Greenhouse gas dynamics in boreal, littoral sediments under raised CO 2 and nitrogen supply

SUMMARY 1. The effects of increasing CO 2 and nitrogen loading and of a change in water table and temperature on littoral CH 4 , N 2 O and CO 2 fluxes were studied in a glasshouse experiment with intact sediment cores including vegetation (mainly sedges), taken from a boreal eutrophic lake in Finland. Sediments with the water table held at a level of 0 or at −15 cm were incubated in an atmosphere of 360 or 720 p.p.m. CO 2 for 18 weeks. The experiment included fertilisation with NO 3 – and NH 4 + (to a total 3 g N m −2 ). 2. Changes in the water table and temperature strongly regulated sediment CH 4 and cCO 2 fluxes (community CO 2 release), but did not affect N 2 O emissions. Increase in the water table increased CH 4 emissions but reduced cCO 2 release, while increase in temperature increased emissions of both CO 2 and CH 4 . 3. The raised CO 2 increased carbon turnover in the sediments, such that cCO 2 release was increased by 16–26%. However, CH 4 fluxes were not significantly affected by raised CO 2 , although CH 4 production potential (at 22 °C) of the sediments incubated at high CO 2 was increased. In the boreal region, littoral CH 4 production is more likely to be limited by temperature than by the availability of carbon. Raised CO 2 did not affect N 2 O production by denitrification, indicating that this process was not carbon limited. 4. A low availability of NO 3 – did severely limit N 2 O production. The NO 3 – addition caused up to a 100‐fold increase in the fluxes of N 2 O. The NH 4 + addition did not increase N 2 O fluxes, indicating low nitrification capacity in the sediments.