Article de synthèse sur les questions de dégradation et de restauration des terres, résumé des séminaires de master de Marc Bied-charreton, à paraître prochainement dans une revue de l'Université d'Auvergne.
Marc BIED-CHARRETON (...)
Agriculture sector is a major source of greenhouse gas, contributing directly 14% of total global emissions. When combined with related changes in land use including deforestation (for which agriculture is a major driver), agriculture’s contribution rises to more than one-third of total GHG emissions. But in the same time the potential for technical mitigation in this sector is high and with 74% of it is in developing countries. Many of the technical options are readily available and could be deployed immediately. It consists mainly in reducing emissions of carbon dioxide through the reduction of the rate of deforestation and forest degradation, adoption of improved management practices of rainfed and irrigated croplands, animal productions, livestock’s waste, and rehabilitation of degraded lands.
The capacity of natural and managed agro-ecosystems remove carbon dioxide from the atmosphere in a manner that is not immediately remitted into the atmosphere is known as carbon sequestration: Carbon dioxide is absorbed by vegetation through photosynthesis and stored as carbon in biomass and soils. Forests and grasslands can store large amounts of carbon in their vegetation and root systems for relatively long period of time. However, soils are a larger terrestrial pool for organic carbon.
Even if the service provide by soil in sequestering carbon is important, and essential, in the context of mitigation, this should not be dissociated of other ecosystems services link with the maintenance of carbon in soils. Soil carbon sequestration allows for the replenishment of soil organic matter and, thus, provides several other benefits including improved soil structure and stability that leads to reduced soil erosion, improved soil biodiversity, increased nutrient holding capacity, increased nutrient use efficiency, increased water holding capacity, increased crop yields and is linked with improved food security. Moreover, carbon in soil consolidates the resilience of cropping systems against both excess of water and lack of water, as well as biodiversity increases resilience to changing environmental conditions and stresses. Therefore it strengthens the capacity to face extreme events (climate adaptation). Most of mitigation and adaptation solutions are inter-related and both must be planned together.
Soil carbon sequestration is good for soil quality, both at short-term and long-term. It is a cost-effective and environmentally-friendly process that can be achieved through land management practices adapted to specific context: the ability to sequester carbon depends on climate, soil type, vegetation cover and land management practices. Full implementation of sequestering practices is still facing some limitation and barriers such as the monitoring and reporting issue in ex-post analysis. But also ex-ante estimates still face the lack of methodologies or approaches that would help project designers and policy makers to integrate significant mitigation effects in agriculture and forestry development projects. The recent developments of carbon footprints both from the private sectors, NGOs and international development organisms, such as the World Bank, the Global Environmental Facilities and the Food and Agriculture Organization of the United Nations, illustrate the need to consider carbon both as an agri-environmental indicator in agriculture policy building and as a performance indicator in policy analysis (e.g. The FAO EX-ACT initiative at www.fao.org/tc/exact/en).
In arid and semi-arid regions, soil degradation is widespread and most of the drylands are already degraded or are at high risk of degradation. Consequently to the natural constraints drylands soils contain very small amount of carbon (typically bellow 1 percent). Thus maintaining a minimum soil organic matter level and thus minimum soil carbon content is critical to maintain these soils functioning. It is evident that in arid en semi-arid regions soil carbon sequestration is more important for the side-benefits, brought in terms of economic and social aspect, than the absolute amount of carbon sequestered! It appears crucial for these regions to enable mechanisms to recognize the carbon side-benefits (erosion control, fight against desertification and land degradation, increased water efficiency, livelihoods of people living in drylands…)
Bernoux M. 2011. Soil carbon: at the crossroads of the conventions. In Panel 2: REDD+ and ‘Cancùn agreements’: what are the perspectives and hurdles for the land. UNCCD - Land Day 4, 11 June 2011, Bonn, Germany.