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.

In this study, effects of elevated air temperatures on thermal and hydrologic process of the shallow soil in the active layer were investigated. Open-top chambers (OTCs) were utilized to increase air temperatures 1-2A degrees C in OTC-1 and 3-5A degrees C in OTC-2 in the alpine meadow ecosystem on the Qinghai-Tibetan Plateau. Results show that the annual air temperatures under OTC-1 and OTC-2 were 1.21A degrees C and 3.62A degrees C higher than the Control, respectively. The entirely-frozen period of shallow soil in the active layer was shortened and the fully thawed period was prolonged with temperature increase. The maximum penetration depth and duration of the negative isotherm during the entirely-frozen period decreased, and soil freezing was retarded in the local scope of the soil profile when temperature increased. Meanwhile, the positive isotherm during the fully-thawed period increased, and the soil thawing was accelerated. Soil moisture under different manipulations decreased with the temperature increase at the same depth. During the early freezing period and the early fullythawed period, the maximum soil moisture under the Control manipulation was at 0.2 m deep, whereas under OTC-1 and OTC-2 manipulations, the maximum soil moisture were at 0.4-0.5 m deep. These results indicate that elevated temperatures led to a decrease of the moisture in the surface soil. The coupled relationship between soil temperature and moisture was significantly affected by the temperature increase. During the freezing and thawing processes, the soil temperature and moisture under different manipulations fit the regression model given by the equation theta (V)=a/{;1+exp[b(TS+c)]}+d.

Ecosystem responses to current global climate change can be predicted through experimental climate simulations. One such simulation method is the open-top chamber (OTC). The effects of OTCs on environmental factors are potentially complex, and recognizing the numerous interactions among these factors is crucial for the proper use of chambers. We studied the effects of OTCs on microclimatic factors including ambient temperature, relative humidity, soil temperature, and soil moisture. Plant abundance responses were also assessed. Our study involved the construction of 20 OTCs (1 m in diameter and 0.75 m in height; made of clear acrylic plastic) and 20 control plots on substrates with and without Sphagnum moss, at post-fire and logging sites of the transitional mixedwood-boreal forest in the southern part of James Bay region, Quebec. Experimental trials were also conducted to test the effects of OTCs on snowmelt in the Montreal region. Our results suggest that OTC treatment is most evident in terms of increased daytime maximum temperatures (2A degrees C to 3A degrees C), and cooler (up to similar to 2.4A degrees C), drier (up to 10% volumetric moisture content) soils. Advanced thawing of the insulating snow cover and exposure of soil in the OTCs to low spring temperatures appeared to prolong soil freeze and result in cooler soils. Earlier snowmelt probably also led to earlier onset and overall increased evaporation of meltwater in the OTCs, leading to drier soils. Plant abundance responses to OTC treatment differed depending on plant species. Overall, open-top chambers provide an effective and simple method of climate change simulation, but it is highly advisable that the complex interactive effects, both desired and undesired, are well understood and appreciated before using OTCs for experimental climate simulation.