Atmospheric pressure drives changes in the vertical distribution of biogenic free-phase gas in a northern peatland

Atmospheric pressure (P_atm) is known to regulate methane emissions from northern peatlands. However, recent conceptual models differ in how gas production and release occurs in shallow (defined here as less than 1 m depth) versus intermediate to deep peat soils (i.e., more than 1 m depth). We used ground penetrating radar (GPR) measurements to non-invasively estimate the vertical distribution of free-phase gas and the dependence of this distribution on atmospheric pressure in a northern peatland. Variations in the travel time of the electromagnetic wave to three interfaces in the peat column were used in conjunction with deformation rod data and a petrophysical model relating electromagnetic wave velocity to free-phase gas content to model changes in the vertical distribution of free-phase gas over time. We found a negative linear relation between changes in free-phase gas content and changes in P _atm for shallow peat soils and a positive linear relation for deeper soils. Our results suggest that (1) free-phase gas content confined in deep peat soils is larger and less variable to changes in P_atm than gas in shallow/intermediate peat soils; (2) increases in P_atm result in gas release from shallow peat soils into the atmosphere (i.e., rapid ebullition); and (3) decreases in P_atm result in upward gas movement from intermediate layers to replenish shallow layers. Our results suggest that changes in P_atm drive changes in the vertical distribution of free phase gas in peat soil and regulate methane ebullition from peat soils to the atmosphere. Our data shows a relationship between free phase gas and depth that may be due to changes in peat properties or increasing water pressure with depth.