Wind Erosion of Agricultural Soils and the Carbon Cycle - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 162

and energy in the global context (Shao
et al ., 2011). In longer timescales, dust also
has modifying influences on the climate
(Martínez-Garzia et al ., 2011). Deserts are the
major global source of dust, but also the sur-
rounding semi-arid steppe ecosystems have
changed in the last decades from former
sinks into substantial source regions. Causes
have always been inappropriate land use such
as the destruction of vegetation by overgrazing
or the conversion from grazing land to crop-
land. Because steppe soils represent one of
the largest carbon stocks, its degradation and
the loss of this area as a sink have a signifi-
cant influence on the global carbon balance.
On agricultural land, wind erosion is a
soil-degrading process, but quantitative data
are rather approximate estimates. Heavy sand
and dust storms attract attention by disturb-
ing the public once in a while, but in general,
the processes mostly happen unnoticed.
Chepil (1960) has already pointed out that
annual average soil losses of up to 40 t ha - 1
are possible without any visible indications
of soil movement. Wind erosion has been
recognized as a gradual soil-degradation pro-
cess, which removes predominantly the fin-
est and most valuable particles of a soil,
such as silt and clay particles, as well as the
soil organic matter. In addition to the creep-
ing degradation, single wind-erosion events
may result in soil losses of more than 100 t ha − 1
and cause considerable on-site and off-site
damages (Funk et al ., 2004; Goossens and
Riksen, 2004; Hoffmann et al ., 2011). In the
last decades, the spatial extent of wind ero-
sion has increased, caused mainly by changes
of agricultural land use and inappropriate
farming practices. Increasing demands on
food production have expanded arable land
use to marginal sites such as natural grass-
land or forest. Some more factors favouring
wind erosion on arable land are:

• Overgrazingisasigniicantcausative
factor in the semi-arid regions, where
no other type of land use is possible
(Frielinghaus and Schmidt, 1993; van
Lynden, 1995; Riksen et al ., 2003;
Hoffmann et al ., 2008a).

Wind erosion has been regarded mainly as a
soil-removing process, but also soil-particle
sorting, long-distance nutrient transport, fer-
tilizing aquatic and terrestrial ecosystems
far away from the origin or the increase of
soil heterogeneity are parts of the problem,
with local to global consequences on soil
properties and the carbon cycle as well.
Erosion and deposition processes both take
place on large areas and are therefore diffi-
cult to identify. In contrast to water erosion,
where the eroded material follows deter-
mined paths, wind-eroded material is widely
dispersed over the landscape. These direct
and indirect effects are difficult to evaluate
entirely and the current ongoing debate is
whether soils, affected by erosion, act as a
source or a sink for carbon. In agricultural
soils, organic carbon is concentrated in the
top layer (0-30 cm) and therefore vulnerable
to wind erosion in its particulate form (par-
ticulate organic matter: POM) because of
its lower density compared to the mineral
particles (Lal, 2003). Cultivated organic soils
are most susceptible to erosion and to mas-
sive decomposition processes. Mineral soils
cover the wide range from high-erodible
sandy soils to non-erodible loam and clay
soils. The erodibility of a soil is attributed
mainly to the texture and organic matter
content, which in turn influence the water-
holding capacity and the ability of the soil to
produce aggregates or crusts (Chepil and
Woodruff, 1961). In general, sandy soils are
highly erodible because they dry quickly,
form only a few, weak aggregates and con-
tain a great part of particles in the most
erodible fraction of 80-200 μm in diameter.
Loamy soils are more resistant against wind
erosion but have a greater potential for dust
production (particles < 60 μm) if they erode.
Due to the low net primary production of
most regions affected by wind erosion, re-
moved carbon (C) can be regarded as an irre-
trievable loss at the eroded site (Yan et al ., 2005).

• Ahigherlevelofmechanizationhasled
to larger fields and, in return, to the re-
moval of hedges and other landscape
structures.
• Thedrainageofarablelandcausesfaster
drying of the soil surface, resulting in
organic matter decomposition and de-
creasing soil aggregate stability.