Influence of elevated carbon dioxide on soil-atmosphere exchange of methane and nitrous oxide in a coniferous forest

Methane and N₂O fluxes were measured for 2 years in laboratory and field studies at the Duke Forest free-air CO₂enrichment (FACE) site, an aggrading pine forest that has been continuously fumigated with CO ₂ to simulate future global environments by maintaining the atmospheric concentration at 560 μl l−1, or 200 μl l −1 above ambient. The primary objective was to determine the influence of elevated CO₂ on the soil-atmosphere exchange of the “greenhouse” gases CH₄ and N₂O flux at an ecosystem scale, relative to ambient CO₂ controls.; Methane consumption in CO₂-enriched plots was reduced 16 to 30%, compared to ambient CO₂ plots. The negative effect of FACE on rates of CH₄ consumption increased with both increasing temperature and time since initiation of fumigation. Evidence of a timelag for the effect of CO₂ treatment was found, for the reduction in soil CH₄ consumption was apparent only after 2 years of exposure to an enriched CO₂ atmosphere. Results of laboratory incubations corroborated field studies, showing net methane oxidation was 46% lower in soils collected under elevated CO₂, as compared with controls, despite similar moisture, pH, temperature, and NO₃− and NH ₄+ concentrations. Thus, rising atmospheric CO₂ may reduce atmospheric CH₄ consumption in temperate forest soils, which would result in higher levels of atmospheric CH₄ and a positive feedback to the greenhouse effect.; The effect of CO₂ enrichment on N₂O flux in situ was analyzed with soil moisture, soil temperature, and season as covariates. Results indicated a significant CO₂ x moisture x temperature interaction. At high moisture, soils at low temperatures under elevated CO₂ responded with greater N₂O emissions than controls, whereas high moisture soils at high temperatures responded with lower N₂O emissions than controls. Also, a significant CO2 x temperature x season interaction showed that increasing temperature in FACE soils led to greater N₂O production than in control soils in the winter and lower N₂O production in the warmer seasons. Overall, time-integrated N₂O fluxes in 1998 and 1999 were similar between elevated and ambient CO₂ plots. Compensatory shifts in nitrification and denitrification, as governed by soil moisture and temperature, may explain the variable effect of FACE and the resultant lack of change in N₂O flux at the soil-atmosphere interface.