Controlled Freeze-thaw Experiments to Study Biogeochemical Process and its Effects on Greenhouse Gas Release in Arctic Soil Columns

Abstract:

Greenhouse gas release associated with permafrost thawing is one of the largest uncertainties in future climate prediction. Improvement of such prediction relies on a better representation of the interactions between hydrological, geochemical and microbial processes in the Arctic ecosystem that occur over a wide range of space and time scales and under dynamic freeze-thaw conditions. As part of the Next Generation Ecosystem Experiments in the Arctic (NGEE-Arctic), we conducted controlled laboratory freeze-thaw experiments to study greenhouse gas release in vertical permafrost soil columns with vertically heterogeneous hydrological, geochemical and microbial properties. The studies were performed using soil cores collected from the NGEE Barrow, AK site. Two cores collected next to each other with very similar soil structures were used for the experiment. One of the cores was destructively sampled for baseline characterization, and the second core was used for the freeze-thaw experiments. The core extends from the ground surface into the permafrost with roughly 40 cm of active layer. The column was instrumented with various sensors and sampling devices, including thermocouples, geophysical (electrical) sensors, and sampling ports for solids and fluids. The headspace of the soil column was purged with CO₂ free air and the gas samples were collected periodically for greenhouse gas analysis. Our initial tests simulated seasonal temperature variation from ~ -10°C to +10°C at the ground surface. Our results demonstrated that temperature and geophysical data provided real time information on the freeze thaw dynamics of the column and the surface greenhouse gas fluxes correlated with the freeze thaw stages and associated hydrological and biogeochemical processes in the vertical soil column. For example, surface fluxes data revealed an early burst of GHG concentrations during the initial thawing of the surface ice rich layer of the soil, indicating the presence of trapped gases from previous season activities. In addition, the dynamics of the surface flux are closely related to the changes of the geochemical and microbial conditions during the freeze thaw processes.