Oxidation of atmospheric methane in Northern European soils, comparison with other ecosystems, and uncertainties in the global terrestrial sink

This paper reports the range and statistical distribution of oxidation rate s of atmospheric CH4 in soils found in Northern Europe in an international study, and compares them with published data for various other ecosystems. It reassesses the size, and the uncertainty in, the global terrestrial CH4 sink, and examines the effect of land-use change and other factors on the o xidation rate. Only soils with a very high water table were sources of CH4; all others wer e sinks. Oxidation rates varied from 1 to nearly 200 mu g CH4 m(-2) h(-1); annual rates for sites measured for greater than or equal to 1 y were 0.1-9 .1 kg CH4 ha(-1) y(-1), with a log-normal distribution (log-mean approximat e to 1.6 kg CH4 ha(-1) y(-1)). Conversion of natural soils to agriculture r educed oxidation rates by two-thirds - closely similar to results reported for other regions. N inputs also decreased oxidation rates. Full recovery o f rates after these disturbances takes > 100 y. Soil bulk density, water co ntent and gas diffusivity had major impacts on oxidation rates. Trends were similar to those derived from other published work. Increasing acidity red uced oxidation, partially but not wholly explained by poor diffusion throug h litter layers which did not themselves contribute to the oxidation. The e ffect of temperature was small, attributed to substrate limitation and low atmospheric concentration. Analysis of all available data for CH4 oxidation rates in situ showed simil ar log-normal distributions to those obtained for our results, with general ly little difference between different natural ecosystems, or between short -and longer-term studies. The overall global terrestrial sink was estimated at 29 Tg CH4 y(-1), close to the current IPCC assessment, but with a much wider uncertainty range (7 to > 100 Tg CH4 y(-1)). Little or no information is available for many major ecosystems; these should receive high priority in future research.