The sustained increase in atmospheric CO2 concentration observed over the past century, and projected to continue into the next, is of great significance for atmospheric CH4. Effects of elevated CO2 on microbial methane cycling are potentially mediated by its effects on plant physiology, which include enhancement of carbon assimilation, belowground carbon allocation, and water use efficiency. To determine the importance of such changes for methane cycling, belowground factors impacting soil CH4 consumption were investigated at the Free Air Carbon Transfer and Storage (FACTS)-I site in the Duke Forest, North Carolina, in which plots have been exposed to ambient (370 ppm) or elevated (ambient + 200 ppm) CO2 since August 1996. CH4 fluxes at the soil surface, porespace concentrations of CH4, O2, and CO2, soil moisture, soil temperature, and soil pH were simultaneously measured over 24 months. Porespace CH4 concentrations were highest (1.98 ¿ 0.25 ppm) at the soil surface and decreased to 0.65 ¿ 0.22 ppm at 30 cm, indicating that methanotrophic activity was depleting CH4 in the upper soil layers and creating a gradient to draw atmospheric CH4 into the soil. This was confirmed by surface CH4 flux measurements, which averaged -1.54 ¿ 0.65 ¿mol/m2/h. Under elevated CO2, porespace CH4 was 25--30% higher in the upper 70 cm of soils; CH4 fluxes from the atmosphere into soil were diminished by ~25%; soil CO2 increased by 10--70%; and volumetric soil moisture was greater by up to 40% during some seasons. Statistical modeling revealed that soil moisture strongly predicted variability in surface CH4 fluxes and that soil CO2 and soil moisture both predicted variability in soil CH4. Results also indicated that a portion of the net CH4 sink inhibition in elevated CO2 soils could be attributable to alterations in soil biological processes, suggesting that changes in the CH4-cycling microbial ecology had taken place.