Avoided Land Degradation and Enhanced Soil Carbon Storage: Is There a Role for Carbon Markets? - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 366

studies on land-use change in the tropics,
expanding substantively from Guo and
Gifford (2002), found that the highest C org
losses were caused by conversion of primary
forest into cropland (-25%) and perennial
crops (-30%), but forest conversion into
grassland also reduced C org stocks by 12%. If
it were a simple additive system, one might
thus expect conversion of grasslands to per-
ennial crops to lead to a decrease of C org by
about 18%. The discrepancy may be due to
the non-average starting points for the land-
use transitions measured. The time frame
over which land-use effects are measured is
unclear, and can be responsible for at least
part of the discrepancies. Another recent meta-
analysis (Powers et al ., 2011) focused on
'paired plot' literature and found little con-
sistency in C org change, with both 'forest to
grassland' and 'grassland to forest' conver-
sions leading to statistically significant C org
gain; this may raise doubts on the selection
bias in the results that get published. Both
reviews confirm that complete data sets that
combine soil bulk density and soil organic
carbon are scarce, and that spatial extrapola-
tion is affected by unbalanced representation
of tropical soils. Bruun et al . (2009) exposed
common misunderstandings in the role of
swiddening in this respect.
These numbers provide a perspective
on the possibility of soil C storage beyond
peatlands to be a significant component of
plans to reduce net anthropogenic C emis-
sions. Net effects can be a modest, but sig-
nificant, contribution to agreed emission
reduction (which was a modest 5% for the
first Kyoto commitment period), but only if
the efforts and associated finance used to
invest in terrestrial C is 'additional'.

stock change. The changes in aboveground
C stock involved in deforestation can be of
the order of 200 Mg C ha - 1 , while the man-
agement swing potential of soil C stocks in
agriculturally managed soils is an order of
magnitude smaller, around 20 Mg C ha - 1
(Buysse et al ., 2013), unless peat soils are
involved. Given these orders of magnitude,
it is unlikely that soil C attracts the atten-
tion of market-based mechanisms separate
from changes in aboveground C stocks. If
soil C protecting practices would reduce
yields, the indirect negative effects of such
land management on anthropogenic atmos-
pheric CO 2 concentrations by increasing
rates of forest-to-agriculture conversion can
easily surpass the positive effects on local C
storage. If the practices increase yields, it
probably does not require support to be
readily adopted.
A further distinction is usually made
between 'avoided degradation' and 'assisted
restoration', recognizing that changes in
above- and belowground C storage tend to be
in the same direction. Comparing above-
ground C storage transition curves (linked
to forest and tree cover transitions) to the
dynamics of the belowground C storage
in (agro-)ecosystems, we expect a reduced
'management swing potential', a more tem-
porally buffered dynamics and a time lag for
the effects of tree cover change to impact on
soil organic matter via root and associated
symbiont turnover (van Noordwijk et al .,
1998b) ( Fig. 31.4 ). The concept of manage-
ment swing potential was recently intro-
duced as the difference in footprint of the
best and worst modes of production (Davis
et al ., 2013). There is empirical evidence of
a recovery of soil carbon in intensive rice-
based cropping systems in East and South-
east Asia that is not related to the use of
trees but to an increase in the number of
crops per year and associated increase in
root biomass inputs to the soil (Minasny
et al ., 2011).

Can soil C be separated from
aboveground land-use effects?

The development of economic incentives
for land use-related C stock change has so
far focused on changes in tree cover, with
rules focused on tree planting, deforestation
and forest degradation, with some attention
to peatlands as hotspots of belowground C

Offsets and additionality

Additionality is usually discussed on the
supply side, with the burden of evidence on