Site variation in methane oxidation as affected by atmospheric deposition and type of temperate forest ecosystem

Factors controlling methane oxidation were analyzed along a soil acidity gradient (pH(H2O) 3.9 to 5.2) under beech and spruce forests in Germany. Mean annual methane oxidation ranged from 0.1 to 2.5 kg CH4 ha(-1) yr(-1) and was correlated with base saturation (r(2) = 0.88), soil pH (r(2) = 0.77), total nitrogen (r(2) = 0.71), amount of the organic surface horizon (r(2) = 0.49) and bulk density of the mineral soil (r(2) = 0.43). At lower pHs the formation of an organic surface horizon was promoted. This horizon did not have any methane oxidation capacity and acted like a gas diffusion barrier, which decreased the methane oxidation capacity of the soil. In contrast, on sites at the higher end of the pH range, higher burrowing activity of earthworms increased macroporosity and thereby gas diffusivity and methane oxidation. Gas diffusivity was also affected by litter shape: broad beech leaves reduced methane oxidation more than spruce needles. An increase in methane oxidation of most soil samples following sieving indicates that diffusion is the main limiting factor for methane oxidation. However, this,,sieving effect" was less in soils with a pH below 5 than in soils with a pH above 5, which we attribute to a direct effect of soil acidity. We discuss our results using a hierarchical concept for the ,,short-term" and ,,long-term" controls on methane oxidation in forest ecosystems.