Geoscience Reference
In-Depth Information
is emitted as carbon dioxide. Any disturbance to the trees
by poor forestry practices, fires or disease will lead to soil
warming, and more carbon dioxide will inevitably be
released into the atmosphere.
Soil organic matter
Twice the mass of carbon is held in soil organic matter
as in above-ground vegetation or in the atmosphere,
therefore changes in soil carbon content have a large
effect on the global carbon budget (see
Chapter 21).
The likelihood of climate change being reinforced by
accelerated CO
2
emissions from soils, owing to rising
temperature and enhanced decomposition, has been
debated for some time. It is 'debatable' because the
evidence for this positive feedback mechanism is based on
small-scale laboratory and field experiments, and from
computer modelling studies. Now, however, a unique
database of soil carbon values for British soils for
1978-2003 is available from work at the National Soil
Resources Institute (NSRI), Cranfield University (Bellamy
et al
. 2005). It shows that during this period carbon was
lost from the soils of England and Wales at an average rate
of 0.6 per cent yr
-1
relative to 1978 values. Relative rate of
decline is directly proportional to soil carbon content, and
is over 2 per cent yr
-1
in peaty soils and peats with carbon
contents over 10 per cent.
Over the same period, the mean temperature of
England and Wales increased by about 0.5
Soil organisms
Soil organisms play a central role in organic decompo-
sition, nutrient cycling, and structure/porosity formation,
all of which affect soil fertility and sustainability. Soil
organisms are very sensitive to changes of temperature,
moisture and vegetation type. They are also affected by
increases in the biomass of fine roots and in litter supply.
The impact of climate change will be large and rapid
because soil organisms migrate only slowly. Experiments
using controlled atmospheres show that the universal
response to higher CO
2
levels is increased below-ground
growth, especially of fine roots. This increases the
numbers and activities of soil bacteria, giving enhanced
production of soil aggregates, increased acidification and
weathering. The nitrogen cycle is stimulated, leading
to increased nitrogen fixation, and greater nitrogen
mineralization and denitrification. Stimulation of soil
fungi brings increased importance to mycorrhizal associa-
tions. The biodiversity of the soil microbial community
under climate change depends very much on the quantity
and quality of the soil organic matter. Any decrease in its
quantity and quality would quickly lead to a decline in
biodiversity.
Soil organisms are one of the least understood compo-
nents of soils. Their importance is recognized, but there
are gaps in knowledge about their numbers, biodiversity
and feedback mechanisms. Most studies have been in the
laboratory or in the greenhouse, using artificial soils, well
supplied with nutrients. When given elevated carbon
dioxide, such systems respond with increased biomass, but
these are artificial conditions because there is neither
chance nor time for microflora to evolve. It is difficult to
extrapolate these conditions to natural ecosystems, where
there will be more time for microflora to respond to
increased carbon in the root system.
C, an increase
which will increase rates of organic matter decomposition
by soil microbes, and will also interact in a complicated
way with changes in soil moisture brought about by
changing rainfall and evapotranspiration. Warmer, drier
soils might have reduced rates of decomposition if soil
moisture were to become limiting to soil organisms, but
in wet, anaerobic soils, evaporation increases the depth to
water table, and promotes increased decomposition at the
higher temperatures.
The loss of carbon appears to be independent of
present land use, rainfall and soil texture, and therefore
climate change is strongly implicated. Some of the carbon
will be emitted to the atmosphere as carbon dioxide and
methane, and some leached into drainage water and
ground water, both of which record increases in dissolved
organic carbon (DOC) in recent years. Changes in
agriculture (drainage, conversion of pasture to arable,
and increased stocking rates) contribute to carbon loss,
as also do changes in non-agricultural uses (afforestation,
increased erosion, and increased moor burning in upland
Britain). However, as with many aspects of environmental
change, there are insufficient long-term data to analyse
these effects.
In countries where forests are a major land-use cover,
e.g. Canada, with 35 per cent forest cover, the mass of
carbon stored in forest soils is enormous. As long as the
forest canopy provides shade and lowers soil temperatures
the soil acts as a carbon store, collecting more carbon than
Thawing of permafrost
A special case of soil impact is the thawing of permafrost
as a result of climate warming in arctic/subarctic and
alpine/subalpine regions (see
Chapter 15,
pp. 371-83).
Most arctic countries have produced maps to show how
the continuous and discontinuous permafrost zones
retreat northward under various climate change scenarios.
However, as no country has mapped permafrost in any
detail, these maps are very speculative. Melting of the
upper layer of the permafrost will progressively increase
