Chemistry Reference
In-Depth Information
formation:
26
CL, O, R, P, T (CLimate, Organisms, Relief, Parent-material, all
as a function of Time). However, there is growing recognition that
anthropogenic activity has a strong influence on soil development and
formation, which is more commonly being referred to as anthropogenic soil
change.
27
Estimates for the next 50 years indicate that mankind is moving to a
global density of 1 person for each 0.01 km
2
of reasonably biologically
productive land.
28
This increase in population pressure means that we must
continue to extract more from our soils to support the growing demand. This
section focuses on how mankind's interaction with the land, both directly and
indirectly, results in soil change, the consequent alteration of soil stocks-and-
flows, and the delivery of ecosystem services on time-scales of relevance to
policy making.
d
n
1
r
2
n
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1
2.1 Direct Drivers of Soil Change
Tillage, traffic, irrigation, fertilisation and pesticide application are perhaps the
main drivers of direct anthropogenic soil change and are mostly related to food
production. Of the Earth's terrestrial land surface, ca. 134 million km
2
(Mkm
2
), arable agriculture was estimated to cover 15 Mkm
2
, and managed
grazing
29
28 Mkm
2
, whilst the amount of land irrigated was estimated to be 2.7
Mkm
2
in 2000.
30
This means that ca. 38% of the Earth's ice-free land surface is
currently used for agriculture, with this land replacing forests, savannas and
grasslands.
31
2.1.1 Physical and Biogeochemical Soil Change as a Result of
Food Production
Tillage changes a range of soil properties including structure, density, porosity,
moisture, aeration, greenhouse gas emission
32
and temperature; chemical
properties, e.g. carbon content; and biological structure, e.g. earthworm
population and fungal/bacterial ratio.
33
Figure 3, reprinted from ref. 31, shows
the impact of tillage on soil organism groups, indicating that the larger soil
organisms, such as worms, spiders and beetles, are much more susceptible to
tillage activities. A recent meta-analysis of data indicated that when land use
changed from forest to crop it resulted in a 42% reduction in soil C stock,
whilst from pasture to crop was a 59% reduction.
34
Not only does this mean
that this C is released into the atmosphere, but it also means that soil structure
declines, as does the ability of the soil to retain nutrient and water stocks.
Moreover, soil organic matter provides energy for the soil biota which are
primarily responsible for the biogeochemical cycling in soils and the delivery of
important ecosystem regulating services such as filtration and waste disposal.
Traffic, both by vehicles and animals, causes soil compaction which increases
soil strength and decreases soil physical fertility through decreasing storage
and
supply
of
water
and
nutrients.
The
global
extent
of
vehicular
soil
compaction is estimated to affect 0.68 Mkm
2
of land;
35
that due to animals is
