World’s Soil Resources

Near-surface soil water content has long been recognized to have a significant effect on the threshold wind
velocity needed for wind erosion (Akiba, 1933; Chepil, 1956). Soil water acts to bind particles together to resist
the shearing force of wind on the particles. In addition, soil water affects vegetative growth, which also affects
wind erosion. Research has shown a time-dependent change in the controlling factors for sediment emission
and transport from soil water to wind speed (Wiggs, Baird and Atherton, 2004). The change of controlling
factors was found to be very sensitive to the prevailing water conditions and, for the sandy soil tested, took
place in a very short period of time. They found the soil water content where wind erosion commenced was
between 4 and 6 percent (Wiggs, Baird and Atherton, 2004). However, the effect of soil water on wind erosion
of dry soils is also sensitive to changes in air relative humidity (Ravi et al., 2006). Recent work on atmospheric
dust concentrations have confirmed this sensitivity, finding that dust concentration increased with relative
humidity, reaching a maximum around 25 percent and thereafter decreased with relative humidity (Csavina et
al., 2014). Climate-induced changes in hydrology and water may produce profound changes in wind erosion
and dust emissions as the soil erodibility is altered.
6.1.8 | Vegetation effects
The effect of vegetation on wind erosion is complex. In native conditions, the wind influences patterns
of vegetation and soils and these patterns, in turn, affect wind erosion at patch to landscape scales (Okin,
Gillette and Herrick, 2006; Okin et al., 2009; Munson, Belnap and Okin, 2011). In agricultural systems, the
vegetation is manipulated by managers and its effects vary spatially and temporally from non-managed
systems. The protective effects of vegetation are well known. A wide variety of methods and models has been
devised to describe the protective effects of vegetation. In general, as vegetation height and cover increase,
wind erosion of erodible land decreases. Vegetation affects wind erodibility by: (1) acting to extract momentum
from the wind and thereby reducing the wind energy applied to the soil surface; (2) directly sheltering the soil
surface from the wind by covering part of the surface and reducing the leeside wind velocity; and (3) trapping
windborne particles, so reducing the horizontal and vertical flux of sediment (Okin, Gillette and Herrick,
2006). Trapping of sediment leads to redistribution of nutrients and modifies surface soil properties such as
water infiltration rate and soil bulk density.
Vegetation cover affects nutrient removal, which in turn affects plant productivity. A study of the effects
of grass cover on wind erosion in a desert ecosystem found increased wind erosion removed 25 percent of the
total soil organic carbon and nitrogen from the top 5 cm of soil after only three windy seasons (Li et al., 2007).
Studies of agricultural crops on severely eroded cropland found 40 percent reductions in cotton and kenaf
yields and 58 percent reduction in grain yield in sorghum (Zobeck and Bilbro, 2001). The eroded areas in this
study had statistically significantly less phosphorus than the adjacent non-eroded areas. Climatic changes
that reduce the cover of vegetation in drylands will increase wind erosion and dust emissions, and likely result
in increased soil degradation and reduced plant productivity.
6.1.9 | Alteration of nutrient and dust cycling
Recognition of a dust cycle, along with other important cycles such as the energy, carbon and water cycles,
has become an emerging core theme in Earth system science (Shao et al., 2011). Dust cycles are dependent
upon the soil and climate systems within which they operate. The dust cycle is a product, in part, of the soil
system. As dust is transported globally, it interacts with other cycles by participating in a range of physical
and biogeochemical processes. The dust carries important nutrients to otherwise sterile soils and so may
improve productivity (Chadwick et al., 1999; Mahowald et al., 2008). Dust may also transport soil parent
material (Reynolds et al., 2006), trace metals (Van Pelt and Zobeck, 2007), soil biota (Gardner et al., 2012) and
toxic anthropogenics (Larney et al., 1999) among ecosystems. Although the fact is not widely recognized, the
global dust cycle is intimately tied to the global carbon cycle (Chappell et al., 2013). Wind and water erosion
both redistribute soil organic carbon within terrestrial, atmospheric and aquatic ecosystems. This carbon is
selectively removed from the soil. This was recently demonstrated in an Australian study where the soil organic
carbon in dust was from 1.7 to over seven times that of the source soil (Webb et al., 2012). Changing climate will
alter these cycles, producing complex and uncertain environmental effects.
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