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Title: Methane Uptake in a Semi-Arid Grassland Affected by Elevated CO2 and Warming: Role of Methanotroph Activity and Gas Diffusion

Author
item Dijkstra, Feike
item VON FISCHER, JOSEPH - Colorad0 State University
item Morgan, Jack
item Follett, Ronald

Submitted to: ASA-CSSA-SSSA Annual Meeting Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: 4/1/2009
Publication Date: 11/1/2009
Citation: Dijkstra, F.A., Von Fischer, J.C., Morgan, J.A., Follett, R.F. 2009. Methane Uptake in a Semi-Arid Grassland Affected by Elevated CO2 and Warming: Role of Methanotroph Activity and Gas Diffusion. ASA-CSSA-SSSA Annual Meeting Abstracts. #51843.

Interpretive Summary:

Technical Abstract: Semiarid rangelands represent a significant global sink for methane (CH4) where methane uptake is controlled by methanotroph activity and the diffusivity of CH4 into the soil. Because increasing soil moisture causes diffusivity to fall but methanotroph activity to rise, methane uptake rates show a hump-shaped response to soil moisture, with a distinct optimum soil moisture level. Both CO2 and temperature affect soil moisture, and the direction of methane uptake response may depend on if the system is below or above the soil moisture optimum. We studied the effects of atmospheric CO2 (ambient vs. 600 ppm), and temperature (ambient vs. 1.5/3.0 ºC warmer day/night) on CH4 uptake during the growing season of 2007 and 2008 in a full factorial semiarid grassland field experiment in Wyoming, USA (Prairie Heating And CO2 Enrichment, PHACE), using static chamber techniques. We also added the inert gas sulfur hexafluoride to the chambers to determine the diffusivity of the soil, and applied a reaction-diffusion model to determine methanotroph activity from our gas diffusivity and CH4 uptake measurements. The effects of warming and elevated CO2 were most strongly expressed in dry soils (< 13% v/v soil moisture): warming significantly reduced CH4 uptake, but elevated CO2 increased CH4 uptake. The optimum soil moisture level for methane uptake occurred around13% v/v. Under drier conditions (i.e., during much of the growing season), warming-induced decreases and elevated CO2-induced increases in soil moisture affected methanotroph activity more than diffusivity. On the few occasions when soils were wetter than 13% water content, soil moisture had stronger effects on diffusivity, and warming and elevated CO2 effects on CH4 uptake were small. Our results indicate that global climate change is likely to alter methane uptake in semiarid grasslands differently from mesic ecosystems because semi-arid grasslands are usually below the optimum soil moisture level for methane uptake.