Climate Change and Soil Carbon Impacts - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 238

answer. Instead, we should focus our re-
search efforts on improving our prediction
of factors that determine the size and direc-
tion of change and the land management
practices that can be implemented to pro-
tect and enhance SOC stocks, as discussed
in Smith et al . (2008b).

vulnerability of soils to future SOC loss under
global warming. As such, soil C sequestration
can, in many respects, be regarded as a
'win-win' and a 'no regrets' option (Smith
and Powlson, 2003; Smith and Trines, 2007;
Smith et al ., 2007).

Drawbacks associated with soil carbon
sequestration as a climate mitigation
measure

The Role of Soils in Mitigating
Climate Change

While there are many advantages to in-
creasing soil C stocks, and 'win-win' and
'no regrets' options can be identified, there
are a number of issues associated with soil
C sequestration that make it a risky climate
mitigation option (Smith, 2005, 2008a).
These issues are: (i) saturation of the C sink
(the C is only removed from the atmos-
phere until the soil reaches a new equilib-
rium soil C level; Smith, 2005); (ii) non-
permanence (C sinks can be reversed at
any stage by poor soil management; Smith,
2005); (iii) leakage/displacement (e.g. in-
creasing soil C stocks in one area leads
to soil C losses in another; IPCC, 2000);
(iv) verification issues (can the sinks be
measured and at what depth?; Smith,
2004); and (v) total effectiveness relative to
emissions reduction targets (only a fraction
of the reduction can be achieved through
sinks; IPCC WGI, 2007). These issues are
discussed briefly below.

Increasing soil carbon stocks to
combat climate change (soil carbon
sequestration)

Carbon stocks in the soil can be increased in
managed ecosystems by optimizing 'best
management practices'. There have been
numerous reviews of management to in-
crease soil C stocks (Lal et al ., 1998; Lal,
2004; Smith, 2008b), so a full review is not
presented here. Increased C stocks in the
soil increases soil fertility, workability and
water-holding capacity, and reduces erosion
risk (Lal, 2004). Increasing soil C stocks can
thus reduce the vulnerability of managed
soils to future global warming (Smith et al .,
2008b; Smith and Olesen, 2010). Manage-
ment practices effective in increasing SOC
stocks include improved plant productivity
(through nutrient management, rotations,
improved agronomy), reduced/conservation
tillage and residue management, more
effective use of organic amendments, land-
use change (crops to grass/trees), set-aside,
agroforestry, optimal livestock densities
and legumes/improved species mix (Smith
et al ., 2008b). While these measures have
the technical potential to increase SOC
stocks by about 1- 1.3 Pg year - 1 (Lal, 2004;
Smith et al ., 2008b), the economic poten-
tials for SOC sequestration are estimated to
be 0.4, 0.6 and 0.7 Pg C year - 1 at carbon
prices of US$0- 20, 0- 50 and 0- 100 t - 1
CO 2 -equivalent, respectively (Smith et al .,
2008b). A small loss of C from permafrost or
peatlands could offset this potential seques-
tration (Joosten et al ., 2014), but the in-
crease in SOC engendered by improved
management is expected also to reduce the

saturation of the carbon sink . The C sink
can be defined as the annual removal of C
from the atmosphere into soil. When a C se-
questration measure is first implemented,
the change in soil C is large to begin with,
but slows over time as the soil approaches a
new equilibrium (see Fig. 20.1) (Smith,
2005). Sink strength therefore decreases
over time until the soil reaches a new equi-
librium. This phenomenon is termed 'sink
saturation'. Compared to reduced emissions
of other greenhouse gases, which can con-
tinue indefinitely, C sequestration in soils
(and indeed in vegetation) is therefore time
limited and finite (Smith, 2005). Improved
management needs to be maintained indef-
initely to maintain the higher soil C stocks,
but with no additional sink benefit.