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 Soils
around the world contain approximately 2000 billion tonnes of carbon
in various forms at any one time. About 300 billion tonnes can be
found as detritus in the top soil, with this carbon rich material
decomposing at varying rates depending on factors such as temperature
and soil conditions.
During this decomposition some of the carbon in soil
detritus is respired by the decomposing organisms (often fungi and
bacteria), with the carbon being returned to the atmosphere as carbon
dioxide.
The rest of the detritus carbon can be converted into
'modified soil carbon' which decomposes more slowly and so keeps
the carbon from the atmosphere for longer. A small amount of this
carbon is further decomposed to so called 'inert' carbon, as noted
in the Biomass Burning Carbon dioxide
page, and this can remain locked away from the atmosphere
for over a thousand years.
Human Impact
As discussed in the page on Land-use
change related carbon dioxide emissions, man's conversion
of soils from natural to agricultural use has led to substantial
losses in the soil carbon sink. Greater soil disturbance, such as
that caused by ploughing, can cause rapid respiration and loss of
large amounts of soil carbon which would otherwise decompose more
slowly.
Potential for control
Sensitive land-use practice is key to better balancing the soil
carbon sink, and perhaps reversing recent trends of loss of carbon
from soils. Farming practices such as 'no-till', whereby agricultural
land is used without the soil disturbance and carbon loss which
comes with ploughing, are becoming more widespread and land-use
remains a key area of research in studies of man-made greenhouse
gas emissions and strategies to reduce them.
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