Submitted to: Biology and Fertility of Soils
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/5/2007
Publication Date: 9/29/2007
Citation: Mclain, J.E., Ahmann, D.M. Increased moisture and methanogenisis contribute to reduced methane oxidation in elevated co2 soils. Biology and Fertility of Soils. 44:623-631.
Interpretive Summary: Atmospheric concentrations of the greenhouse gas CO2 have risen sharply in recent decades. In the same time period, concentrations of methane (CH4), a greenhouse gas 26 times more potent than CO2, have more than doubled. The CH4 increase would be even more dramatic without the activity of soil microorganisms that destroy CH4 before it reaches the atmosphere. In recent years, several studies have been published showing that elevated atmospheric CO2 inhibits the activity of CH4-consuming microorganisms in soils. This study was conducted to determine the environmental factors contributing to the observed decreases in soil CH4 consumption. Soil samples were obtained over 11 months at the Free Air Carbon Transfer and Storage (FACTS)-I site in the Duke Forest, North Carolina, where plots have been exposed to ambient (370 ppm) or elevated (570 ppm) atmospheric CO2 since August 1996. We found that the decreased CH4 uptake corresponded to increased soil moisture. In addition, a small percentage of the soil samples showed microbial CH4 production, and 80% of these came from elevated CO2 plots, suggesting that CH4 production must be considered as a contributor to decreased net CH4 consumption under elevated CO2. This work will contribute to the development of improved predictive models to quantify the contributions of soils to current and future climate change.
Technical Abstract: Understanding the mechanisms controlling carbon dioxide-induced soil change has taken on a new immediacy with recent awareness of the biogeochemical linkages in the cycles of carbon dioxide (CO2) and methane (CH4) and the roles these compounds will play in mediating future climate change. Factors impacting soil CH4 consumption were investigated using laboratory incubations of soils collected at three depths (0-5, 15-20, and 25-30 cm) over 11 months at the Free Air Carbon Transfer and Storage (FACTS)-I site in the Duke Forest, NC. Plots at this site have been exposed to ambient (370 ppm) or elevated (ambient + 200 ppm) CO2 since August 1996. Nearly 90% of the 352 activity measurements showed net CH4 consumption, confirming that methanotrophic bacteria were active in these soils. Soil moisture, measured as percent water-holding capacity (%WHC) was significantly (p < 0.01) higher in the 25-30 cm depth of elevated CO2 soils over the length of the study, but moisture levels were equal between CO2 treatments in shallower soils. Increased soil moisture corresponded to decreased net CH4 oxidation, as elevated CO2 soils also oxidized less CH4 at the 25-30 cm depth, while net methanotroph activity at the 0-5 and 15-20 cm depths was statistically equal between treatments. Soil moisture content predicted (p < 0.05) net methanotroph activity in the 0-5 and 15-20 cm depth of ambient CO2 soils, but the relationship between soil moisture and net CH4 oxidation was not significant in elevated CO2 soils at any depth, suggesting that additional factors were strongly influencing net CH4 oxidation in elevated CO2 soils. More than 6% of the activity assays showed net CH4 production and of these, 80% contained soils from elevated CO2 plots, suggesting that CH4 production may contribute to decreased net CH4 consumption under elevated CO2 in otherwise aerobic soils.