Effect of long-term free-air CO ₂ enrichment on the diversity and activity of soil methanogens in a periodically waterlogged grassland

As atmospheric CO ₂ levels continue to rise researchers now identify concomitant changes in plant biomass and diversity, which are postulated to alter the quality and quantity of the organic carbon entering the soil. In anoxic soils, CH ₄ is the end product of the degradation of organic carbon and the system's terminal electron sink. Some soils (such as wetlands) are usually waterlogged and therefore constitute permanent CH ₄ sources, while others (hydromorphic soils) are only occasionally saturated with water and alternate between acting as net CH ₄ sources or sinks. Since methanogenesis is ultimately dependent on soil organic carbon, we hypothesized that a long term alteration of the latter will cause changes in type and magnitude of the former. To test this, we studied the effect of 10 years of atmospheric CO ₂ enrichment on the methanogenic potential and community in a hydromorphic temperate grassland soil at the experimental Free Air Carbon dioxide Enrichment (FACE) site in Giessen, Germany. While all soils demonstrated methanogenic potential, we detected no significant changes in CH ₄ production rates, lag times, methanogenic pathways, diversity, or population sizes in soils that were exposed to either 20 or 30% elevated ambient CO ₂. Our findings suggest that the methanogenic potential of the soil and the methanogenic community might be insensitive to changes in atmospheric CO ₂ concentrations, at least not on a decadal timescale. Thus, if our results can be extrapolated to other temporarily flooded or even wetland ecosystems, the often-observed increase in CH ₄ emissions under elevated CO ₂ may simply be due to an increase in labile-C input via living root and increasing fresh litter deposition, but not due to shifts in the microbial population characteristics. This could make it easier to model and extrapolate the global effect of elevated CO ₂ on terrestrial CH ₄ emissions.