A moderate increase in the annual CH4 efflux by raised CO2 or NH4NO3 supply in a boreal oligotrophic mire

Natural wetlands release about 20% of global emissions of CH4, an effective greenhouse gas contributing to the total radiative forcing. Thus, changes in the carbon cycle in wetlands could have significant impacts on climate. The effect of raised supply of CO2 or NH4NO3 on the annual CH4 efflux from the lawn of a boreal oligotrophic mire was investigated over two years. Ten study plots were enclosed with mini-FACE rings, five vented with CO2-enriched air and the other five with ambient air. In addition, five plots were sprayed with NH4NO3 so that the cumulative addition of N was 3 g m(-2) y(-1); and five plots were controls. The CO2 enrichment (target concentration 560 ppm(v)) increased CH4 efflux about 30-40%, but half of this increase seemed to be caused by the air-blowing system. The increasing atmospheric concentration of CO2 would promote CH4 release in boreal mires, but the increase in CH4 efflux would be clearly smaller than that reported in studies made in temperate or subtropical temperature conditions. Addition of N enhanced the annual release of CH4 only slightly. At least over the short-term, the increase in N deposition would have little effect on CH4 effluxes. The increase in CH4 release would probably increase radiative forcing and thus accelerate climate change. However, CH4 effluxes are only a small part in the whole matter balance in mires and thus further studies are needed to define the net effects of raised supply of CO2 or N for carbon accumulation, trace gas fluxes and radiative forcing.