Modelling the three-dimensional spatial distribution of soil organic carbon (SOC) at the regional scale (Flanders, Belgium)

The rate of exchange of CO2 between the soil and the atmosphere depends on the stability of the organic carbon stored in the soil. Recent studies show that carbon stored in the subsoil is characterised by larger turnover times than the carbon stored in the topsoil. Consequently, identification of the depth at which high/low amounts of SOC are stored is essential for applying sustainable management of the soil in the light of global warming and related threats. This study investigates the depth distribution of SOC in relation to land use and soil type based on a large dataset for Flanders (Belgium). Soil type determines the SOC content along the entire profile. On the contrary, land use appears to have a strong influence on SOC content in the top layers of the profile, but doesn't play a significant role at the bottom of the profile (> 1 m depth). SOC content near the surface of the profile is remarkably higher in fine (clay) textured soils than in coarse (sand) textured soils and tends to increase by increasing soil wetness under sand and silt textured soils. SOC at the bottom of the profile increases as well by increasing soil wetness, but only in fine (clay and silt) textured soils. The rate of decline of SOC content with depth depends on texture and land use. Under forest this decline is remarkably fast, although less so in the more sandy soil types. The overall model predicts the distribution of SOC density by depth using land use and soil type information and allows in its integrated form the estimation of SOC stocks that can be represented on SOC maps until a reference depth free of choice. Applying this model, based on a three-dimensional spatial distribution approach, the total amount of SOC stored in Flanders is calculated at 62.20 ± 0.72 Mt C for the top 0.3 m and 103.19 ± 1.27 Mt C for the top 1 m.