Chemical and Physical Controls, and Microbial Implications, on Methane Fluxes During an Unusually Warm Spring Thaw in Barrow, Alaska

Abstract:

The magnitude of warming predicted for the Arctic is much higher than elsewhere on the planet. Methane emissions from Arctic wetlands may contribute to a positive feedback loop in global atmospheric warming dynamics. Understanding the inputs, outputs, pool size and pertinent controls on production and release will be critical for making accurate predictions about how Arctic methane cycling will affect global climate change. The goal of the research presented here is to investigate the springtime shoulder season with regard to methane flux, physical and chemical soil characteristics, and abundances of methanogenic precursors. This time period coincides with rapid lowering of the thaw depth and water table with the onset of the growing season. Significant shifts in soil organic carbon constituents and alternative electron acceptors occur during this time period as well as a sharp increase in methane flux observed at the landscape level. From June 16th to June 27th, 2015, on the Barrow Environmental Observatory in Barrow, Alaska, methane and carbon dioxide chamber fluxes were measured and soil chemical properties were recorded on alternating days. Additionally, soil pore water was collected every day as well as thaw depth and water table measurements. This study was replicated across 20 plots representing 4 microtopographic polygon features. Interestingly, this study site experienced exceptionally high late-spring / early-summer air temperatures, making the data presented here useful for predicting future conditions under a warming Arctic climate scenario. Results from this study indicate a sharp increase in methane flux with increasing thaw depth, followed by a plateau in methane flux with a lowering of the water table and an increase in soil conductivity. Methane flux dynamics may be due to release of frozen soil methane stores during thaw down, a change in the concentration and oxidation state of alternative electron acceptors, a nutrient pulse of methanogenic precursors in soil pore water, stimulation and/or inhibition of methanogenic and methanotrophic microbial groups, or a combination of some or all of these factors. Chemical analysis of pore water samples is ongoing and should shed light on the role of alternative electron acceptors and methanogenic precursors during this important transition period.