Agriculture Reference
In-Depth Information
Table 19.2.
Global extent of severe water and wind erosion. (From Lal, 2003.)
Land area affected by severe erosion (Mha)
Total as a
per cent of
total land use
Region
Water erosion
Wind erosion
Total
Africa
169
98
267
16
Asia
317
90
407
15
South America
77
16
93
6
Central America
45
5
50
25
North America
46
32
78
7
Europe
93
39
132
17
Oceania
4
16
20
3
World
751
296
1047
12
•
Water erosion
occurs when topsoil is
removed and rills and gullies are formed;
waterborne SOC losses also include
leaching and translocation as dissolved
(DOC) or particulate organic carbon
(POC). Not all effects are, however,
negative as water erosion may improve
downstream soil fertility, as in river
deltas that would not exist without up-
land erosion.
•
Wind erosion
is caused by a decrease of
vegetation cover due to overgrazing or
land conversion to arable agriculture,
leading to loss of topsoil, deflation hol-
lows and dunes, and overblowing.
•
Chemical degradation
includes the loss
of organic matter by oxidation and min-
eralization following; for example, till-
age or drainage.
•
Physical degradation
includes subsid-
ence of organic soils by consolidation,
oxidation, compaction and shrinkage
(DenHaan
et al
., 2012).
• deforestation,slashandburntraditions
and burning of crop remains, which
hampers soil carbon replenishment;
• drainageofwetlands;thatis,applying
'semi-desert' agriculture to organic soils;
worldwide peatland drainage causes
carbon-rich peats to disappear at a rate
20
times faster than it took them to ac-
cumulate (Joosten, 2010);
• changingtheCqualityofthecropbio-
mass compared to the native vegetation;
• removalofharvestedproductsandde-
creasing crop residues by more efficient
harvesttechniquesandincreasingthe
production of silage;
• tillage,levellingandotherformsofsoil
disturbance, which destroys physical
structure, exposes SOC to decompos-
ition and enables erosion of the C-rich
topsoil by water and wind;
• summerfallowingandcleancultivation;
• soilcompactionbyheavymachinery,
high livestock and agricultural mechan-
ization (repetitive hoeing and ploughing,
resulting in a compacted layer at hoe/
plough depth);
• increasedmicrobialrespirationbecause
of higher fertilization levels; and
• excessiveuseofpesticidesandother
chemicals (Harrison
et al
.,1993;Lal,
2002;McLauchlan,2006;Smith,2008;
Joosten
et al
., 2012).
Since the start of settled agriculture
around
10,000
years ago, agricultural ac-
tivities have led to fundamental changes
in the pools and fluxes of carbon (McLauch-
lan,2006;McNeillandWiniwarter,2008).
Traditional agricultural systems are based
on 'fertility mining'; that is, enhancing
the decomposition of SOC, which releases
plant nutrients and allows the land to
produce more than its natural capacity
(Janzen, 2006; Lal, 2012). Agricultural
practices that lead to SOC depletion
include:
These factors may be self-amplifying; for
example, when changing vegetation and
soil reflectance alters the temperature re-
gime of the soil, or when decreasing soil
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