Ancient Low-Molecular Weight Organic Acids in Permafrost Fuel Carbon Dioxide Upon Thaw

Northern permafrost soils contain an estimated 1,672 Pg of carbon (C), nearly twice that of the present atmosphere. Current and projected amplification of polar warming threatens to destabilize and thaw these frozen, organic carbon (OC) rich soils. Upon thaw, mobilized permafrost OC can enter streams and rivers, which are now recognized as important processors of terrestrial organic matter and conduits for carbon dioxide (CO2) to the atmosphere. In this study, a series of biodegradation experiments sampled at high temporal resolution were used to assess the quality and mineralization of permafrost C in a simulated aquatic setting. Over a 200 hour incubation period, dissolved organic carbon (DOC) concentrations decreased by an average of 53%, whereas aqueous dissolved inorganic carbon (DIC) concentrations increased by an average of 660%. Ion Chromatography revealed that 87% of the DOC lost were low molecular weight organic acids, acetate and butyrate. Specific UV Absorbance at 254 nm increased over the incubation period, corresponding to the loss of these low-molecular weight organic molecules. This finding is corroborated by the simultaneous loss of fluorescence signatures attributed to low-molecular weight phenols. Collectively, these results are among the highest biolability values reported for permafrost C and are the first to directly link CO2 production with low-molecular weight DOC loss. The high biolability and rapid respiration documented in these experiments suggests that permafrost C is quickly metabolized upon thaw and outgassed to the atmosphere as CO2 in upstream ecosystems, either during transit through soils or within headwater streams. This finding may help to account for both the elevated levels of CO2 in Arctic headwater streams and the lack of an aged isotopic signal indicative of the mobilization of permafrost to the mouths of higher order Arctic rivers