A moderate increase in the annual CH 4 efflux by raised CO 2 or NH 4 NO 3 supply in a boreal oligotrophic mire

Summary Natural wetlands release about 20% of global emissions of CH 4 , 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 CO 2 or NH 4 NO 3 on the annual CH 4 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 CO 2 ‐enriched air and the other five with ambient air. In addition, five plots were sprayed with NH 4 NO 3 so that the cumulative addition of N was 3 g m −2  y −1 ; and five plots were controls. The CO 2 enrichment (target concentration 560 ppm v ) increased CH 4 efflux about 30–40%, but half of this increase seemed to be caused by the air‐blowing system. The increasing atmospheric concentration of CO 2 would promote CH 4 release in boreal mires, but the increase in CH 4 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 CH 4 only slightly. At least over the short‐term, the increase in N deposition would have little effect on CH 4 effluxes. The increase in CH 4 release would probably increase radiative forcing and thus accelerate climate change. However, CH 4 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 CO 2 or N for carbon accumulation, trace gas fluxes and radiative forcing.