Assessment of extreme wind erosion and its impacts in Inner Mongolia, China
Highlights
► We quantified extreme wind erosion on a fallow field by sizing a sediment fan. ► The average wind erosion rate was up to 340 t ha−1 within a single spring. ► The average dust production at the field amounted up to 136 t ha−1. ► Croplands contribute considerably to the total dust production in Inner Mongolia. ► Abandonment of tillage is strongly recommended to prevent extreme wind erosion.
Introduction
Wind erosion is a natural geological process and comprises the detachment, movement and deposition of particles smaller than 1 mm by strong winds (Skidmore, 1986). The impact of bouncing particles moved in the saltation mode can initiate the rolling of coarser sand particles and the emission of dust and its suspension in the atmosphere (Gillette, 1977, Shao and Raupach, 1993). Brought about by grain size sorting and the selective removal of fine particles, organic matter and nutrients, the accumulation of coarse mineral grains is a typical on-site effect of wind erosion. This process, gradually reduces organic matter content and with it the productivity of arable soils (Lal, 1998, Pimentel and Kounang, 1998). Major off-site effects of suspended particles by soil dust emission are the reduced air quality and impacts on infrastructure, particularly in populated regions (Saxton et al., 2001).
In North China, heavy and continuous grazing is responsible for ongoing grassland degradation and dust emission by wind (Hoffmann et al., 2008, Liu et al., 2003, Wang et al., 2005). Widespread dust emissions in the semi-arid grasslands of North China often cause or boost East Asian dust storms, which frequently reach humid regions several hundred kilometers away (Xu, 2006). Tanaka and Chiba (2006) estimated that the average annual soil dust emissions for East Asia are 214 Tg revealing its significance in the global dust budget.
About 10 million ha of croplands in China have been affected by desertification between the years 1949 and 2000 (China Water Resource News, 2001). The loessial soils of Inner Mongolia are fertile and of high potential productivity. It is therefore common to use parts of the grassland as cropping land despite the limited water supply and low spring temperatures. During the spring, both the soil erodibility of fallow soils and climatic erosivity (high wind speeds and low precipitation) are often increased. Cropping fields of Inner Mongolia are generally of large dimension and exceed 100 ha, a large proportion of which lies fallow during the windy spring season. While grass vegetation fragmented by heavy grazing provides little shelter from wind erosion, bare and ploughed soils are even more susceptible. Zhao et al. (2006) found that soils under crops in the ‘Horqin Sandy Lands’ in Inner Mongolia are degraded significantly and that plant growth was severely constrained by wind erosion. Frequent severe wind erosion events caused reductions in the clay, carbon, nitrogen and phosphorous contents of the top soil (Zhao et al., 2006). Wind erosion induced by intensive farming is therefore responsible for soil degradation and plays thus an important geo-ecological role in semi-arid grasslands of the world.
Quantifying sediment fluxes by wind erosion is difficult although most of the available field measurements currently facilitate evaluations of wind erosion models. To date, only few field-based studies have quantified loss of matter from cropping lands in semi-arid grasslands (Table 1): for an cropping land in the Horqin Sandy Land, Li et al. (2004) found transport rates of the wind erosion of 232 kg ha−1 d−1 on average and a maximum of 1254 kg ha−1 d−1. In a Chinese study, the annual soil loss from cropping land in semi-arid climates through wind erosion was estimated by grain size analysis (Dong and Chen, 1997). The estimated annual loss of 14–41 t ha−1 in this study is two to four times higher than that measured for grassland. On dry farmland in the Qinghai-Tibet-Plateau, an annual average wind erosion rate of about 31 t ha−1 were estimated by using the radionuclide 137Cs-method (Yan et al., 2001). At fields with conventional tillage in the semi-arid Shanxi Province (North China) wind erosion rates of around 30 t ha−1 were measured, compared to 6 t ha−1 on no-tillage fields (Cai et al., 2002).
Different studies on croplands in the Sahel zone estimated soil material losses by wind erosion. Bielders et al. (2000) measured erosion rates between 5 and 6 t ha−1 a−1, with highest soil loss of 8.3 t ha−1 from a single event and with a potential annual material loss of 79 t ha−1. Average material losses of 14 t ha−1 during 1 month and 45 t ha−1 for four wind erosion events were measured by Sterk et al. (1996) and Sterk and Stein (1997). Buerkert and Lamers (1999) measured a soil loss of 270 t ha−1 from a bare cropping land within a time span of 21 months. Deposition of 130 t ha−1 was observed over an adjacent residue-mulched cropping land during heavy storms between May and July. These transport rates however account for both wind and water erosion. At a fallow sandy field in Northeast Germany, soil loss rates of up to 105 t ha−1 by a single extreme event were measured (Funk et al., 2004).
