[permaculture] Soil Carbon Sequestration
Charles de Matas
cdematas at hotmail.com
Fri Jul 6 14:31:42 EDT 2007
Yoeman seems to think nuclear is necessary also, which I don't agree with.
I agree with Paul's statement: "I would suggest that the alt. ag. methods
resulted in a recovery of soil organic matter, but that regeneration to
forest would have provided a greater rate."
To sequester enough CO2 in the soil we would have to completely reverse
deforestation worldwide. Wetlands and tropical forests that have high
rainfall would have to be completely protected from human activity. I don't
see any signs of this happening soon.
Anyway here is the passage from Dr Youngs book:
Agroforestry and the greenhouse effect
It has been suggested that the wider adoption of agroforestry could help to
reduce the build-up of atmospheric carbon dioxide and other greenhouse
gases (Dixon et al. 1994; Schroeder, 1994; Dixon, 1995, 1996). Possible
means by which such a reduction could come about are:
- taking up carbon into trees on farms;
- increasing soil carbon;
- indirectly, by reducing pressure for forest clearance.
Widespread adoption of the growing of more trees on farms would lead to a
temporary increase in carbon storage, followed by its maintenance at the
higher level. The same changes would follow increases in soil organic matter
brought about by the adoption of agroforestry. Potential changes of this
kind are, however, small in relative magnitude. Soils are the largest pool
of terrestrial carbon, estimated at 2200 Bt (billion tonnes=10^9 tonnes) in
the top 100cm. Of this total, 1500 Bt is organic carbon and 700 Bt is in
the form of carbonates; about one-third is in the tropics. An estimated 900
Bt is in topsoils, 0-30 cm (Batjes, 1996). Let it be supposed that a very
widespread adoption of agroforestry and other methods of improved soil
management in the tropics led to a relative increase in topsoil carbon of
10%. Since tropical soils contain only 280/2200 or 13% of world soil
carbon, this relative change would amount to a relative increase in soil
carbon storage of about 1%. The contribution from the temperate zone would,
on a realistic assessment of current potential for adoption, be very much
smaller. The effect of agroforestry in raising soil organic-matter levels
is therefore in the right direction in this respect, but small in relative
The indirect effects through the other environmental means noted above can
be very much greater. In particular, through reducing land pressure and
thus forest clearance, it can help to reduce a major source of carbon
dioxide emissions. Taking the same assumptions on agroforestry and forest
clearance as made above, the potential for a reduction of forest clearance
by 5 Mha/year would lead to a lowering of carbon emissions from forest
burning from 1.6 to 1.1 Bt/year (Dixon, 1995). The assumptions on which
this estimate is made are very optimistic, but sufficient to show that a
contribution of significance is possible.
Batjes, N.H. (1996) Total carbon and nitrogen in the soils of the world.
European Journal of Soil Science 47, 151-163.
Dixon, R.K. (1995) Agroforestry systems: sources or sinks of greenhouse
gases? Agroforestry systems 31, 99-116
Dixon, R.K. (1996) Agroforestry systems and greenhouse gases. Agroforestry
Today 8(1), 11-14.
Dixon, R.K., Winjum, J.K., Andrasco, K.J. et al. (1995) Integrated land use
Climatic Change 30, 1-23.
Schroeder, P. (1994) Carbon storage benefits of agroforestry systems.
Agroforestry Systems 27, 89-97.
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