Climate Change Mitigation - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 119

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Climate Change Mitigation

Martial Bernoux* and Keith Paustian

Abstract
Terrestrial ecosystems play a major role in regulating the concentrations of three greenhouse gases
(CO 2 , CH 4 and N 2 O), of which CO 2 is the most important in terms of the impact on the global radiative
balance. Soils play a major role in the global carbon (C) cycle and CO 2 dynamics; thus, management of
soil carbon appears essential and more and more inevitable.
The capacity of natural and managed agroecosystems to remove carbon dioxide from the atmos-
phere in a manner that is not immediately re-emitted into the atmosphere is known as carbon seques-
tration: carbon dioxide is absorbed by vegetation through photosynthesis and stored as carbon in
biomass and soils, and released through autotrophic and heterotrophic respiration. Forests, croplands
and grasslands can store large amounts of carbon in soils for relatively long periods. Soils are the larger
terrestrial pool of organic carbon. Moreover, soil carbon sequestration is beneficial for soil quality, both
over the short term and long term, and can be achieved through land management practices adapted
to the specific site characteristics. The ability of soils to sequester carbon depends on climate, soil
type, vegetation cover and land management practices.
According to the fourth assessment report of the Intergovernmental Panel on Climate Change
(IPCC), the total technical greenhouse gas (GHG) mitigation potential of agriculture (considering all
gases and sources) is estimated to be in the range 4.5- 6 Gt CO 2 -equivalent year - 1 by 2030. Estimates
indicate that many of these options are of relatively low cost and generate significant co-benefits in the
form of improved agricultural production systems, resilience and other ecosystem services. Moreover,
many of the technical options are readily available and could be deployed immediately. About 90% of
this potential can be achieved by soil C sequestration through cropland management, grazing land
management, restoration of organic soils and degraded lands, and water management in rainfed and
irrigated croplands. In most cases, such management practices include the management of organic
residues produced on site or coming from outside the field or the farm. It has been estimated that the
global world production of residues in the agriculture sector is about 3.8 Pg C and, to date, the use of
this resource has not been optimized; a large part is still being burned.
Over the past two decades, other practices have been tested and are still controversial, such as
biochar or chipped ramial wood application in cultivated fields. Biochar is a stabile carbon amend-
ment, produced from pyrolysis of biomass, which may increase biomass productivity as well as se-
quester C from the source biomass. The scientific validation of these practices is still incomplete.