SUMMARYFluxes of NO from three different soils have been studied by a flow-through system in the laboratory as a function of gas flow rate, of NO mixing ratio, and of incubation conditions. The dependence of net NO fluxes on gas flow rates and on NO mixing ratios could be described by a simple model of simultaneous NO production and NO uptake. By using this model, rates of gross NO production, rate constants of NO uptake, and NO compensation mixing ratios could be determined as function of the soil type and the incubation condition. Gross NO production rates were one to two orders of magnitude larger under anaerobic than under aerobic conditions. NO uptake rate constants, on the other hand, were only 5-8 times larger so that the compensation mixing ratios of NO were in a range of about 1600-2200 ppbv under anaerobic and of about 50-600 ppbv under aerobic conditions. The different soils exhibited similar NO uptake rate constants, but the gross NO production rate and compensation mixing ratio was significantly higher in an acidic (pH

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