Carbon dioxide and methane fluxes from the transitional zone of a Virginia ephemeral wetland

Abstract:

The spatial and temporal controls mediating the switch between anaerobic and aerobic respiration within soils located in transitional zones adjacent to ephemeral wetlands remains unclear. As ephemeral wetlands dry down, a soil moisture gradient develops in adjacent transitional zones resulting in changes to the soil environment—moving from anoxic to oxic conditions. Under oxic conditions, aerobic decomposition and CO₂ fluxes should dominate, while under anoxic conditions, anaerobic decomposition and CH₄ emissions should be more prominent.

To investigate the spatial controls and temporal dynamics of anaerobic and aerobic respiration we ran three 20 m transects starting from the late spring peak wetland edge (June 1, 2014 max. lake extent) of Lake Arnold, an ephemeral wetland located at Blandy Experimental Farm in Boyce, Virginia. At 10 m intervals along each transect, high-resolution soil moisture and temperature sensors were installed at three depth levels in the soil (5 cm, 20 cm, and 50 cm). Soil surface CO₂ efflux was measured weekly at 5 m intervals using a portable, infra-red gas analyzer and surface chamber (EGM-4 and SRC-1; PP Systems; Amherst, MA). CH₄ emissions were sampled weekly using a non-steady state chamber at 10 m intervals along each transect and analyzed in the lab using gas chromatography. Redox potential was measured weekly at two soil depths (5 cm and 20 cm) at 5 m intervals using platinum electrodes and a Ag/Cl reference electrode.

Lake Arnold water levels decreased at a rate of 18.16 mm day^-1 during the month of July. Preliminary results show a distinct drop in soil moisture at 5 and 20 cm depths at the 0 and 10 m distances along each transect. At 50 cm, soil moisture shows no distinct trend. Late July measurements of redox potential ranged from -196 mV to 865 mV and was correlated with soil moisture (R² = 0.52). Rates of soil CO₂ efflux were diminished at volumetric water contents (VWC) above 45% (ranging from 2.45 – 7.3 µmol CO₂ m^-2 sec^-1). Below 45% VWC, soil CO₂ efflux rates ranged from 4.5 – 9.6 µmol CO₂ m^-2 sec^-1.