Page 141 - Materials Chemistry, Second Edition

Life Cycle Assessment: Principles, Practice and Prospects
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Figure 10.1 Schematic of forest biomass circulation system (numerals show tonnes of carbon per
hectare per year) (cited in Horne and Matthews 2004).

Soils release various gases, particularly when disturbed or the overlying canopy and plant
matter is changed, and these can include potent greenhouse gases such as nitrous oxide (N O)
2
and methane (CH ) in addition to carbon dioxide. Of course, a shift from agriculture to
4
forestry may lead to positive greenhouse gas changes, as forests involve less soil disturbance
than agricultural land.
Time scales, land use prior to and after planting the sequestration crop, the fate of the biomass
after harvesting and soil dynamics are all potentially important factors in the calculation of
‘deemed’ net carbon emission reductions or offsets. Clearly, carbon accounting of bio-sequestra-
tion projects is contentious and there are various factors which should be taken into account in
deeming any carbon offsets. In the following discussion, the example of forestry is used, although
the main discussion points are also likely to apply to other forms of bio-sequestration.

10.2.1 Forests as carbon sinks
Forests are in a state of continuous carbon flux. During natural growth and regeneration,
equilibrium is maintained through the carbon dioxide cycle – fixed by plant growth, emitted
to air, locked up in soil and removed through weathering. Following forest harvesting and

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