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

Earth hummocks are the most common patterned ground feature in the Subarctic region and subduction of organic matter by cryoturbation (i.e., mixing of soil layers by repeated freezing–thawing) physically protects soil organic carbon (SOC) from decomposition. Climate models predict that Subarctic regions will experience disproportionate rates of warming, which may accelerate rates of decomposition of stored SOC and thus cause increased release of CO2. Our objectives in this study were to characterize and develop relationships between the chemistry and bioavailability of SOC in the horizons of Subarctic earth hummocks. Biodegradability was assessed in a controlled mineralization study in the laboratory. The chemical composition of soil organic matter (SOM) was characterized by X-ray absorption near-edge structure (XANES) spectroscopy at the carbon (C) K-edge and its thermal stability was determined by Rock-Eval 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.