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

Methane (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 Cm-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. Plain Language Summary: Much attention has been directed toward methane (CH4) dynamics in peatlands and wet ecosystems at high latitudes, which are considered net CH4 sources which intensify the greenhouse effect and lead to further warming. However, few studies have hitherto investigated CH4 fluxes in subarctic heath ecosystems, which likely exhibit both CH4 production and uptake. Therefore, climate-induced changes in CH4 exchange and the overall carbon balance are largely unknown. In this unique long-term field experiment, we investigated the response of biological CH4 uptake (microbial CH4 consumption) to increased summer warming by open-top chambers and deciduous leaf litter input in a wet heath ecosystem in northern Sweden, representative of a large proportion of the tundra landscape. We found that leaf litter addition significantly increases CH4 uptake rates due to a pronounced soil drying effect, which is intensified in combination with warming. Warming enhances CO2 release, while CH4 uptake is controlled by soil moisture. The study demonstrates the sensitivity and capacity of a wet heath ecosystem to function as a net CH4 sink. However, it was also shown that higher summer temperatures might shift the ecosystem toward a net carbon source due to an increase in CO2 release, thereby enhancing the greenhouse effect.