Agriculture Reference
In-Depth Information
growth, inadequate (self-)regulation and in-
creasing demand for food has led to over-
grazing in many African savannah ecosys-
tems (Lal, 2002; Oztas, 2003; Smet and
Ward, 2006). The intensive use of these
grasslands has led to the loss of valuable
grass species (Glasscock et al ., 2005), deg-
radation of land cover, decreased carbon
stocks and the associated decline of soil bio-
logical, chemical and physical quality
(Teague, 2004; Noellemeyer et al ., 2006;
Savadogo et al ., 2007; Steffens et al ., 2008;
Gili et al ., 2010). Grazing reduced the total
organic carbon content in the soils of the
semi-arid rangelands of central Argentina,
and specifically in more sandy textures, the
carbon loss was noticeable (Fig. 3.4 ) .
Semi-arid grasslands have also been
converted extensively to arable agriculture
(Priess et al ., 2001; Nosetto et al ., 2005;
Coutinho et  al ., 2008). Especially in warmer
regions, these conversions have triggered rapid
and massive losses of soil organic carbon
(Zach et al ., 2006; Noellemeyer et al ., 2008).
case of extreme degradation of cropland re-
sulting from a combination of environmen-
tal factors and improper farming practices.
The area was covered by dense native
grasses with deep rooting systems that had
stabilized the loess (glacial windblown)
soils and maintained soil organic matter.
Deep ploughing to produce wheat crops
disrupted this cover and reduced the soil
organic matter contents and water-stable
aggregates, and exposed tiny loose particles
at the bare soil surface to wind erosion.
There were initially good yields associated
with a wet period in the 1920s, but when
this area experienced a major drought in
the 1930s, the dry soil was blown away as
dust. An estimated 12 cm of topsoil was
lost from 4   Mha within the dust bowl re-
gion (Montgomery, 2007). The resulting
collapse of the farming sector led to the lar-
gest population migration in the history of
the USA, involving hundreds of thousands
of refugees (Worster, 2004).
Elsewhere, soils formed on loess have
also exhibited widespread erosion, such as
in the semi-arid regions of South America
and South-east Asia (Chapter 13, this vol-
ume). Although erosion is a natural phe-
nomenon in a semi-arid climate, it has been
enhanced significantly by removal of the
natural vegetation and cultivation (Shi and
Shao, 2000).
Stage II: Low or collapsing
soil productivity
There are a number of examples of very rapid
soil fertility collapse (Stage II). The dust bowl
era in the great plains of the central USA is a
25
Grazed
Non-grazed
20
15
10
5
0
85
85
70
70
55
55
Sand (%)
Fig. 3.4. Total organic carbon (TOC) content of surface 20 cm soil in different textures of the central
Argentinean semi-arid rangelands under grazing and ungrazed. (From Gili et al ., 2010.)
 
 
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