Summary This paper reports the range and statistical distribution of oxidation rates 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 oxidation rate. Only soils with a very high water table were sources of CH4; all others were sinks. Oxidation rates varied from 1 to nearly 200 μg CH4 m−2 h−1; annual rates for sites measured for ≥1 y were 0.1–9.1 kg CH4 ha−1 y−1, with a log‐normal distribution (log‐mean ≈ 1.6 kg CH4 ha−1 y−1). Conversion of natural soils to agriculture reduced oxidation rates by two‐thirds –‐ closely similar to results reported for other regions. N inputs also decreased oxidation rates. Full recovery of rates after these disturbances takes > 100 y. Soil bulk density, water content and gas diffusivity had major impacts on oxidation rates. Trends were similar to those derived from other published work. Increasing acidity reduced oxidation, partially but not wholly explained by poor diffusion through litter layers which did not themselves contribute to the oxidation. The effect of temperature was small, attributed to substrate limitation and low atmospheric concentration. Analysis of all available data for CH4 oxidation rates in situ showed similar log‐normal distributions to those obtained for our results, with generally 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.

Abstract. While much is known about process level control on N20 production by nitrification and denitrification, knowledge of the environmental controls responsible for site variation in annual N20 fluxes on ecosystem level is low. Our goal was to improve existing concepts of controls on N20 fluxes. We measured N20 emission weekly or biweekly during lyear in 11 temperate forest ecosystems using closed chambers. We identified three types of forest with different temporal emission patterns: forest with seasonal, event-based and background emission patterns. Comparison of annual data sets from literature showed that most temperate forests had low N20 emissions throughout the year (background emission pattern) with mean annual fluxes of 0.39 +0.27 kg N ha -1 yr -1 (n = 21). Event-based emission patterns were observed during frost/thaw periods and after rewetting. Highest fluxes up to 72 kg N ha-1 were emitted from a drained alder forest with organic soil in 46 weeks, followed by well drained tropical and temperate forests with seasonal emission patterns and fluxes between 2 -6 (n = 3) and 1 -5 kg N ha-1 yr-1 (n = 4), respectively. Seasonal emission patterns were explained by combined effect of high annual precipitations; broad leave trees; amount and structure of organic upper horizon; high mineral bulk densities; and plant community. These state variables reduce gas diffusivity so that oxygen demand by microorganism and roots exceeded oxygen supply during wet and warm periods (>10 ø C). The resultant upper mean level was about 100 I•g N20-N m-2 h-1 in both temperate and tropical forests. Annual N20 losses of the seasonal emission type were controlled by both duration and upper mean level of the periods with high emissions. We conclude that "short-term controls" of climate determine the duration of high emissions, whereas "long-term controls" by state variables determine the difference between background and seasonal emission types.

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Fluxes of methane from field observations of native and cropped grassland soils inColorado and Nebraska were used to model CH4 oxidation as a function of soil water content, temperature, porosity, and field capacity (FC). A beta function is used to characterize the effect of soil water on the physical limitation of gas diffusivity when water is high and biological limitation when water is low. Optimum soil volumetric water content (Wovt) increases with FC. The site specific maximum CH 4 oxidation rate (CH4max) varies directly with soil gas diffusivity (Dom) as a function of soil bulk density and FC. Although soil water content and physical properties are the primary controls on CH4 uptake, the potential for soil temperature to affect CH4 uptake rates increases as soils become less limited by gas diffusivity. Daily CH4 oxidation rate is calculated as the product of CH4max, the normalized (0-100%) beta function to account for water effects, a temperature multiplier, and an adjustment factor to account for the effects of agriculture on methane flux. The model developed with grassland soils also worked well in coniferous and tropical forest soils. However, soil gas diffusivity as a function of field capacity, and bulk density did not reliably predict maximum CH4 oxidation rates in deciduous forest soils, so a submodel for these systems was developed assuming that CH4max is a function of mineral soil bulk density. The overall model performed well with the data used for model development (r 2 = 0.76) and with independent data t¾0m grasslands, cultivated lands, and coniferous, deciduous, and tropical forests (r 2 = 0.73, mean error < 6%). radiatively active trace gas is produced biologically during fermentation in anaerobic environments and consumed by reaction with OH-in the atmosphere and by microbial oxidation in soils. Although oxidation in soils accounts for -10% of the global CH4 sink of 350-480 TgC yr -•, the CH4 consumed annually in soils approximately equals or exceeds the net yearly increase of CH4 in the atmosphere [Prather et al,, 1995]. A major goal of this research is to improve estimates of the contributions of natural and managed ecosystems to the terrestrial CH4 sink. Methane is produced in water-logged soils as an end product of organic matter decomposition. In aerated soils, CH 4 may be oxidized by methanotrophs and other CH4 oxidizing microbes [Davidson and Schimel, 1995]. CH 4 produced in saturated soil layers may be oxidized to CO2 in drier surface layers before diffusing out of the soil [Conrad, 1989]. Atmospheric CH 4 may also diffuse into soil and be oxidized. On an annual basis, rice paddies and natural wetlands are net producers of CH 4, while grasslands and forests are net consumers. Our model is designed to simulate CH 4 oxidation in soils that are usually net sinks of atmospheric CH4. Controls of atmospheric CH 4 uptake include soil water [Adamsen and King, 1993], temperature [Whalen and Reeburgh, 1996], texture [Boeckx et al., 1997], microbial population [Willison et al., 1997] and mineral...

This study focuses on the microbial N cycle in the acid soil of a beech forest that falls in the upper range of the N saturation continuum. Our objectives were: (1) to quantify microbial N cycling under long‐term N‐saturated and limed conditions and (2) to determine the factors controlling the differences in microbial N cycling. Our study site has a long history of high N deposition: ≥25 kg N·ha–1·yr–1 since measurements began in 1971. This was further enhanced by 11 yr (1983–1993) of fertilization (140 kg ammonium sulfate‐N·ha–1·yr–1) to create an N saturation plot. Another plot was limed with 30 Mg/ha dolomitic limestone in 1982. In 1999–2000, gross rates of microbial N cycling were measured using 15N pool dilution techniques. Despite the chronic high N deposition, the control plot showed a tightly coupled microbial N cycle; NH4+ and NO3– immobilization rates were comparable to gross N mineralization and nitrification rates, respectively. These were supported by low levels of NH4+, NO3–, and dissolved organic N (DON) in percolate. Liming increased gross N mineralization and nitrification rates but did not cause similar increases in microbial biomass or NH4+, and NO3– immobilization rates. In addition, NO3– immobilization rates were somewhat less than gross nitrification rates; relatively high levels of NO3– and DON in percolate were also observed. The N‐saturated plot suggested an uncoupled microbial N cycle; NH4+ immobilization rates were lower than gross N mineralization rates, and NO3– immobilization rates were somewhat less than gross nitrification rates. These were corroborated by high levels of NH4+, NO3–, and DON in percolate. The reduced NH4+ and NO3– immobilization rates in the N‐saturated plot could be attributed to the measured decreases in microbial biomass, and the low microbial biomass was likely due to decreases in the supply of labile C. Our study demonstrates that while hydrological N input/output budgets can indicate whether or not a forest ecosystem is in a state of N saturation, the microbial N cycle can provide quantitative information on key processes that govern N losses. Corresponding Editor: E. A. Holland.