Seasonal variations in methane fluxes in response to summer warming and leaf litter addition in a subarctic heath ecosystem

Abstract

AbstractMethane (CH4) is a powerful greenhouse gas controlled by both biotic and abiotic processes. Few studies have investigated CH4 fluxes in subarctic heath ecosystems, and climate change‐induced shifts in CH4 flux and the overall carbon budget are therefore largely unknown. Hence, there is an urgent need for long‐term in situ experiments allowing for the study of ecosystem processes over time scales relevant to environmental change. Here we present in situ CH4 and CO2 flux measurements from a wet heath ecosystem in northern Sweden subjected to 16 years of manipulations, including summer warming with open‐top chambers, birch leaf litter addition, and the combination thereof. Throughout the snow‐free season, the ecosystem was a net sink of CH4 and CO2 (CH4 −0.27 mg C m−2 d−1; net ecosystem exchange −1827 mg C m−2 d−1), with highest CH4 uptake rates (−0.70 mg C m−2 d−1) during fall. Warming enhanced net CO2 flux, while net CH4 flux was governed by soil moisture. Litter addition and the combination with warming significantly increased CH4 uptake rates, explained by a pronounced soil drying effect of up to 32% relative to ambient conditions. Both warming and litter addition also increased the seasonal average concentration of dissolved organic carbon in the soil. The site was a carbon sink with a net uptake of 60 g C m−2 over the snow‐free season. However, warming reduced net carbon uptake by 77%, suggesting that this ecosystem type might shift from snow‐free season sink to source with increasing summer temperatures.