Mapping spatial and vertical repartitions of soil carbon stocks, additional storage potential and storage dynamics at the regional scale

Soils constitute a carbon reservoir that can help to mitigate climate change, or conversely accelerate greenhouse gas emissions if not managed properly. Soils are heterogenous and dynamic systems, which physico-chemical properties impact their current soil organic carbon (SOC) stocks and their capacity to store more carbon. Land-use planning aiming to preserve and increase SOC stocks should therefore be aware of the spatial repartition of various soil types and of the SOC dynamics therein.

This project aims to map the effect of soil typology on the spatial and vertical repartition of soil carbon stocks, additional storage potential and storage dynamics at a regional scale to improve guidance of SOC storage strategies. The study site is a 320 km² temperate rural region in NE France. Eight dominant soil types are defined, notably Calcaric cambisols in the agricultural valleys, deep silty and acidic soils in the forested plateaus, shallow rocky calcaric soils on the hillslopes, and deep clay-rich hydromorphic soils in the alluvial valleys.  

Based on logarithmic fits of soil carbon data extracted from 197 full-depth soil profiles, mean soil organic carbon stocks are obtained as a function of depth for each represented soil type and land cover. The additional storage potential corresponds to the difference between the current stock and the maximum stock, as estimated by the fit of the upper 25% of the soil carbon content data.  Finally, a depth-dependent SOC dynamic model using multilayer soil modules is used to simulate SOC stock evolution. Results are mapped by combining the spatial information given by a pedological map and a map of land covers.

Median soil carbon stocks over the full soil profile range from 78 to 333 tC ha^-1, of which 59 to 148 tC ha^-1 are in the topsoil (0-30 cm). The lower stocks are found in the shallow, rocky cultivated soils, and the highest stocks in the gleysols under grasslands. The additional storage potential varies from 19 tC ha^-1 for shallow, rocky forest soils to 197 tC ha^-1 for cultivated gleysols. SOC build-up is heterogenous and depends on the mean residence time of carbon in the represented soil types.

Maps of carbon stocks show the areas to preserve to avoid C losses, and maps of additional storage capacity for different time horizons show areas in which to implement carbon storage practices. Going forward, the association of carbon stock mapping and modelling should allow us to estimate at which depths and over which timescales.