Preservation of fire-derived carbon compounds and sorptive stabilisation promote the accumulation of organic matter in black soils of the Southern Alps

Cryptopodzols are black soils that occur under forests dominated by chestnut trees (Castanea sativa) in Southern Switzerland. Their soil organic carbon (SOC) stocks reach an average of 150 t C ha−1 and are thus among the highest of European forest soils.We investigated the processes leading to the accumulation and stabilisation of SOC in these soils by analysing three Cryptopodzols and one Cambisol for charred organic matter content (macrocharcoal and BPCA), the amounts of Fe and Al, and the colour and SOC content in bulk soil and density fractions. The results showed that charred organic matter produced by frequent fires in the area for more than 10,000 years is highly abundant in Cryptopodzols: the stocks of macrocharcoal and BPCA-C amount to up to 31 t ha−1 and 17 t ha−1, respectively. These high amounts of charred organic matter are responsible for the dark soil colour and high SOC concentrations that are, however, also closely related to Fep and Alp concentrations. We concluded that the occurrence of charcoal across the whole profiles of Cryptopodzols seems to be the dominating factor, although both the formation of organo-metallic or organo-mineral complexes in the subsoil and the high abundance and stability of charred organic matter are responsible for the high SOC stocks in Cryptopodzols.

Research highlightsOur study on SOM stabilisation Cryptopodzols of Southern Switzerland suggested that organo-mineral, or organo-metallic complexation and the recalcitrance of charcoal particles are responsible for stabilising SOM. The presence of free particulate charcoal and its significant correlation to SOC is an evidence that not only sorption to mineral surfaces but also molecular recalcitrance is an important factor. BPCA-C, however, strongly interacts with metals that are organically bound, and contributes through this mechanism to the stabilisation of SOM. We conclude that fire-derived C contributes significantly to the black colour and to the high C stocks in southern Switzerland. In addition, statistical evidence suggests that sorption processes stabilise plant and, also very likely, fire-derived SOC, mainly in the subsoil. The conclusions are based on the following results:
► charcoal, which has been present in the soils for 13,000 years, and BPCA-C can be detected in much higher concentrations and stocks in the Cryptopodzols than in the Cambisol;
► charred organic matter, measured as BPCA, intensifies the dark soil colour;
► the amount of SOC and BPCA-C correlated well with the concentrations of pyrophosphate-extractable, organically bound Al and Fe fractions in the soils;
► SOC was mainly present in the light fraction of topsoils, but also in the medium and heavy fractions in the subsoil horizons.