1. Any cultivation of "biomass crops" or trees for the purpose of energy
production is almost certain to increase atmospheric carbon. Land will
sequester the maximum amount of carbon per acre when the land is left
in the local "climax community" of forest or grassland. Any sustained
harvesting of organic matter for energy production reduces the per-acre
stock of stored carbon by at least the amount harvested per acre for as
long as the harvesting continues.
2. In most ecosystems (other than rain forests), most of the
sequestered carbon will be in the debris layer and in the soil organic
matter. Most cultivation and harvesting methods for energy crops, as for
other crops, increase soil exposure (especially when plowing and after
harvesting) and result in semi-permanent reduction of soil organic matter
as well as loss of the harvested crop.
3. In extreme cases, biomass harvesting can reduce the soil organic
matter to negligible amounts, and the "crop" may be totally removed at
harvest before replanting. In such a system, the carbon storage of the
land would be at most a small fraction of what it would be in a natural
state, and that storage would be completely removed and consumed
every few years. The undesirable consequences could be compounded
by erosion, sedimentation, loss of species diversity, etc. Theoretically,
then, a "renewable biomass energy system" could almost completely
eliminate the carbon storage value of the land producing it.
3. These unwanted consequences could be limited by harvesting
perennial trees or other plants in such a way that the surface debris
"mulch" and the root systems are left intact for natural regeneration, thus
helping preserve the unharvested part of the sequestered carbon.
4. Harvesting energy from the organic RESIDUES of human activity has
much less potential to reduce natural stores of sequestered carbon than
biomass cropping has. However, there is an opportunity cost where
energy is harvested from wood, paper, yard trimmings, or similar
materials (that could have been re-used or recycled to preserve their
status and utility as stored carbon or returned to natural carbon stores in
the form of mulch or compost).
5. There are any number of variables that affect the storage of carbon
and the impact of greenhouse gas releases. Methane is a much more
potent form of greenhouse gas than CO2, so any practice that leads to
large uncombusted methane releases is a particular problem. Peat bogs,
one of the few natural systems that sequester more and more carbon
year after year, also release a fraction of that carbon in the form of
methane. I doubt anyone has ever calculated the net greenhouse effect
of peat bogs.
6. Aerobic composting releases a large part of the composted organic
matter as CO2 -- at least where a lot of high-carbon materials are put
through the composting process for an extended period of time. Some
thought might be given to using such materials in composting only to the
extent they are needed as bulking agents for high-nitrogen residues, and
thus preserving them as much as possible to be used as mulches. As
mulches, they will do more to protect and regenerate soil organic matter
than the same material would do after being converted to a finished
compost (and being largely lost to C02).
7. Surplus organic matter is generally more valuable to help restore lost
organic matter and microbial life in depleted soils than it is for the few
calories of heat energy it would yield. There are potentially many
specific instances in which using such biomass as an energy source is
justifiable, but careful thought should be given before committing large
land areas or large stocks of residual biomass to energy uses.
Watershed Management Team