Aggregate formation and organo-mineral association affect characteristics of soil organic matter across soil horizons and parent materials in temperate broadleaf forest

Precise assessment of soil organic carbon (OC) storage requires understanding of soil type and depth specific differences in organic matter (OM) stabilization. Therefore, we aimed to analyze OC dynamics down the soil profile and to clarify the effect of depth on the importance of aggregate formation and mineral adsorption for OC storage in mature beech forest soils developed from different parent materials. Aggregate size and density fractions were separated from samples of top and subsoil horizons, which were quantified and analyzed by infrared spectroscopy. We also determined the microbial biomass C (C_mic) and the amount of C decomposed within incubation experiments (CO₂-C) for the bulk soil samples. OC stabilized via aggregate formation and mineral association significantly increased with soil depth. However, this stabilized pool seemed to fuel the labile OM stronger in the subsoil than in the topsoil because the CO₂-C/SOC and CO₂-C/C_mic ratios increased with depth. Measured differences in the magnitude of the detected stabilization and destabilization patterns were attributed to parent material and soil horizon, indicating pronounced spatial and vertical heterogeneity in the contribution of soils under temperate broadleaf forest to terrestrial C sequestration. Considering such site and depth specific differences will improve assessment and modelling of soil OC storage for areas covered with the same forest type but with high pedogenetic diversity.