The Global Challenge for Soil Carbon - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 7

productivity and creates more physically
cohesive soil to resist soil losses by phys-
ical erosion and by protecting occluded
organic matter within the larger aggre-
gates. Carbon that enters soil is removed
from the atmosphere; any gains in soil car-
bon mitigate greenhouse gas emissions,
with caveats about impacts on the N cycle
and N 2 O production and the production of
CH 4 from the anaerobic decomposition of
organic matter in waterlogged soils.
The factors that control soil carbon levels
offer clues to strategies that can maintain and
increase soil carbon content. Increasing car-
bon levels may be achieved by reducing soil
carbon losses by measures to reduce physical
erosion by wind or overland water flow, meas-
ures to prevent the mechanical disturbance of
aggregates and measures to increase the water
content of organic soils. Increasing input of
soil carbon can be achieved by measures that
increase the aboveground production of vege-
tation, the increased allocation of carbon
below ground through greater root density
and associated carbon input and microbial
biomass, increased plant residue return to soil
and the addition of imported organic matter
such as compost.
Soil carbon is lost rapidly when soils
are disturbed through land-use conversion
from grassland and forest to arable, and
when land is drained. However, building up
soil carbon is slow. The risks of losing soil
carbon are great because of the potential
consequences of:

There is considerable knowledge and
data on the role of soil organic matter in
specific soil functions, particularly related
to biomass production, water and contam-
inant filtration, and CO 2 emissions. There is
considerably less known about the inter-
actions between soil organic matter, bio-
diversity, transformations of nutrients and
soil structure, and the physical stability of
soil structure and aggregates. The know-
ledge of the role of soil carbon, and the ex-
isting methods and innovation potential to
manage it effectively for this wide range of
benefits, is collectively substantial but is
fragmented between many different discip-
lines. The subsequent chapters of this volume
seek to summarize this wide knowledge base
and showcase regional examples of beneficial
management of soil carbon with the potential
to expand such practices greatly worldwide.
Beneficial management of soil carbon
offers the opportunity not only to avoid the
negative consequences but also to enhance
the wide range of available soil functions and
ecosystem services. For these reasons, pol-
icies are essential that encourage protecting,
maintaining and enhancing soil carbon levels.
A new focus on soil carbon at all
levels of governance for soil management
would better enable the full potential of
soil ecosystem services to be realized. This
advance is urgent and essential. To meet
successfully the ' 4 × 40' challenge laid out
in the introduction (Godfray et al ., 2010),
there is significant opportunity through soil
carbon management to help meet the de-
mand for food, fuel and clean water world-
wide. It is also an essential step towards
soil management that establishes enhanced
soil functions that last - in order to meet
the needs of future generations; not only to
meet the demands anticipated in the com-
ing four decades.

• the loss of soil fertility and agricultural
production;
• increased greenhouse gas emissions
and accelerated climate change; and

• diminished soil functions across the

full range of the ecosystem services de-
scribed above.

References

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Lair, G.J., Kram, P. et al . (2012) Soil processes and functions across an international network of Critical