Soil organic matter dynamics in deep horizons under different land covers and pedoclimatic environments, using the Rock-Eval® thermal analysis

Soil constitutes the second-largest carbon sink after oceans. While topsoils (>30 cm) are generally carbon-rich, subsoils may represent a crucial long-term carbon reservoir, storing carbon for centuries to millennia scales due to the greater stability of soil organic matter. However, subsoils remain understudied, leaving significant knowledge gaps in how carbon dynamics between topsoil and subsoil layers respond to management and pedoclimatic factors, both affecting soil carbon sequestration. DeepHorizon is an EU Horizon-funded project funded involving 20 partners across Europe. DeepHorizon seeks to improve our understanding, monitoring and assessment of the subsoil by focusing on subsoils up to 2 m deep at forty sites in EU representing various soil types, pedoclimatic regions and land covers (forest, cropland, grassland and wetland). Subsoil carbon often occurs in low concentrations, making it challenging to quantify it accurately using standard elemental analysis. In addition, subsoils are often enriched with components from the parent material, such as carbonates, which further increase analytical uncertainty and require additional sample preparation. The project employs the Rock-Eval® thermal method, based on carbonaceous gas emissions under a controlled temperature gradient up to 850 °C through successive pyrolysis and oxidation conditions. This method has gained attraction in recent years for soil organic matter characterization and is particularly effective in detecting small carbon contents, while distinguishing different soil carbon pools. The tested land-cover types and management practices influenced the vertical distribution of carbon pools with varying thermal stability. Results from all forty sites across soil depths will be further analyzed and discussed.