ARS | Publication request: Reduced net atmospheric CH4 consumption is a sustained response to elevated CO2 in a temperate forest

Submitted to: American Geophysical Union
Publication Type: Abstract Only
Publication Acceptance Date: November 13, 2009
Publication Date: December 14, 2009
Citation: Whalen, S., Dubbs, L., Fischer, E.N., Phillips, B.L. 2009. Reduced net atmospheric CH4 consumption is a sustained response to elevated CO2 in a temperate forest [abstract]. American Geophysical Union.

Technical Abstract: We compared nearly continuously from 1998 until 2006 rates of soil atmosphere CH4 exchange at permanently established sampling sites in a temperate loblolly pine (Pinus taeda) forest exposed to ambient (control plots; approx. 380 uL L-1) or elevated (ambient + 200 uL L-1) CO2. Net atmospheric CH4 consumption (flux from the atmosphere to the soil) from static chambers was observed most of the time at rates ranging to 4.5 mg m-2 d-1. However, we infrequently found net CH4 production (flux from the soil to the atmosphere) at lower rates, with a maximum of 0.08 mg m-2 d-1. Annual rates of net CH4 consumption in control plots (183 to 204 mg m-2 y-1) were 8 to 30% higher than comparable values for CO2-enriched plots. Differences between treatements were significant (p<0.05) in five of the study years and nearly significant (p=0.10) in the sixth. Volumetric soil water content was significantly higher in five of six study years at CO2-enriched sites and a mixed effects model identified a significant soil moisture x CO2 interaction on net atmospheric CH4 consumption. Laboratory experiments show generally higher potential rates of CH4 production and lower rates of CH4 consumption in soils from CO2-enriched sites while soils exposed to organic acids demonstrably produced by loblolly pine under elevated CO2 sometimes reduced rates of CH4 consumption. Increased soil moisture at CO2-enriched sites likely increased diffusional resistance of surface soils to atmospheric CH4 amd increased the frequency of anaerobic microsites supporting methanogenesis, while organic root exudates may have also periodically reduced rates of atmospheric CH4 consumption. A decline in atmospheric CH4 consumption of the magnitude observed here (approx. 15%) across all forest biomes gives a decrease of 3.6 Tg CH4 y-1, a value that is not inconsequential as it represents nearly 10% of a model estimate (Ridgwell et al. 1999) of 38 Tg CH4 y-1 for the total sink.