Chemistry Reference
In-Depth Information
have exacerbated rates of erosion.
5
Various properties of soil influence its
resistance to erosion. Surface cover by vegetation decreases the impact of
raindrops on soil and buffers overland flow.
50
Soil particles are enmeshed by
roots and fungal hyphae that are broken by mechanical disruption from soil
cultivation.
51
Compounds exuded by soil biota increase the adhesion and
hence aggregation of soil particles.
52,53
Rearranging soil particles and stirring
in oxygen by cultivation increases the rate at which organic compounds are
mineralised, resulting in decreased amounts of soil organic carbon.
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Erosion
resistance and soil organic carbon are closely correlated properties of soil.
d
n
1
r
2
n
g
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2.2.1 Extent of Erosion
Agriculture accounts for about three quarters of soil erosion globally. As
Figure 1 illustrates, the problem is widespread, with about 80% of agricultural
land plagued by moderate-to-severe erosion.
50
These figures are worrying and
have resulted in the phrase 'Peak Soil' coming into use. Civilisations have
fallen due to soil erosion. From 4000-2000 BC, Uruk in southern
Mesopotamia had up to 80 000 residents and was the first recorded city.
55
Overgrazing by livestock and forestry prompted erosion and poor soil
drainage. Flooding was commonplace, with extreme events turning vast areas
of the surrounding terrain into a sea with the city serving as a refuge. It was the
birth of the story of Utnapishtim, a flood hero similar to Noah. By 2000 BC,
food shortages due to soil erosion and salinisation caused Uruk to become
uninhabitable. The inhabitants of Easter Island, the Mayans and others had a
similar fate.
In modern agriculture, a global compilation of erosion studies found that
ploughed fields eroded 1-2 orders of magnitude more rapidly than soil was
formed.
13
The human-induced impact caused by soil cultivation and vegetation
removal was evident from this broad analysis of available data. Cultivating soil
for agriculture caused a median increase in soil erosion that was 18 times
greater than the level experienced under natural vegetation. The rate of soil
loss versus soil formation has been characterised to predict longer term trends
in soil sustainability. In Europe the upper limit of soil erosion based on
formation rates ranges from 0.3 to 1.4 t ha
21
yr
21
.
56
The actual level of soil
erosion in Europe, however, is 3 to 40 t ha
21
yr
21
, which is comparable to
global estimates of soil loss rates at 10-40 times greater than the rate of soil
formation.
50
Of the 1966 Mha of land estimated to be physically degraded, water erosion
is responsible for 1100 Mha and wind erosion for 550 Mha. Extrapolating the
degree of degradation further, approximately 225 Mha of soil has been
degraded by water erosion to such an extent that it is unsuitable for
agriculture. It is estimated that the primary cause of soil lost through wind
erosion results from overgrazing, accounting for 60% of soil losses.
The stripping of soils of vegetative cover for construction or mining
activities can cause erosion of soil from 20 to 500 t ha
21
yr
21
.
50
Most countries
