Although ammonia-oxidizing archaea (aoa) and bacteria (aoB) coexist in most non-acidic agricultural soils, the factors that influence their relative contributions to soil nitrification activity remain unclear. a 2-to 4-d whole soil microcosm assay was developed, utilizing the aliphatic c8 alkyne 1-octyne to inactivate aoB-driven nitrification activity without impacting aoa nitrification activity. responses of aoa-and aoB-supported net nitrifi-cation activities (accumulation of no2-+ no3-) to different concentrations of extractable nH4 + were examined in four diverse, paired cropped and non-cropped Oregon soils sampled in summer and winter. Maximum aoasupported net nitrification rates were significantly higher in non-cropped (3.7 mg n kg-1 soil d-1) than in cropped soils (0.9 mg n kg-1 soil d-1) and in summer (3.1 mg n kg-1 soil d-1) compared with winter soils (1.6 mg n kg-1 soil d-1). the nH4 + concentration required to significantly stimulate aoB nitrification activity was significantly higher in cropped soils (67 mg n kg-1 soil) than in non-cropped soils (12 mg n kg-1 soil). Maximum aoB activity was significantly higher in cropped (8.6 mg n kg-1 soil d-1) than in non-cropped soils (2.9 mg n kg-1 soil d-1) and in summer (7.8 mg n kg-1 soil d-1) compared with winter soils (3.8 mg n kg-1 soil d-1). this study revealed that aoa-and aoB-supported nitrification rates in cropped and non-cropped soils respond differently to season and nH4 + concentration and raises the possibility that aoa and aoB nitrification activities might be differentially managed to improve n use efficiency. © Soil Science Society of America, 5585 Guilford Rd., Madison WI 53711 USA. All Rights reserved.
