Methane Uptake in a Semiarid Grassland Affected by Elevated CO2 and Warming.

Semiarid rangelands represent a significant global sink for methane (CH₄) where CH₄ 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.  The optimum soil moisture level for methane uptake occurred around 24% water filled pore space (WFPS).  We observed no significant CO₂ effect on CH₄ uptake, but CH₄ uptake significantly decreased with warming in both years (on average by 25 and 13% in 2007 and 2008 respectively).  The effects of warming and elevated CO₂ were most strongly expressed at times when soils were dry (< 24% WFPS): warming significantly reduced CH₄ uptake, but elevated CO₂ increased CH₄ uptake.  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.