Sustainability: Humanity Perspective - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 767

layers is maintained, may prevent the decomposition of the organic carbon pool in deep
soil layers in response to future changes in temperature. Any change in land use and agri-
cultural practices that increases the distribution of fresh carbon along the soil profile could
however stimulate the loss of ancient buried carbon (Fontaine S, Barot S, Barre P, Bdioui
N, Mary B, Rumpel C. Stability of organic carbon in deep soil layers controlled by fresh
carbon supply. Nature, 2007; 450: 277-U10). Managed forest stands on 80-year rotations
stored only half the carbon of old-growth forests (Janisch JE and Harmon ME. Succes-
sional changes in live and dead wood carbon stores: implications for net ecosystem
productivity. Tree Physiology. 2002; 22:77 e 89.). Therefore, if all the forests were converted
to productive woody biomass field, there would be a net CO 2 input into the atmosphere
(if the ocean and open land did not take in the then available excess CO 2 ). A well-
managed woody biomass land will have a net CO 2 contribution to atmosphere if it was
converted from natural forest. Still the net CO 2 contribution to the atmosphere would
be the lowest when steady state (biomass harvesting rate equal to growth rate) was
reached than other crop operations simply because of its unparallel high standing
mass. When steady state is reached (80 years into the future for an 80-year rotation forest
scheme), woody biomass becomes truly carbon neutral, although the CO 2 level in the
atmosphere will be higher than what we know today. The consequence of not starting
using woody biomass today would be simply increasing the CO 2 in the atmosphere to
an even higher level.
Fig. 15.12 shows that the initial carbon debt (of 9 tons or 9 Mg) due to the use of woody
biomass instead of fossil energy is shown as the difference between the total carbon har-
vested for biomass (20 Mg-C) and the carbon released by fossil fuel burning (11 Mg-C) that
produces an equivalent amount of energy. The carbon dividend is defined as the portion of
the fossil fuel emissions (11 Mg-C) that are offset by forest growth at a particular point in
time. After the 9 Mg-C biomass carbon debt is recovered by forest growth (year 32), atmo-
spheric GHG levels fall below what they would have been had an equivalent amount of
energy been generated from fossil fuels. This is the point at which the benefits of burning
biomass begin to accrue, rising over time as the forest sequesters greater amounts of carbon
relative to the typical harvest.
Fig. 15.12 shows that zero net CO 2 emission would not occur after 100 years if one
wishes the biomass to grow to its original level before harvesting. Fig. 15.11 (and
Fig. 15.12 ) shows that biomass incremental growth is slow initially, very rapid in the
middle of the biomass life span, and then slows down dramatically when saturation
biomass is nearly reached. Is it economical or beneficial to harvest the woody biomass
when saturation biomass has reached? If we choose this route, the productivity of the
forest would be very low (incremental growth realized is nearly zero or negligible).
This would be the scenario that woody biomass would not be renewable. A healthy
woody biomass industry would be one that maximizes the productivity of the forest
(if other parameters are fixed). This would not be corresponding to the case of near
zero incremental growth. Referring in Fig. 15.12 ,harvestingatyear70,forexample,would
maintain a high-biomass productivity for the stand. However, either increased size of the
stand or more biomass removal at harvest must be employed to receive the amount of
woody biomass needed to generate that the extra 20 Mg-C required for the given amount
of energy.