Soil moisture plays a vital role in regulating the direction and magnitude of methane (CH4) fluxes. However, it remains unclear whether the responses of CH4 fluxes to climate warming exhibit difference between dry and moist ecosystems. Based on standardized manipulative experiments (i.e., consistent experimental design and measurement protocols), here we explored warming effects on growing season CH4 fluxes in two alpine grasslands with contrasting water status on the Tibetan Plateau. We observed that experimental warming enhanced CH4 uptake in the relatively arid alpine steppe, but had no significant effects on CH4 emission in the moist swamp meadow. The distinct responses of CH4 fluxes were associated with the different warming effects on biotic and abiotic factors related to CH4 oxidation and production processes. Warming decreased soil water-filled pore space (WFPS) and increased the pmoA gene abundance and CH4 oxidation potential in the alpine steppe, which together led to a significant increase in CH4 uptake at this alpine steppe site. However, warming-induced enhancement in CH4 oxidation potential might be counteracted by the simultaneously increased CH4 production potential in the swamp meadow, which could then result in insignificant warming effects on CH4 emission at this swamp meadow site. Based on a meta-analysis of warming effects on CH4 fluxes across the entire Tibetan Plateau, we found that the entire alpine grasslands could absorb an extra 0.042 Tg CH4 (1 Tg = 10(12) g) per growing season if soil temperature increased by 1 degrees C. These findings demonstrate that warming effects on CH4 fluxes differ between two alpine grasslands with contrasting moisture conditions and the entire alpine grasslands may not trigger a positive CH4 feedback to climate system with moderate warming.

Arctic ecosystems are characterized by a wide range of soil moisture conditions and thermal regimes and contribute differently to the net methane (CH4) budget. Yet, it is unclear how climate change will affect the capacity of those systems to act as a net source or sink of CH4. Here, we present results of insitu CH4 flux measurements made during the growing season 2014 on Disko Island (west Greenland) and quantify the contribution of contrasting soil and landscape types to the net CH4 budget and responses to summer warming. We compared gas flux measurements from a bare soil and a dry heath, at ambient conditions and increased air temperature, using open-top chambers (OTCs). Throughout the growing season, bare soil consumed 0.22 +/- 0.03g CH4-Cm-2 (8.1 +/- 1.2g CO2-eqm(-2)) at ambient conditions, while the dry heath consumed 0.10 +/- 0.02g CH4-Cm-2 (3.9 +/- 0.6g CO2-eqm(-2)). These uptake rates were subsequently scaled to the entire study area of 0.15km(2), a landscape also consisting of wetlands with a seasonally integrated methane release of 0.10 +/- 0.01g CH4-Cm-2 (3.7 +/- 1.2g CO2-eqm(-2)). The result was a net landscape sink of 12.71kg CH4-C (0.48 tonne CO2-eq) during the growing season. A nonsignificant trend was noticed in seasonal CH4 uptake rates with experimental warming, corresponding to a 2% reduction at the bare soil, and 33% increase at the dry heath. This was due to the indirect effect of OTCs on soil moisture, which exerted the main control on CH4 fluxes. Overall, the net landscape sink of CH4 tended to increase by 20% with OTCs. Bare and dry tundra ecosystems should be considered in the net CH4 budget of the Arctic due to their potential role in counterbalancing CH4 emissions from wetlands - not the least when taking the future climatic scenarios of the Arctic into account.

Aerated forest soils are a significant sink for atmospheric methane (CH4). Soil properties, local climate and tree species can affect the soil CH4 sink. A two-year field study was conducted in a deciduous mixed forest in the Hainich National Park in Germany to quantify the sink strength of this forest for atmospheric CH4 and to determine the key factors that control the seasonal, annual and spatial variability of CH4 uptake by soils in this forest. Net exchange of CH4 was measured using closed chambers on 18 plots in three stands exhibiting different beech (Fagus sylvatica L) abundance and which differed in soil acidity, soil texture, and organic layer thickness. The annual CH4 uptake ranged from 2.0 to 3.4 kg CH4-C ha(-1). The variation of CH4 uptake over time could be explained to a large extent (R-2 = 0.71, P < 0.001) by changes in soil moisture in the upper 5 cm of the mineral soil. Differences of the annual CH4 uptake between sites were primarily caused by the spatial variability of the soil clay content at a depth of 0-5 cm (R-2 = 0.5, P < 0.01). The CH4 uptake during the main growing period (May-September) increased considerably with decreasing precipitation rate. Low CH4 uptake activity during winter was further reduced by periods with soil frost and snow cover. There was no evidence of a significant effect of soil acidity, soil nutrient availability, thickness of the humus layer or abundance of beech on net-CH4 uptake in soils in this deciduous forest. The results show that detailed information on the spatial distribution of the clay content in the upper mineral soil is necessary for a reliable larger scale estimate of the CH4 sink strength in this mixed deciduous forest. The results suggest that climate change will result in increasing CH4 uptake rates in this region because of the trend to drier summers and warmer winters. (C) 2009 Elsevier Ltd. All rights reserved.