Climate Change and Soil Carbon Impacts - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 236

between global warming and other aspects
of global change, including other climatic
effects (e.g. changes in water balance, fire
regimes, pests and diseases), changes in at-
mospheric composition (e.g. increasing at-
mospheric carbon dioxide concentration),
land-use change (Smith et al ., 2005, 2006)
and arable land management. In addition to
responding to climate change, soils could
also play an important role in climate
change mitigation; if C can be sequestered
in soils, this could be a significant mechan-
ism for reducing atmospheric CO 2 concentra-
tions (Smith et al ., 1997, 2000). In this short
review, we outline recent evidence of poten-
tial responses of soils to climate change, and
then present recent evidence on the possible
role of soil C sequestration in climate mitiga-
tion and discuss some limitations associated
with this method of climate mitigation. This
review is limited to mineral soils and does
not cover peatlands and permafrost soils,
since the role of peatlands in climate change
has been reviewed recently (Joosten et al .,
2014) and space precludes dealing with all
soil types in similar depth.

Schimel et al ., 2001). Soil C pools are smaller
now than they were before human interven-
tion. Historically, soils have lost between
40 and 90 Pg C globally through cultivation
and disturbance (Schimel, 1995; Houghton,
1999; Houghton et al ., 1999; Lal, 1999). The
size of the pool of soil organic carbon
(SOC) is large compared to gross and net
annual fluxes of C to and from the terres-
trial biosphere.
Small changes in the SOC pool could
have dramatic impacts on the concentration
of CO 2 in the atmosphere. The response of
SOC to global warming is, therefore, of crit-
ical importance. One of the first examples of
the potential impact of increased release of
terrestrial C on further climate change was
given by Cox et al . (2000). Using a climate
model with a coupled C cycle, this study
showed that the release of terrestrial C under
warming would lead to a positive feedback,
resulting in increased global warming. Since
then, a number of coupled climate C cycle
(so-called C4) models have been developed.
However, there remains considerable un-
certainty concerning the extent of the terres-
trial feedback, with the difference between
the models amounting to differences in the
atmospheric CO 2 concentration of ~ 250 ppm
by 2100 (Friedlingstein et al ., 2006). This is
of the same order as the difference between
fossil fuel C emissions under the IPCC
SRES (2000) emissions scenarios. It is clear
that better quantifying the response of ter-
restrial C, a large proportion of which de-
rives from the soil, is essential for under-
standing the nature and extent of Earth's
response to global warming.

The Impact of Climate Change
on Soils

Soils in the global carbon cycle

Globally, soils contain about 1500 Pg ( 1 Pg = 1
Gt = 10 15 g) of organic C (Batjes, 1996), about
three times the amount of C in vegetation
and twice the amount in the atmosphere
(IPCC WGI, 2001). The annual fluxes of CO 2
from atmosphere to land (global net pri-
mary productivity (NPP)) and land to at-
mosphere (respiration and fire) are each of
the order of 60 Pg C year - 1 (IPCC WGI, 2001).
During the 1990s, fossil fuel combustion
and cement production emitted 6.3 ± 1.3 Pg
C year - 1 to the atmosphere, while land-use
change emitted 1.6 ± 0.8 Pg C year - 1 (IPCC
WGI, 2001; Schimel et al ., 2001). Atmos-
pheric C increased at a rate of 3.2 ± 0.1 Pg C
year - 1 , the oceans absorbed 2.3 ± 0.8 Pg C
year - 1 , with an estimated terrestrial sink of
2.3 ± 1.3 Pg C year - 1 (IPCC WGI, 2001;

The response of soils to future
climate change

Despite suggestions during the 1990s that
climate change could lead to massive losses of
C from the world's soils, more recent studies
have suggested that climate change impacts
on soil C could be less significant (Smith
et al ., 2005, 2006; Ciais et al ., 2010;
Gottschalk et al ., 2012). The level of SOC in a
particular soil is determined by many factors,
including climatic factors (e.g. temperature