Relative activity of ammonia oxidizing archaea and bacteria determine nitrification-dependent N2O emissions in Oregon forest soils

Using oxic soil slurry incubations supplemented with either the selective AOB inhibitor 1-octyne or the nonspecific nitrification inhibitor acetylene, we investigated the relative contributions of ammonia oxidizing archaea (AOA) and bacteria (AOB) to nitrification-dependent N2O production in six Oregon forest soils (pH 3.7-5.3), collected from three different sites (Cascade Head, H.J. Andrews, and McDonald Forest), each under two different tree species (Douglas-fir, Pseudotsuga menziesii and red alder, Alnus rubra). We found that nitrification-dependent N2O production across a range of acid forest soils depends on the site-specific contribution of AOB and AOA to nitrification because of their inherently distinct N2O yields. The latter were determined to be similar to 0.15 +/- 0.01% for AOB and 0.06 +/- 0.01% for AOA. Soil pH regulated the nitrifier-specific N2O production by affecting the nitrifier activity. ADA-dependent N2O production dominated at low pH (<4.5), while AOB-dependent N2O production was favored in less acidic soils. Increase in relative contribution of the AOB nitrifying activity, favored by the increase in soil pH, was accompanied by higher total N2O yields.