Modeling soil organic carbon fluxes attributable to conversion of forest to corn cropping in a landscape of southwest France

Recent concern about rising levels of atmospheric CO2 has directed attention to the stores of organic carbon in soils and to changes caused by human activities. In southwest France, thick, humic, loamy soils have developed from Quaternary silty alluvial deposits. Most forest lands on these soils have been converted to continuous intensive corn cropping. The spatial distribution of C stores under forest and the loss of C upon this conversion have been modeled in previous studies. The objective of this study was to determine if a spatial and temporal analysis of this conversion makes possible reliable estimates of the organic carbon fluxes attributable to this conversion. Forest clearing showed a sigmoid temporal progression. The analysis of changes in land use provided a useful tool to improve the prediction of soil organic carbon fluxes to the atmosphere. From 1948 to 1984, carbon release to the atmosphere was about 2.2 Tg, that is 6.35 kg.m(-2). Temperate soils may be an important source of atmospheric carbon when they are high in carbon content initially and then cultivated intensively.