Conversion of hardwood forests to spruce and pine plantations strongly reduced soil methane sink in Germany

Well-drained forest soils are thought to be a significant sink for atmospheric methane. Recent research suggests that land use change reduces the soil methane sink by diminishing populations of methane oxidizing bacteria. Here we report soil CH4 uptake from 'natural' mature beech forests and from mature pine and spruce plantations in two study areas of Germany with distinct climate and soils. The CH4 uptake rates of both beech forests at Solling and Unterhig were about two-three times the CH4 uptake rates of the adjacent pine and spruce plantations, indicating a strong impact of forest type on the soil CH4 sink. The CH4 uptake rates of sieved mineral soils from our study sites confirmed the tree species effect and indicate that methanotrophs were mainly reduced in the 0-5 cm mineral soil depth. The reasons for the reduction are still unknown. We found no site effect between Solling and Unterhig, however, CH4 uptake rates from Solling were significantly higher at the same effective CH4 diffusivity. This potential site effect was masked by higher soil water contents at Solling. Soil pH (H2O) explained 71% of the variation in CH4 uptake rates of sieved mineral soils from the 0-5 cm depth, while cation exchange capacity, soil organic carbon, soil nitrogen and total phosphorous content were not correlated with CH4 uptake rates. Comparing 1998-99, annual CH4 uptake rates increased by 69-111% in the beech and spruce stands and by 5-25% in the pine stands, due primarily to differences in growing season soil moisture. Cumulative CH4 uptake rates from November throughout April were rather constant in both years. The CH4 uptake rates of each stand were separately predicted using daily average soil matric potential and a previously developed empirical model. The model results revealed that soil matric potential explains 53-87% of the temporal variation in CH4 uptake. The differences between measured and predicted annual CH4 uptake rates were less than 10%, except for the spruce stand at Solling in 1998 (17%). Based on data from this study and from the literature, we calculated a total reduction in the soil CH4 sink of 31% for German forests due in part to conversion of deciduous to coniferous forests.