different wind velocities during the wind tunnel experiments

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3.3 Nutrient loss through aeolian processes

The average SOM, TK, TP, and TN contents of the materials eroded during the wind tunnel experiments are 37.88, 14.49, 0.31, and 1.89 g kg−1, respectively. The corresponding values measured in the initial sediment samples are 46.95, 15.20, 0.52, and 2.65 g kg−1, respectively (Table 2). These results show that compared with the initial surface soil samples, there are some slight decreases in nutrient content for the transported materials, which may be due to the fact that transported materials contain a greater proportion of low-nutrient, coarse-grained material. Statistical analysis indicates that there are significant positive correlations between wind velocity and removal of TP and TK, while no similar correlation is obtained between wind velocity and removal of SOM and TN (Fig. 6). Our results show that aeolian processes influence the correlation between nutrient contents and particle compositions. For example, there are significant correlations between particle size and the fine fraction content of SOM, TK, and TN, and the correlation coefficients are significantly higher for the transported materials (Table 3). This indicates that the fine fraction of the transported materials may have a higher nutrient content than the initial surface soils. The results of the wind tunnel experiments show that a high wind velocity (≥16 m s−1) results in losses of SOM and TN totaling 7.56 and 0.38 kg ha−1, respectively, and under very high wind velocities (≥22 m s−1), the losses were 26.99 and 1.41 kg ha−1, respectively.

Table 2 Summary of total soil organic carbon (SOM), total phosphorous (TP), total nitrogen (TN), and total potassium (TK) contents of sedimentary material removed during the wind tunnel experiments
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Fig. 6
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Wind velocity versus normalized nutrient content. Nutrients include organic matter, total potassium, total phosphorous, and total nitrogen

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Table 3 Pearson’s correlation coefficients between particle size fraction and soil organic matter (SOM), total nitrogen (TN), total carbon (TC), and total phosphorous (TP) of both the initial soil sample and the material transported during the wind tunnel experiments
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4 Discussion

Given the importance of the Qinghai–Tibet Plateau for regional ecology, water supply in Asia, and the global climate system, land degradation in this region has attracted much attention. Previous studies have focused mainly on either the effects of climate change on temporal trends in land degradation in this region (e.g., Feng et al. 2011; Wang et al. 2015) or on the influence of one or two plant species on regional ecological rehabilitation (e.g., Tian et al. 2015). At present, the extent and magnitude of land degradation on the plateau remain largely unknown, and the causes of degradation remain uncertain. The impacts of human activities such as cultivation and animal husbandry are still not sufficiently understood (e.g., Harris 2010). Some of the sites sampled in the Gonghe Basin comprised sediments containing 70.9% fine fraction (<100 μm) (Table 4). The substantial fine fraction content and vegetation cover (e.g., Livingstone and Warren 1996) ensure that high wind velocity has a low impact on intact surface soils. It is only after conversion to agriculture that the surface soils become completely disaggregated and large quantities of sediment can be eroded by wind in this region. In addition, the average contents of SOM, TK, TP, and TN in surface soil samples are 28.5, 18.7, 0.4, and 1.5 g kg−1, respectively. The results of the wind tunnel experiments indicate that aeolian processes can trigger substantial loss of nutrients that are critical to maintaining healthy plant communities.

Table 4 Summary (mean ± SD) of the particle size fractions, soil organic matter (SOM), total potassium (TK), total phosphorus (TP), and total nitrogen (TN) content of initial soil samples
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Over the past decades, soil nutrient loss and desertification on the Qinghai–Tibet Plateau have mainly occurred in the 1970s and mid-1980s when the intensities of human activities were low (Dong and Chen 2002; Liu et al. 2008). However, although wind velocity decreases and temperature increases over recent decades have been suitable for ecological rehabilitation (Wang et al. 2013, 2015), increased cultivation and grazing intensity have destroyed soil structure and resulted in the export of large quantities of fine fraction and concomitant nutrient loss from the soil, which may trigger widespread desertification in the region. For example, in 1987, the total sowing area and the meat output in this region were 61,330 ha and 17,336.1 t, and in 2012, it was 93,760 ha and 53,587.0 t, respectively (Fig. 7), which showed over the past decades there was significant increase of cultivation and grazing. In this region, the increase in human activities combined with the influence of aeolian processes has led to large amounts of nutrients that are transported away from the surfaces, which consequently triggers the desertification. Therefore, although the global warming trend has fostered ecological rehabilitation on the Qinghai–Plateau from the 1990s to the present (e.g., Wang et al. 2015), significant land degradation occurrence linked to human activity after the 1990s has been observed in this region (Li et al. 2013; Yin et al. 2014).

Fig. 7
figure 7

Temporal trends in human activity around the Gonghe Basin (indicated by total cultivated area and total meat output). Data are from the Qinghai Province Statistical Yearbook

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5 Conclusions

A combination of high wind velocities and human activity (e.g., conversion to cropland and increased grazing intensity) may trigger soli nutrient loss and rapid desertification of the Gonghe Basin, Qinghai–Tibet Plateau. Our wind tunnel results demonstrate that after grassland soil surfaces are disaggregated by cultivation and animal trampling, sediment removed by low to medium wind velocities (≤12 m s−1) range from 6.13 to 129.87 g m−2 and wind erosion depth varied from 0.21 to 1.72 mm. High wind velocity (≥16 m) removes between 294.02 and 1012.73 g m−2 of sediment, and the wind erosion depth varies between 4.09 and 11.27 mm. Furthermore, high wind velocities result in losses of total SOM and TN of 7.56 and 0.38 kg ha−1, respectively, and very high wind velocities (≥22 m s−1) result in losses of SOM and TN of 26.99 and 1.41 kg ha−1, respectively. The data show no significant correlation between wind velocity and the proportion of SOM and TN removed; however, positive correlations are observed between wind velocity and the removal of TK and TP. These results indicate that aeolian processes may remove proportionally more TK and TP from the soil surface as wind velocity increases, which demonstrate that K and P are enriched within the fine particle fraction and that nutrient loss caused by aeolian processes may result in farmland and grassland degradation and consequently trigger desertification in the region.