Climate Change and Soil Carbon Impacts - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 239

Soil C

Vegetation C

Time since management change

Management change

Fig. 20.1. Decline in sink strength over time. Change in soil and vegetation carbon sequestration showing
large atmospheric carbon removals (sink strength) soon after management change (large vertical arrow
on left-hand side of the figure), but over the subsequent equivalent time removals become smaller as the
soils approach a new equilibrium (smaller arrows as soils gain in carbon). (From Smith, 2012.)

non - permanence . As well as declining over
time, soil C sinks are also reversible. A soil
C stock that has been increased by improved
soil management will lose C rapidly unless
the improved management is maintained.
The rate of C loss is more rapid than the rate
of gain (Smith, 2005). Compared to reduced
emissions of other greenhouse gases, where
an emission reduction is permanent, C se-
questered in the soil (and in vegetation) is
non-permanent, presenting a risk of future
release (Smith, 2005).

area leads to land-use change that causes C
release in another area, in a process termed
'indirect land-use change' (Searchinger
et al ., 2008).

verification issues . Changes in soil C are
small compared to the large stocks of C pre-
sent in the soil, meaning that the change in
C stock can be difficult to measure, present-
ing problems for monitoring, reporting and
verification (MRV) (Smith, 2004). If the
value of the C removed from the atmosphere
is less than the cost of measuring the change,
MRV costs can make soil C less cost com-
petitive with greenhouse gas reduction
measures that are less expensive to demon-
strate (Smith, 2004).

leakage / displacement . Increasing soil C stocks
does not necessarily lead to a decrease in
atmospheric CO 2 concentrations (Powlson
et al ., 2011). It is possible, for example, to
enhance soil C stocks in one area by apply-
ing large inputs of organic matter. If, how-
ever, the organic matter applied to the area
gaining in C would otherwise have been ap-
plied in another area, the other area would
lose C (i.e. the emissions are displaced; also
termed 'leakage' where emissions occur out-
side the greenhouse gas accounting boundary;
IPCC, 2000). In this example, the impact
across the two areas would be neutral, lead-
ing to no net atmospheric C removal. An in-
crease in soil C stocks in this case, does not
constitute a genuine decrease in atmos-
pheric CO 2 concentrations (Smith, 2005).
Displacement/leakage also occurs where
land-use change to increase C stocks in one

total effectiveness relative to emission reduction
targets . Soil C sequestration is an import-
ant climate mitigation strategy, but it is not
a panacea for greenhouse gas emissions re-
duction. Only a fraction of the reduction
can be achieved through sinks (IPCC WGI,
2007). Soil C sequestration, therefore, needs
to be considered alongside many other
greenhouse gas emissions reduction strat-
egies across all sectors.
The problem of attempting to use soil
and vegetation to sequester C as a climate
mitigation measure has been summarized
succinctly by W.H. Schlesinger as 'trying to
sequester the geosphere in the biosphere'.