Sequestration of organic carbon in West African soils by Amenagement en Courbes de Niveau

摘要

A recent Intergovernmental Panel on Climate Change (IPPC) report concludes that global warming, while already a global crisis, is likely to become even more devastating. The scientific consensus is that global warming is caused by increases in greenhouse gases including carbon dioxide. The Sahel of West Africa sterns to be more adversely affected by such climate changes, leading to reduced and more sporadic rainfall. In addition, food security in the region is tenuous and fragile, due to adverse climate change, but also due to the historical mining of nutrients and carbon. With the adoption of the Kyoto accords, at least by some countries, sequestered carbon (C) has become a tradable commodity. This provides a double incentive to increase soil organic carbon in the C-depleted and degraded soils of West Africa - return C to improve soil quality and assist in removing CO_2 from the atmosphere to assist in mitigating climate change. A challenge, however, remains to determine which agricultural systemscan actually sequester C. The technology called Amenagement en courbes de niveau (ACN), which can be roughly translated as 'Ridge-tillage, has given crop yield increases of 30 to 50%. To date, there has only been anecdotal evidence suggesting that Amenagement en courbes de niveau leads to increased soil organic C. The objectives of the study reported here were to determine whether the technology has the potential to sequester C in West African soils, and, if so, how much. In this study, soil organic Cwas measured by combustion methods in soils sampled at 0-20 and 20-40 cm depths in a series of experiments in Mali, Senegal and The Gambia. Soil organic C was measured in three very different types of experiments, all including Amenagement en courbes deniveau technology, resulting in three methods of measuring C sequestration. Our results indicate that the Amenagement en courbes de niveau technology significantly increased maize yields by 24% by weight in the Gambia experiment while soil organic C wasincreased by 26% in The Gambia, by 12% in Siguidolo, Mali, and by 14% in peanut systems of Nioro, Senegal. These increases in soil organic C are likely due to three factors: (1) reduced erosion and movement of soil, (2) increased crop growth resulting from the greater capture of rainfall, and (3) increased growth and density of shrubs and trees resulting from the increased subsoil water, resulting in turn from the increased capture of rainfall, and reduced runoff. Measuring soil C on fields that were successively placed under Amenagement en courbes de niveau management and the use of replicated experimental plots appear to be the best methods to quantify the C sequestration potential of the practice. These results indicate that this soil and water conservation technology not only harvests water and increases food production, but also increases soil organic carbon. This technology thus is a successful technique to sequester C in soils and if carried out in a large region may both offset CO_2 emissionsand help mitigate climate change.