Methane Transmission and Oxidation throughout the Soil Column from Three Central Florida Sites

Abstract:

When methane (CH₄) is generated in anoxic soil sites, it may be subsequently re-oxidized to carbon dioxide (CO₂). Understanding the controls on, and magnitudes of, these processes is necessary to accurately represent greenhouse gas production and emission from soils. We used a laboratory incubation to examine the influence of variable conditions on methane transmission and oxidation, and identify critical reaction zones throughout the soil column. Sandy soils were sampled from three different sites at Disney Wilderness Preserve (DWP), Florida, USA: a depression marsh characterized by significant surface organic matter accumulation, a dry pine flatwood site with water intrusion and organic horizon at depth (200+ cm); and an intermediate-drainage site. Contiguous, 30-cm long cores were sampled from N=4 random boreholes at each site, from the surface to the water table (varying from 90 to 240 cm). In the lab, each core was monitored for 50 hours to quantify baseline (pretreatment) gas fluxes before injection with 6 ml CH₄ (an amount commensurate with previous field collar measurements) at the base of each core. We then monitored CH₄ and CO₂ evolution for 100 hours after injection, calculating per-gas and total C evolution. Methane emissions spiked ~10 hours after injection for all cores, peaking at 0.001 µmol/g soil/hr, ~30x larger than pre-injection flux rates. On a C basis, CO₂ emissions were orders of magnitude larger, and rose significantly after injection, with elevated rates generally sustained throughout the incubation. Cores from the depression marsh and shallower depths had significantly higher fluxes of both gases. We estimate that 99.1% of the original CH₄ injection was oxidized to CO₂. These findings suggest either that the methane measured in the field at DWP originates from within a few centimeters of the surface, or that it is produced in much larger quantities deeper in the profile before most is subsequently oxidized. This highlights the need for better understanding and modeling the multiple processes that result in soil-atmosphere CO₂ and CH₄ fluxes.