A Lumped Bubble Capacitance Model Controlled by Matrix Structure to Describe Layered Biogenic Gas Bubble Storage in Shallow Subtropical Peat

Methane (CH4) accumulates in the gaseous phase in peat soils, being released to the atmosphere at rates higher than those for diffusion and plant-mediated pathways. An understanding of the mechanisms regulating gas bubble storage in peat remains incomplete. We developed a layered capacitance model to compare the bubble storage ability of peat over different depths. A peat monolith (0.395 m x 0.243 m x 0.247 m) was collected from the U.S. Everglades and kept submerged for 102 days from a condition of minimum bubble storage to bubble saturation. Time-lapse electromagnetic wave velocity and power spectrum data were used to estimate changes in both gas content and relative average dimensions of stored bubbles with depth. Bubble capacitance, defined as the increase in volumetric gas content (m(3) m(-3)) divided by the corresponding pressure (Pa), ranges from 3.3 x 10(-4) to 6.8 x 10(-4) m(3) m(-3) Pa-1, with a maximum at 5.5 cm depth Bubbles in this hotspot were larger relative to those in deeper layers, while the decomposition degree of the upper layers was generally smaller than that of the lower layers. X-ray computed tomography on peat sections identified a specific depth with a low void ratio, and likely regulating bubble storage. Our results suggest that bubble capacitance is related to (1) the difference in size between bubbles and peat pores, and (2) the void ratio. Our work suggests that changes in bubble size associated with variations in water level driven by climate change will modify bubble storage in peat soils.