Reduced atmospheric CH4 consumption by temperate forest soils under elevated CO2

Impacts of the projected increase in the atmospheric CO₂ concentration on other biogeochemical cycles are uncertain. We previously reported for the calendar years 1999 and 2000 a 16% to 30% decrease in atmospheric methane (CH₄) consumption in a temperate loblolly pine (Pinus taeda) forest experimentally exposed to elevated CO₂ (550 ppm). The reasons for the decline are unclear, as is the time trajectory for reduced CH₄ consumption. Elevated photosynthesis has been demonstrated to be a transient response (2 yr duration) to elevated CO₂ in a tundra ecosystem. Consumption by upland soils is the only known terrestrial sink for CH₄ accounting for 30 Tg yr^-1. A sustained response of this magnitude will significantly impact the atmospheric CH₄ cycle. Our current research is directed at: (1) extending the observational period for soil CH₄ consumption at fixed sites to determine the long-term trajectory for this activity; and (2) identify the factor(s) responsible for the reduced CH₄ consumption under projected future atmospheres.

The observed reduction in CH₄ consumption appears to be a sustained response to elevated CO₂. Relative to unfumigated controls, CH₄ consumption was reduced by 13% (2001-2002), 19% (2004), and 25% (2005 to date). Known controls of CH₄ consumption by soil microbes include O₂ supply, temperature, soil pH, soil moisture, the CH₄ supply rate, the presence of organic compounds (monoterpenes, aqueous extracts), and abundance and ratios of common ions. Our laboratory experiments indicate that changes in the chemical composition of leachate from aboveground plant materials and the organic soil layer, as well as chemicals identified as components of root exudates, had no impact on the CH₄ oxidizing community. Rates of CH₄ consumption by soil samples amended with throughfall, leaf leachate, duff leachate, and chemicals identified in root exudates from CO₂-enriched and control plots were not significantly different. It is most likely that the decline in CH₄ consumption may be related to the rate of supply of CH₄ to the subsurface zone of oxidation, as soil moisture was significantly higher in CO₂ enriched plots in 2004. Higher soil moisture may impede CH₄ atmospheric diffusion into the soil and also lead to anoxic microzones where CH₄ production is possible.