Students and faculty at the University of Florida have been discussing the carbon cycle of biofuelsversus gasoline. Patrick O'Donoughue wrote this interesting synopsis:
The original question was whether plants, specifically
biofuel plants, absorb only atmospheric carbon over the course of their life.
If plants absorb carbon from the soil in addition to atmospheric carbon, then
biofuels would not be carbon neutral. In this case, the combustion of the
biofuel would re emit CO2 that was originally absorbed from the atmosphere and
emit new CO2 obtained from the soil.
After discussion, the general consensus is that plants do
not uptake any carbon from the soil. However, the situation contains a number
of complexities.
The first complexity is that the soil bacteria, when
converting organic matter into nutrients for the plant, respire carbon dioxide.
Thus, the decomposition of organic matter in the soil produces a flux of CO2 to
the atmosphere.
However, in mature forests and landscapes, the terrestrial
carbon cycle is in equilibrium; the amount of CO2 emitted from decomposition of
organic matter in addition to CO2 emitted from fires, is equal to the amount
absorbed in photosynthesis by the total landscape.
Therefore, the micro carbon cycle associated with the growth
of a biofuel plant, decomposition of organic matter, and combustion of the fuel
is essentially carbon neutral.
The big picture, though, is not necessarily carbon neutral.
The farming techniques, production of farm tools, inorganic fertilizer, use of
fossil fuel in tractors/maintenance equipment, processing of the biofuel,
production of digester equipment, transportation, distribution, etc. all
account for possible outputs of CO2.
But the biggest consideration is the question of how the
landscape changes to become a farm for biofuels. You can consider the vegetation
on the land as a store for atmospheric carbon. Some vegetation acts as a larger
storage tank. For instance, dense forests store more CO2 than grassy
landscapes. Thus, when humans change the landscape to create a farm for
biofuels, if the biofuel crops store less CO2 than the original landscape
vegetation, then they will not absorb all of the CO2 emitted by the original
vegetation (through decomposition, combustion, etc.). The question though, is
whether the CO2 from the original vegetation is actually emitted into the
atmosphere. For instance if it was a forest, the trees could be used for
furniture, preventing the CO2 stored in the trees from being reintroduced into
the atmosphere. This could then be considered a carbon negative process,
perhaps even a carbon sequestration technique. This is not a viable
sequestration technique, since the land would be depleted if the organic matter
was not returned to the ground. In the long term though, after the timber has
reached the end of its useful life, the CO2 is returned to the atmosphere, by
fire or by decomposition in the soil, where the bacteria respire the CO2.
The changing landscape accounts for another output of C02
that makes the production of biofuels carbon positive.
The total positive flux of C02 associated with biofuels is
miniscule in comparison to that of fossil fuels.
- Patrick O'Donoughue
(Patrick adds that he has "no concrete data to support the
statement that biofuels emit less carbon than fossil fuels when
considering the total life of the two fuels. The only way to do this is
through a life cycle assessment. I am unaware of any sources of an
assessment. I will do some more research to try and come up with
numbers, and perhaps come up with my own number."
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