Persistence of, and the Potential Reasons for, Reduced Atmospheric CH4 Consumption by Temperate Forest Soils Under Elevated CO2

Atmospheric CO2 concentrations have increased an average of 1.65 ppmv per year over the past 19 years {to 2004; Keeling 2005}. The current atmospheric concentration is greater than 360 ppmv. That concentration is expected to continue to increase and, by the end of the present century, will exceed the pre-industrial concentration as much as 250%. Little is known about the influence of increasing concentrations of atmospheric CO2 on other biogeochemical cycles. In a two-year study, Phillips et al.{2001a,b} reported a 17 {yr 2} to 30% {yr 1} decrease in atmospheric CH4 consumption by soils in CO2-enriched plots in a temperate loblolly pine {Pinus taeda} forest, although the reason for the decline was unclear. It is uncertain whether decreased atmospheric CH4 consumption represents a transient or sustained response of forest-soil systems to elevated CO2. The overall objectives of this research are to investigate the persistence of the previously observed decline in atmospheric methane consumption by loblolly pine {Pinus taeda} forest soils under elevated CO2, and to evaluate the reasons for the negative feedback of increased atmospheric CO2 on soil CH4 consumption. More specifically, field observations, and laboratory and field experiments have been employed between elevated and ambient CO2 treatments; identify differences in the chemical composition of forest plant exudates between elevated and ambient CO2 treatments that affect net CH4 consumption; and qualitatively and quantitatively compare the methane oxidizer and producer communities between elevated and ambient CO2 treatments. We report here the findings of the first two years of this three-year study.