Reduced atmospheric CH4 consumption by temperate forest soils under elevated CO2
Lindsay Dubbs, University of North Carolina, Chapel Hill, NC; and S. C. Whalen and E. N. Fischer
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.
Joint Poster Session 1, Land-Atmosphere Interactions (Joint with 18th Conference on Climate Variability and Change and 20th Conference on Hydrology)
Tuesday, 31 January 2006, 9:45 AM-11:00 AM, Exhibit Hall A2
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