Forest soils in France are sequestering substantial amounts of carbon.

The aim of this study was to assess whether French forest soils are sources or sinks of carbon and to quantify changes in soil organic carbon (SOC) stocks over time by resampling soil in long-term forest monitoring plots. Within each plot, and for each survey, soils were sampled at five points selected in five subplots and divided into layers. Composite samples were produced for each layer and subplot, then analysed for mass, bulk density and SOC. Linear mixed models were used to estimate SOC changes over 15years between two soil surveys carried out in 102 plots in France. A factor analysis and a budget approach were also used to identify which factors and processes were primarily responsible for SOC dynamics. Forest soils throughout France substantially accumulated SOC (+0.35MgCha-1yr-1) between 1993 and 2012. The SOC sequestration rate declined with stand age and was affected by stand structure. Uneven-aged stands sequestered more SOC than did even-aged stands (p<0.001). For the forest floor, the SOC sequestration rate estimated by the budget approach was in agreement with that based on stock comparison. This increasing SOC stock in the forest floor can be explained by recent changes in certain factors affecting litter decomposition (climate and litter quality). For the mineral soil, the budget approach was unable to replicate the observed SOC sequestration rate, probably because SOC stocks were not yet at equilibrium with litter inputs at the beginning of the monitoring period (contrary to our steady-state assumption). This explanation is also supported by the fact that the SOC sequestration rate decreased with stand age. As the SOC sequestration rate declines with stand age and is higher in uneven-aged stands, forest management has the potential to influence this carbon sink.