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Fluxes of methane from field observations of native and cropped grassland soils in Colorado and Nebraska were used to model CH 4 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 (W opt ) increases with PC. The site specific maximum CH 4 oxidation rate (CH 4max ) varies directly with soil gas diffusivity (D opt ) as a function of soil bulk density and FC. Although soil water content and physical properties are the primary controls on CH 4 uptake, the potential for soil temperature to affect CH 4 uptake rates increases as soils become less limited by gas diffusivity, Daily CH 4 oxidation rate is calculated as the product of CH 4max , 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 CH 4 oxidation rates in deciduous forest soils, so a submodel for these systems was developed assuming that CH 4max 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 from grasslands, cultivated lands, and coniferous, deciduous, and tropical forests (r 2 = 0.73, mean error < 6%). |
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