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FAO/UNEP/UN-Energy Bioenergy Decision Support Tool -
MODULE 5: Land Resources
CARBON DEBT
Direct land use change associated with bioenergy production, as
with any land-intensive activity, entails preparation and/or conver-
sion of land, and will release carbon. The recovery of such losses
in carbon depends on the quality of the soils or land in terms
of carbon content and also the effectiveness of the bioenergy
feedstock in reducing GHG emission by fossil fuel substitution.
If the release of carbon is relatively small and the feedstocks
effcient, the sequestration of carbon and the substitution of
bioenergy for fossil fuels will “pay back” this carbon in a few years
or less, after which net savings in carbon begin to accumulate.
However, when high carbon environments are cleared and/or
the biofuels produced have relatively small GHG savings, then
the payback period can be quite long. This payback period has
been termed “carbon debt” (Searchinger et al 2008; Fargione et
al 2008).
The carbon debt for photosynthetically effcient perennial grasses
(e.g. miscanthus, sugarcane) on marginal or degraded lands will
be small or even close to zero. At the other extreme, the burning
of peatlands or clearing of rainforest for biofuels production will
result in a carbon debt of several hundred years. Preservation
of carbon-rich land is thus even more important than substitu-
tion of renewable energy for fossil fuels. The same is true when
carbon-rich lands are cleared for other uses, such as production
of livestock or food, except that in these cases the main goal is
human sustenance rather than energy substitution and climate
mitigation.
CARBON STORAGE
Different types of vegetation and soils store different amounts of
carbon per unit of area: both the carbon stored in the vegetation
and that stored in soils must be considered in order to properly
account for overall GHG emissions. Table 3 gives an indication of
the average and maximum amount of carbon stored in different
vegetation types. A change of land use, such as from forests to
agricultural land, will affect the overall carbon budget. As the plant
litter and soil organic matter gets burned or decomposes, the
carbon is released, and the soil carbon stocks decline. Table 4
shows the average amount of organic carbon stored in different
soil types.
Land conversion results in a release of 20-50 percent of the
organic carbon into the atmosphere, depending on soil type and
management practices (Sombroek et al, 1993). In some cases,
the conversion is especially damaging; the drainage of peat land
releases enormous amounts of carbon stored for centuries as
well as methane and N2O, both of which are more potent GHGs
than CO2. Lower river plains and deltas are also areas that are
extremely rich in organic carbon. Such areas of high carbon
storage should be avoided for bioenergy production as well as for
other uses that might entail such carbon releases.
INDIRECT LAND USE CHANGE (ILUC)
Bioenergy crop production on land already converted for agricul-
tural production can have the same adverse effects as bioenergy
production on virgin land; as food production is displaced,
production tends to move elsewhere, resulting in further land
conversion and carbon releases. The additional demand for
agricultural land arises from underlying factors such as increasing
population, increasing material demands, international trade,
increasing demand for mobility and changing food habits (e.g.
more meat consumption). This phenomenon is often referred to
as indirect land use change (ILUC). The general issue of indirect
impacts extends across all types of biomass and land use, due to
the underlying factors of increased demand; consequently, land
use planning and other general measures that improve land use
effciency will be valuable as mitigation measures for both energy
and non-energy uses.
Table 3: Carbon stored in vegetation (above ground)
Carbon Sequestered
(mt C per ha)
Brief Description
Mean
Maximum
Broad evergreen
115.3
120.0
Broad deciduous closed
91.0
120.0
Broad deciduous open
26.5
90.0
Shrub
14.4
20.0
Herbaceous
32.0
76.7
Sparse herb or shrub
16.9
20.0
Crops
6.2
70.0
Mosaic (trees)
29.9
66.7
Mosaic (shrubs)
18.5
30.0
Source: Nelson and Robertson, 2008
Table 4: Carbon stored in different soil types
Generalized Soil Types
Carbon Sequestered (million tonnes
Organic Carbon per ha)
Desert soils (
Yermosols
); Sandy Soils (
Arenosols
)
20 - 30
Soils of semi deserts (
Xerosols
), Poorly developed soils (
Regosols
), Saline and Sodic
soils (
Solonchaks, Solonetz
), Soils with clay accumulation (
Luvisols, non humic Acrisols,
Podzoluvisols
).
55 -75
Dark cracking clays (
Vertisols
), Tropical soils (
Nitosols, Ferralsols)
, Sandy temperate soils
(
Podzols
), Steppe soils (
Kastanozems, Phaeozems, Chernozems
), Hydromorphic soils
(
Gleysols
), Soils with little development (
Cambisols
), Allusvial soils (
Fluvisols
)
75 – 150
Humus rich tropical soils (
Humic Acrisols
), Cold forest soils (
Greyzems
), Volcanic soils
(
Andosols
)
150 – 250
Peat soils (
Histosols
)
> 500
Source: Sombroek et al, 1993