Terrestrial ecosystems play a major role in regulating the concentrations of three greenhouse gases (CO2, CH4 and N2O), of which CO2 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 CO2 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 atmosphere in a manner that is not immediately re-emitted into the atmosphere is known as carbon sequestration: 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 CO2-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 amendment, produced from pyrolysis of biomass, which may increase biomass productivity as well as sequester C from the source biomass. The scientific validation of these practices is still incomplete. Full participation of the agricultural sector in GHG mitigation still faces some challenges and barriers related to measurement, monitoring and reporting requirements in C offset markets. Further improvements are needed in methodologies and approaches that would help project designers and policy makers to integrate significant mitigation effects in agriculture development projects.
