Effects of Long-Term CO(2) Enrichment on Soil-Atmosphere CH(4) Fluxes and the Spatial Micro-Distribution of Methanotrophic Bacteria

BACKGROUND: Effects of elevated atmospheric CO(2) concentrations on plant growth and associated C cycling have intensively been studied, but less is known about effects on the fluxes of radiatively active trace gases other than CO(2). Net soil-atmosphere CH(4) fluxes are determined by the balance of soil microbially-driven methane (CH(4)) oxidation and methanogenesis, and both might change under elevated CO(2). METHODS AND RESULTS: Here, we studied CH(4) dynamics in a permanent grassland exposed to elevated CO(2) for 14 years. Soil-atmosphere fluxes of CH(4) were measured using large static chambers, over a period of four years. The ecosystem was a net sink for atmospheric CH(4) for most of the time except summer to fall when net CH(4) emissions occurred. We did not detect any elevated CO(2) effects on CH(4) fluxes, but emissions were difficult to quantify due to their discontinuous nature, most likely because of ebullition from the saturated zone. Potential methanotrophic activity, determined by incubation of fresh sieved soil under standardized conditions, also did not reveal any effect of the CO(2) treatment. Finally, we determined the spatial micro-distribution of methanotrophic activity at less than 5× atmospheric (10 ppm) and elevated (10000 ppm) CH(4) concentrations, using a novel auto-radiographic technique. These analyses indicated that domains of net CH(4) assimilation were distributed throughout the analyzed top 15 cm of soils, with no dependence on CH(4) concentration or CO(2) treatment. CONCLUSIONS: Our investigations suggest that elevated CO(2) exerts no or only minor effects on CH(4) fluxes in the type of ecosystem we studied, at least as long as soil moisture differences are small or absent as was the case here. The autoradiographic analyses further indicate that the spatial niche of CH(4) oxidation does not shift in response to CO(2) enrichment or CH(4) concentration, and that the same type of methanotrophs may oxidize CH(4) from atmospheric and soil-internal sources.