Since many soil and climate factors affect the nature and intensity of wind erosion, the amount of material eroded by wind can vary by several magnitudes between locations. In the space of 1 h, these events can relocate the amount of material equivalent to the average annual total. To put outputs from wind erosion models as the ‘Wind Erosion Prediction System’ (WEPS, Hagen, 1991) and the ‘Revised Wind Erosion Equations’ (RWEQ, Fryrear et al., 2001) into a more realistic perspective extreme wind erosion events should be considered based on field measurements (Buschiazzo and Zobeck, 2008, Hagen et al., 1999).
Aeolian sand translocation creates sand patches, sand sheets and small dunes. On vegetated surfaces surrounding eroding source fields, sheets of eroded and accumulated sand develop. Wind erosion is a grain size sorting process. Saltating and creeping sand particles deposit within some meters of the source and fine sand is often transported in a short term suspension for several hundreds of meters. By contrast, large proportions of the very fine silt and clay particles are lost from the eroding site by dust emission. Hagen et al. (2007) reported that about one third of the suspended dust is deposited within the initial 200 m during wind erosion events. Both the covering of soil by coarse particles and dust production by wind erosion are two cost-intensive effects (Hu et al., 1995, Huszar and Piper, 1986).
In spring 2006 severe wind erosion was observed over a fallow cropping land in the upper Xilin River catchment (Xilingol grassland). Since the field was surrounded by grassland, the source and sink areas were clearly identified. A visible effect was the accumulation of loose sand particles. A sand sheet of several centimeters thickness formed near and on the cropping land, consisting of light and loose particles. The reasons for the severe wind erosion events were low precipitation in the month before, the great extent of the land lying fallow, and frequent very strong winds (Hoffmann et al., 2008).
The aim of this study was to quantify soil erosion and dust production after a period of extreme wind erosion by measuring the size and volume of an annual sediment fan. Based on the great amount of sediments we hypothesized that this combination of very strong winds during spring and loose and dry soil surfaces at fallow cropping fields in the semi-arid grasslands of Inner Mongolia could cause extreme erosion events, which significantly exceed the average values calculated by models or described in previous studies. Although cropping fields comprise less than 4% of the regional land use area (Fan et al., 2007), these fields obviously are major dust sources in the Xilingol grassland.
Section snippets
Site description and definition of the wind erosion season 2006
The semi-arid grassland of Xilingol is located in central Inner Mongolia about 400 km North of Beijing (Fig. 1). The region was formed on basalt plateaus and is covered mainly with fine-sand loess. Formed by aeolian deposits, the local soil is prone to the re-mobilization by wind.
Wind erosion was quantified for a cropping land in the upper reaches of the Xilin River (N43°33′, E116°38′, 1260 m asl). The field was used as cropping land since the 1980s and was cultivated with wheat and with rape. The
Size, shape and grain size composition of the sediment fan
The measurements and calculations showed evidence of massive material relocation from the field to the grassland during a single spring season. While the greatest part of fine material was lost by dust emission, it was mainly sand that accumulated in a low angle sediment fan. The sediment fan had a size of 257 ha and a volume of 27,600 m3 (Fig. 5, Table 3). Sediment mass over the total area of the fan was estimated to be 30,100 t, derived from the surface soil density of 1.09 g cm−3.
The thickest
Conclusions
Strong winds and high soil erodibility caused extreme wind erosion in spring 2006 on a fallow cropping field in Inner Mongolia. More than 44,600 m3 of soil material were moved by wind from a 147 ha sized cropping field within a windy and dry period of 2 month. The total wind erosion was between 323 and 340 t ha−1, while between 119 and 136 t ha−1 were emitted as dust. The accumulation of sand at the surface of the cropping and grass-land locally degraded soil quality. Croplands are thought to
Acknowledgments
This study is a part of the Sino-German Research Project MAGIM (MAtter fluxes in Grasslands of Inner Mongolia as influenced by stocking rate) funded by the Deutsche Forschungsgemeinschaft (Forschergruppe 536). We thank Prof. Xing Guo Han for giving us the opportunity to work at IMGERS.
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