The Global Challenge for Soil Carbon - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 6

such that management decisions are made
without full information on the conse-
quences of change. Such market and infor-
mational failure necessarily leads to the
suboptimal allocation of effort to conserve
soil from a social perspective. The failure of
markets and policy to prevent soil carbon
loss and land degradation is therefore a key
component of the global challenge to pro-
vide sufficient life-sustaining resources.

accumulation due to low decomposition
rates. Soil carbon varies substantially geo-
graphically with land cover (Plate 2). For
example, savannah has relatively low soil
carbon content but covers a large area glo-
bally. On the other hand, peatlands have ex-
tremely high carbon content but cover less
than 0.3% of the global land surface. By in-
spection of Plates 1 and 2, it is clear that
degraded land coincides in large part with
Earth's drylands, due to low productivity
from low water availability and relatively
high decomposition due to dry, well-aerated
and warm soils.
From these controls on soil carbon con-
tent, it is clear that predicted changes to
regional as well as global climate in the
coming decades will create important im-
pacts on soil carbon (Schils et al ., 2008;
Conant et al ., 2011). Drier, warmer condi-
tions are expected to coincide with greater
potential for loss of soil carbon and the
associated loss of soil functions. Loss of
permafrost will expose accumulated carbon
in cold regions to much greater rates of
microbial decomposition (Schuur and Abbot,
2011). Furthermore, the demographic drivers
of more intensive land use raise the pro-
spect of greater physical disturbance of soils,
e.g. tilling of grasslands. More intense till-
age and greater areas of mechanical tillage
are expected to coincide with higher loss of
soil carbon due to greater exposure of soil
carbon to O 2 (Powlson et al ., 2011).

Threats to Soil Carbon

The global stocks of soil carbon are under
threat (Table 1.1 and Plate 1), with conse-
quences for the widespread loss of soil func-
tions and an increase in greenhouse gas
emissions from land and acceleration of global
warming (Lal, 2010a,b). In many locations, soil
functions are already compromised. Some of
the consequences include increased erosion,
increased pollution of water bodies from the
N and P loads that arise from erosion, desert-
ification, declining fertility and loss of habi-
tat and biodiversity. The primary control on
the global distribution of soil carbon is rain-
fall, with greater accumulation of soil organic
matter in more humid regions. A secondary
control is temperature, with greater organic
matter accumulation in colder regions when
otherwise sufficiently humid conditions per-
sist regardless of temperature. Under similar
climatic conditions, wetter soils help to accu-
mulate soil carbon by limiting rates of microbial
respiration (Batjes, 2011), since O 2 ingress is
restricted by the gas diffusion barrier created
by greater water content. Relatively drier con-
ditions favour O 2 ingress and aeration of soil,
thus accelerating soil carbon decomposition.
Furthermore, physical disturbance such as
tillage breaks up larger soil aggregates and
exposes occluded carbon within aggregates
to O 2 and biodegradation, thus creating con-
ditions that allow greater soil carbon loss.
With sufficient water, nutrients and O 2
supply, biological processes are relatively
faster at higher temperature; hence, greater
rates of productivity and decomposition. Thus,
warm, humid conditions favour soil carbon ac-
cumulation due to high productivity, while
cool, humid conditions favour soil carbon

Managing Soil Carbon for
Multiple Benefits

Maintaining and increasing soil carbon
content yields substantial multiple benefits.
Greater soil carbon helps to maintain soil
structure by forming stable larger aggre-
gates and larger inter-aggregate pores that
create greater soil permeability and drain-
age for root growth. Smaller interior pores
within aggregates, on the other hand, pro-
vide water-holding capacity to sustain bio-
logical processes. Increasing soil carbon
provides carbon and energy to support mi-
crobial activity, provides a reservoir of
organic N, P and other nutrients for plant