Stabilization of soil organic matter isolated via oxidative degradation

Formation of long residence time carbon pools in soils is crucial for understanding the global carbon cycle. Acting on the assumption that chemical oxidation mimics natural oxidative processes, we treated forest floor, topsoil and subsoil samples from two acid forest soil profiles (Dystric Cambisol, Haplic Podzol) with H₂O₂. The oxidation-resistant organic carbon fraction was dated using ¹⁴C and found to be 500-3900 years older than the bulk soil material. A positive correlation between the oxidation-resistant organic carbon and the amount of total iron oxides (dithionite-extractable Fe) exists for the subsoil samples of the Dystric Cambisol. In subsoils of the Haplic Podzol total iron oxides and the clay content are correlated with the oxidation-resistant organic carbon. This suggests that stabilization of the oxidation-resistant, old carbon fraction is caused by its interaction with iron oxides and/or other clay minerals rather than by chemical recalcitrance. In contrast to the subsoil, the topsoil samples have much higher concentrations of residual carbon relative to their iron oxide contents. Like forest floor horizons they probably contain additional, non-mineral-protected, but chemically recalcitrant, aliphatic organic matter, which is resistant to H₂O₂ treatment.