Soil organic carbon and its sorptive preservation in central Germany

Soils are increasingly viewed as a potential sink for atmospheric carbon. However, their use to meet CO₂ emission reductions is problematic for there are knowledge gaps regarding the mechanisms involved in the sequestration of organic carbon (OC). There is evidence which suggests that OC concentrations are controlled by the mineralogy and related specific surface area (SSA) of a given soil. The goal of this study was to examine the importance of sorptive mechanisms on OC stabilization. The objectives were (i) to determine the SSA, clay mineralogy and dithionite‐ and oxalate‐extractable Fe and Al concentrations of several soils, and (ii) to analyse how these variables are related to OC concentrations. Five soils were sampled and analysed: two Umbrisols, a Stagnic Acrisol, an Anthrosol/Vertisol/Gleysol‐Chernozem and a Gleysol (FAO terminology), all located in Hesse, Germany. Oxalate‐extractable Fe and Al were found to be the best predictors of OC concentrations in the soils examined. Specific surface area correlated significantly with the OC content of the B and C horizons of one Umbrisol and the entire profile of the Anthrosol/Vertisol/Gleysol‐Chernozem. The relationship between SSA and OC concentrations is likely to be restricted to certain soils and might be a product of the sorptive capacity of Fe and Al oxides. We can assume that the available mineral surface area on oxides is a limiting factor in terms of a soil's capacity to sequester organic carbon. As such, attention should be paid to soil mineralogy and how this might limit the use of soils as a sink for atmospheric CO₂.