Conversion of forest to agriculture in Amazonia with the chop-and-mulch method: Does it improve the soil carbon stock?

•The chop-and-mulch method was tested to convert forest to cropland and grassland.•We assessed soil organic carbon masses changes up to 3 years after forest clearing.•Up to 1.5 years after conversion, soil carbon masses were higher in converted soils.•Three years after conversion, grassland had similar stocks than forest soils.•At 3 years, carbon depletion occurred for disk-tillage and even more for no-tillage. Fire-free forest conversion with organic inputs as an alternative to slash-and-burn could improve agro-ecosystem sustainability. We assessed soil carbon mass changes in a sandy–clayey and well-drained soil in French Guiana after forest clearing by the chop-and-mulch method and crop establishment. At the experimental site of Combi, native forest was cut down in October 2008; woody biomass was chopped and incorporated into the top 20cm of soil. After about one year of legume and grass cover, three forms of land management were compared: grassland (Urochloa ruziziensis), maize/soybean crop rotation with disk tillage and in direct seeding without tillage. There were four replicates. We measured 14.16kgm−2 of carbon in 2mm-sieved soil down to 2m depth for the initial forest. Forest clearing did not induce significant soil compaction; neither did any specific agricultural practice. In converted soils, C stocks were measured in the 0–30cm layer after each crop for three years. Carbon mass changes for soil fractions 2mm (soil C pool) in the 0–5, 5–10, 10–20 and 20–30cm soil layers were assessed on an equivalent soil mass basis. One year and 1.5 years after deforestation, higher C stocks (+0.64 to 1.16kgCm−2yr−1) and C pools (+0.52 to 0.90kgCm−2yr−1) were measured in converted soils, compared to those of the forest into the top 30cm of soil. However, the masses of carbon in these converted soils declined later. The highest rates of carbon decrease were measured between 1.5 and 2 years after forest conversion in the