When the effect of water content was minimized, soil CO2 evolution and soil organic matter content were good predictors of aerobic NO. uptake rate constants across a wide range of soil types. Field manure application to a Gleysol stimulated NO. uptake rate constants and lowered NO. compensation points compared to unfertilized or NH4NO3-fertilized soil. This effect lasted for months after manure application. In a laboratory experiment, addition of manure reduced the NO. efflux associated with nitrification of NH4 Cl fertilizer, and manured soils had a greater capacity to remove NO. from polluted air. Evidence is presented that these observations result from NO. oxidation during heterotrophic microbial activity in soil.