Response of soil methanotrophic activity to carbon dioxide enrichment in a North Carolina coniferous forest

Rates of methane oxidation in soils from a forest that was enriched with carbon dioxide at 200 μl l−1 above ambient CO2 for 3 years were compared with rates in control soils from a forest that was aerated with ambient air. Laboratory incubations were performed on homogenized soil samples collected from the 0 to 7.5 and 7.5 to 15 cm soil zones at 8-week intervals from May through October. Methane was oxidized consistently at both depths. A repeated measures analysis of variance indicated that net CH4 consumption was significantly lower in soils collected under elevated CO2, despite similar moisture, temperature, NO3−, and NH4+ contents. The effect of CO2 enrichment on CH4 consumption was greatest in May and lowest in October. Potential differences in the labile fraction of organic matter in soils from CO2-enriched plots were not responsible for lower CH4 oxidation, as rates measured did not change for both ambient and elevated CO2 soils during a 2-week incubation. Net CH4 oxidation rates were unaffected by additions of 0.5 μmol NH4+ g−1 soil, but were significantly reduced in all soils at 1.0 μmol NH4+ g−1. Nitrate addition did not influence net CH4 oxidation at either concentration. Methane production (methanogenesis) was not evident after application of difluoromethane, an inhibitor of CH4 oxidation, indicating that the observed changes in headspace CH4 concentration during lab incubations resulted solely from methanotrophic activity. Methane consumption from May through October for elevated CO2 soils was 47% less overall than for ambient CO2 soils. The contemporary increase in atmospheric CO2 may negatively affect the soil CH4-oxidizing community of upland forest soils and reduce the sink strength of these ecosystems in the atmospheric CH4 budget.