Exploring Fine-scale Drivers of Methane Sources and Sinks in a Boreal Wetland

In warming boreal ecosystems, permafrost thaw exposes soil organic carbon to microbial decomposition and affects land-atmosphere carbon exchange. To monitor whether increasing methane (CH4) emissions are reducing overall carbon sink status, we need information on the magnitudes and primary drivers of CH4 flux (i.e., emission and uptake) at fine spatial scales to reduce uncertainties in carbon budget accounting for landscape-to-regional level assessments. Due to high spatiotemporal heterogeneity in boreal wetlands, it can be challenging to identify the role of surface versus subsurface conditions in regulating the net surface flux. This is especially difficult in thermokarst landscapes with complex talik geometry and sub-permafrost groundwater flow that can lead to CH4 emission hotspots. In this study, we investigate CH4 flux drivers at a wetland site near the Big Trail Lake Complex in Goldstream Valley, a watershed north of Fairbanks, Alaska characterized by discontinuous permafrost, yedoma thermokarst lakes, and abundant wetlands. In July 2021, we sampled a 2-hectare wetland area located 150 meters north of Big Trail Lake and underlain by sub-permafrost groundwater channels. We collected chamber-based CH4 flux, in-situ soil variables, and soil samples for chemical and microbial community analysis. The flux data indicated that CH4 sinks and sources varied over small spatial scales (< 5 m). We found that the CH4 sink/source pattern was partially explained by microtopography and vegetation type, with CH4 sinks tending to have higher elevation and shrub cover. Methane emissions were related to soil moisture at 20 cm depth, but near-surface soil temperature was not identified as a significant driver of CH4 emissions. In ongoing work, we are investigating methanogen and methanotroph abundance and phylogeny as drivers of near-surface CH4 production and oxidation. We are also obtaining new geophysical measurements (August 2022) to identify whether observed CH4 emission hotspots are being driven by deep, underlying permafrost thaw and sub-surface water channeling.