Methane Uptake by Temperate Forest Soils

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Abstract

Net uptake of atmospheric methane by methanotrophic bacteria in forest soils is generally less than 20 kg CH4 ha−1 per year and therefore has almost no impact on the carbon budget of forest ecosystems. However, terrestrial soils are the most important biological sink for atmospheric methane, consuming between 20 and 45 Tg CH4 per year (Smith et al. 2000; Dutaur and Verchot 2007). The methane concentration in the atmosphere increased from about 700 to 1,774 ppb (by volume) during the last 150 years, though the concentration has possibly stabilised during the past decade (IPCC 2007). Methane currently contributes approximately 18% to the anthropogenic greenhouse effect and has a global warming potential 25 times higher than that of CO2 based on a time horizon of 100 years (IPCC 2007). Human activities have reduced the soil sink for atmospheric methane by converting natural forests and grasslands to coniferous plantations, agricultural and urban land (Ojima et al. 1993; Dobbie et al. 1996; Smith et al. 2000; Borken et al. 2003; Borken and Beese 2006) and by causing acid deposition in temperate forests (Brumme and Borken 1999). Undisturbed forest soils generally consume much higher rates of atmospheric methane than agricultural and urban soils, although the methane uptake rates may vary considerably within and among forests over all climatic regions. Comparisons of adjacent forest and agricultural soils showed that converting forest land to agricultural use reduced methane uptake rates by two-thirds in both temperate and tropical regions (Smith et al. 2000). The mechanisms for long-term reduction in methane uptake are not completely understood. Disturbances of the soil structure as well as changes in vegetation, micro-climate, and nutrient status may have diminished the population of methane-utilising bacteria in several ways after land conversion to agricultural use. It is believed that these still unidentified bacteria have an extremely slow growth rate due to the small atmospheric methane concentration. Methanotrophs have a high affinity for methane as indicated by low K m values and low threshold concentrations (Bender and Conrad 1993). Smith et al. (2000) pointed out that it probably takes more than 200 years for methane utilising bacteria to recover after reversion of agricultural land to forest or grassland.