Perennially and annually frozen soil carbon differ in their susceptibility to decomposition: Analysis of Subarctic earth hummocks by bioassay, XANES and pyrolysis

The mineralization bioassay showed that buried organic horizons were less susceptible than surface SOM to biodegradation, and not significantly more susceptible than the adjacent mineral soil. Analysis by XANES showed the accumulation of ketones in buried organic horizons, and the loss of carbohydrate, phenolic and carboxylic compounds. This suggests that ketones can be used as biomarkers for microbially transformed SOM. In contrast, SOM in perennially frozen mineral soils (i.e., below the permafrost table), was more susceptible to biodegradation than that in buried mineral and organic soils in the annually frozen active layer. The SOM in these horizons did not show ketone signals but instead showed strong phenolic content. Analysis by pyrolysis indicated that the thermolabile fraction was related to the bioavailability of C, and that in perennially frozen soils, this fraction contained proportionately higher oxygen-containing functional groups. These results point to a pool of labile SOC, relatively rich in phenolic compounds, in perennially frozen soils which may be susceptible to decomposition in a warming climate. Future warming-induced C losses, therefore, may mostly occur not from annually-frozen SOM buried by cryoturbation, but from perennially-frozen C made accessible by falling permafrost table.