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 365

Topsoil only or land-use effects
on deeper soil layers

for these positions. An example is the rub-
ber agroforest that tends to occur close to
rivers in soils with higher clay + silt content
than the locations where natural forest is
left in the landscape; higher C org data for
rubber agroforest relative to natural forest
turn into lower C org /C ref ratios once corrected
for texture.
A change in the intensity and depth of
soil tillage affects C org distribution in the
soil, rather than the total C stock, and initial
claims that reduced tillage is a C storage op-
tion had to be reconsidered when data for
the whole soil profile were included in the
analysis. When assessed across nitrous oxide,
CO 2 and methane, there may still be positive
effects (Ruan and Robertson, 2013).

Most of the data refer to changes in the top-
soil, which generally are more readily ob-
servable and where changes may occur
more rapidly after land cover and land-use
change. For the total soil-atmosphere flux,
deeper soil layers may be at least as import-
ant (Nepstad et al ., 2002), depending on the
presence of deep-rooted trees that stay green
in dry periods.
In a recent study in Sumatra, an area
where deforestation has almost run its course
over the past three decades, statistically
significant differences in soil C stock with
depth could only be ascertained for the top
10 cm, once the depth data were corrected
for the bulk density under the influence of
land use ( Fig. 31.3) . The bulk density cor-
rection removed about half of the apparent
difference in soil C stocks if only C org data
were available. Correction for differences in
soil texture using the C ref equation of van
Noordwijk et al . (1997) and Hairiah et al .
(2011) helps in separating true land-use ef-
fects from the covariance of land use, positions
in the landscape and the soil texture typical

Default estimates for land-use effects
on soil C stocks

As part of the second IPCC review, Paustian
et al . (1997) summarized known effects of
land-use change on soil carbon across cli-
matic zones and soil types. Subsequent
literature has lead to some refinement. Don
et al . (2011) in a global meta-analysis of 385

0

- 5

- 10

- 15

Natural forest

- 20

Logged-over forest

- 25

- 30

Rubber agroforest

- 35

Perennial crops

- 40

Secondary forest

- 45

Shrub

- 50

0 0.5 1

BD ref *C org /C ref

1.5 2 2.5

Fig. 31.3. Relationship between C org /C ref and the mean depth of sampled soil layers in a deforestation
landscape in Sumatra (Indonesia); the regression lines shown represent 54- 85% of variation, except the
secondary forest line, which represents only 31% of variation at sample level.