Characterization And Modeling Of Microbial Carbon Metabolism In Thawing Permafrost

Increased temperatures in high latitude regions are warming the surface and subsurface, resulting in thawing permafrost. At issue is the potential for increased greenhouse gas (GHG) generation and emission, caused by microbial degradation of vast stores of buried organic carbon. Most global-scale land-surface models lack depth-dependent representations of carbon conversion and GHG transport; therefore they do not adequately describe permafrost thawing or microbial mineralization processes. The current work was performed to determine how permafrost thawing at moderately elevated temperatures and anoxic conditions would affect CO2 and CH4 generation and emission, while refining the resolution of the Community Land Model (CLM4) by parameterizing depth-dependent GHG production processes, with respect to temperature and pH. These enhancements will improve the accuracy of GHG emission predictions and identify key biochemical and geochemical processes for further refinement. Core samples were obtained from a discontinuous permafrost site in Fairbanks, AK with a mean annual temperature of -3.3oC. Each core was sectioned into surface/near surface (0-0.8 m), active layer (annual thawing/freezing, 0.8m-1.6m .), and permafrost (1.6-2.2 m). Core sections were pulverized and used for sediment characterization as well as microcosm construction. Sediment characterization included water content (20-60%), pH (5.5-6.6), total N (0.05-0.25%) and C (0.4-4.1%), and total organic carbon (0.4-3.6%). Surface layer microcosms were constructed aerobically while the active and permafrost layers were constructed anaerobically. The microcosms, 20 g sediment with 38 ml headspace had either in-situ water levels (n=6) or 15 ml sterile water added (n=2) to saturate, and then incubated at -2oC, +3oC, or +5oC for 6 months. At monthly intervals, CO2 and CH4 were quantified by GC. At 6 months, microcosm samples and original core material were analyzed via 454 16S rDNA pyrosequencing to identify changes in the individual microbial populations and community succession. CO2 production was highest in the surface/near surface incubations (4-14%) while CH4 was undetectable. Active layer sediments produced considerably less CO2 (0.2-0.7%) but CH4 was detected up to 0.25%. Concentrations of CO2 found in the deep permafrost incubations were comparable to those in the active layer, while CH4 was considerably higher ranging from 0.2-0.6%. Overall, the CO2 generation rate (0.02-0.12 umol/g/month) was roughly 50 times that of CH4 biogenesis (0.002-0.007 umol/g/month). GHG levels peaked after 4 months, and the pH of all but one of the core sections fell below 5.7, suggesting that acid accumulation may be an important physicochemical parameter in controlling GHG biogenesis. Surprisingly, increasing temperature did not necessarily increase GHG emission rates. New simulations are being performed incorporating these data in CLM4, including a recently introduced methane biogeochemistry module.