[en] This study aimed at determining whether carbon sequestration in agricultural soils could contribute to the reduction of the greenhouse effect through actions aiming at cumulating carbon from agricultural origin, which were the conditions of compliance with the Kyoto protocol, which economic policy tools should be efficient to promote desirable changes, and which are research and reference needs. After an introduction that presents the problematic and the addressed issues, the report proposes an overview of the state-of-the-art of knowledge, references and available tools. It proposes a critical analysis of published results and of data obtained during agronomic tests performed in France and under similar climates, and also proposes values of storage per surface unit according to main changes in land use or envisaged agricultural practices. Then, it addresses the agronomic feasibility of changes which would allow an additional storage (under which climatic conditions, with which associated effects, and with which sustainability?). Simulations are performed to assess a global storage potential of the French metropolitan territory. Possible economic incentive tools for the promotion of additional storages are then analysed.

[en] At the Earth Summit in Johannesburg, declarations by some States raised hopes that the Kyoto Protocol could be implemented in 2003. A reduction in the human induced greenhouse gas emissions is clearly the most sustainable policy to slow down the accumulation of these gases in the atmosphere, which is causing, according to a large scientific consensus, climate change. However, in the case of CO₂ emissions, possible alternatives consist in stocking for some time some additional organic carbon in the biomass and in the soil organic matter. Carbon stocks in the forest biomass (Article 3.3. of the Protocol) have already been the subject of international agreements, with strict quotas being imposed on different countries. Favouring a more or less long-term accumulation of organic matter in soils, through changes in land use and agricultural or forestry practices, is another alternative (Article 3.4.) which was accepted as a mode of application of the Kyoto Protocol. With respect to this alternative, there are no limitations as to the amounts accumulated or the land surface concerned, but no methods for verification have yet been defined. Worldwide, soils contain about 1500 giga-tons of organic carbon. A relatively small increase in these stocks could therefore play a significant role in limiting the net flux of greenhouse gases towards the atmosphere. Changes in land use and agricultural production practices could contribute to this, notably by increasing the magnitude and the duration of organic carbon storage in soils. The residence time of carbon in the soil organic matter is highly variable, as it varies with the rate of carbon mineralisation, through which organic carbon is finally returned to the atmosphere. It is therefore important to determine the potential offered by this pool, as a function of soils, their uses and associated practices. In order to apply the Kyoto Protocol, it is also necessary to know how and with what precision this pool could be measured, and which incentive policies could induce additional accumulations in soils. These challenges are not negligible for France in a European context, because of its large agricultural land and surface area. For this reason, the Ministry for Ecology and Sustainable Development requested an Assessment Report from INRA, which concerns the capacity for organic carbon accumulation in agricultural soils and its time-related dynamics. This Expert Report should be read in the context of a strategy for soil management on a national basis, bringing together the principal actors in a Scientific Interest Group and the implementation of a soil quality monitoring network. At the European level, this report is in line with the recommendations of the recent Communication of the European Commission on soil protection. The main questions for this assessment have been formulated by a Steering Committee and are as follows: - Can we, in France, through actions specifically targeting an increase in organic carbon accumulations in soils, contribute to reduce the greenhouse effect? The answer to this question is of direct interest to the French Inter-ministerial Mission on the Greenhouse Effect, and France in general, so that agriculture can be taken into account in the national plan to combat the greenhouse effect. - How could such actions meet the conditions of compliance with the Kyoto Protocol? It is necessary to propose solutions which will be technically feasible and economically viable. - Which economic policy tools would be effective to promote the changes required? In this respect, it must be possible to integrate the tools in the Common Agricultural Policy. - What are the needs in terms of research and references? What is required to further develop the research results and recommendations arising from MEDD pilot programs concerning the sustainable management of soils and the impacts of climate change? This Assessment Report was drafted by an Expert Group consisting of researchers specialised in soil sciences, agronomy, bio-climatology and public economics applied to agriculture, from INRA and from other French and foreign organisations. This report, through the assessment it has enabled and without omitting any debates and uncertainties, will provide an unquestionable aid for decision-makers. This 'Synthesis', more particularly drafted for Decision-Makers, forms a detailed summary of the report, with cross references to the chapters of the expertise which will provide readers with a more comprehensive information.