Effect of the 2018 European drought on methane and carbon dioxide exchange of northern mire ecosystems.

We analysed the effect of the 2018 European drought on greenhouse gas (GHG) exchange of five North European mire ecosystems. The low precipitation and high summer temperatures in Fennoscandia led to a lowered water table in the majority of these mires. This lowered both carbon dioxide (CO2) uptake and methane (CH4) emission during 2018, turning three out of the five mires from CO2 sinks to sources. The calculated radiative forcing showed that the drought-induced changes in GHG fluxes first resulted in a cooling effect lasting 15-50 years, due to the lowered CH4 emission, which was followed by warming due to the lower CO2 uptake. This article is part of the theme issue 'Impacts of the 2018 severe drought and heatwave in Europe: from site to continental scale'.

Effects of wood ash fertilization on forest floor greenhouse gas emissions and tree growth in nutrient poor drained peatland forests.

Wood ash (3.1, 3.3 or 6.6 tonnes dry weight ha(-1)) was used to fertilize two drained and forested peatland sites in southern Sweden. The sites were chosen to represent the Swedish peatlands that are most suitable for ash fertilization, with respect to stand growth response. The fluxes of carbon dioxide (CO(2)), methane (CH(4)) and nitrous oxide (N(2)O) from the forest floor, measured using opaque static chambers, were monitored at both sites during 2004 and 2005 and at one of the sites during the period 1 October 2007-1 October 2008. No significant (p>0.05) changes in forest floor greenhouse gas exchange were detected. The annual emissions of CO(2) from the sites varied between 6.4 and 15.4 tonnes ha(-1), while the CH(4) fluxes varied between 1.9 and 12.5 kg ha(-1). The emissions of N(2)O were negligible. Ash fertilization increased soil pH at a depth of 0-0.05 m by up to 0.9 units (p<0.01) at one site, 5 years after application, and by 0.4 units (p<0.05) at the other site, 4 years after application. Over the first 5 years after fertilization, the mean annual tree stand basal area increment was significantly larger (p<0.05) at the highest ash dose plots compared with control plots (0.64 m(2) ha(-1) year(-1) and 0.52 m(2) ha(-1) year(-1), respectively). The stand biomass, which was calculated using tree biomass functions, was not significantly affected by the ash treatment. The groundwater levels during the 2008 growing season were lower in the high ash dose plots than in the corresponding control plots (p<0.05), indicating increased evapotranspiration as a result of increased tree growth. The larger basal area increment and the lowered groundwater levels in the high ash dose plots suggest that fertilization promoted tree growth, while not affecting greenhouse gas emissions.