|Von Fischer, Joseph -|
Submitted to: Journal of Geophysical Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 27, 2010
Publication Date: January 27, 2011
Citation: Dijkstra, F.A., Morgan, J.A., Von Fischer, J., Follett, R.F. 2011. Elevated CO2 and Warming Effects On CH4 Uptake in a Semiarid Grassland Below Optimum Soil Moisture. Journal of Geophysical Research 116, G01007, doi:10.1029/2010JG001288. Interpretive Summary: Methane (CH4) is an important greenhouse gas that has a 25 times greater relative global warming potential than CO2. Semi-arid grasslands are an important CH4 sink, but it is unclear how the sink strength of semi-arid grasslands is affected by climate change. Here we report how elevated atmospheric CO2 (600 ppm) and warming (1.5°C during the day and 3°C during the night) affected CH4 uptake in a semi-arid grassland in Wyoming. Methane uptake was not affected by elevated CO2, but significantly decreased with warming. Moreover, CH4 uptake strongly increased under elevated CO2 and showed the strongest reductions with warming when soils were dry. Our results are in contrast to other studies that were done in wetter environments and indicate that in a drier world CH4 uptake in these grassland ecosystems will be more sensitive to elevated CO2 and warming.
Technical Abstract: Semiarid rangelands are a significant global sink for methane (CH4), but this sink strength may be altered by climate change. The uptake of CH4 is sensitive to soil moisture showing a hump-shaped relationship with a distinct optimum soil moisture level. Both CO2 and temperature affect soil moisture, but the direction of methane uptake response may depend on if the system is below or above the soil moisture optimum. Most studies of methane uptake response to climate change have been conducted in mesic environments, where soil moisture levels are typically above the optimum, but little is known about the responses in drier systems with sub-optimal soil water. 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 two growing seasons in a full factorial semiarid grassland field experiment in Wyoming, USA (Prairie Heating And CO2 Enrichment, PHACE). The CH4 uptake was not affected by elevated CO2, but significantly decreased with warming in both years (on average by 25% in the first and 13% in the second season). As expected, we observed typical hump-shaped relationships between CH4 uptake and water filled pores space (WFPS), but surprisingly, the curve shape and optimum soil moisture differed with warming and CO2 treatments, suggesting that elevated CO2 and warming effects cannot be completely ascribed to their effects on soil moisture. Warming showed the strongest reduction and elevated CO2 the strongest increase in CH4 uptake when soils were below optimum moisture (optimum around 24% WFPS), indicating that these effects are particularly strong when soils are dry. We conclude that the directional effects of elevated CO2 and warming on CH4 uptake in semiarid grasslands can be opposite to their effects in mesic ecosystems because semiarid grasslands are often below optimum soil moisture for methane uptake.