Nitrification Responses of Soil Ammonia-Oxidizing Archaea and Bacteria to Ammonium Concentrations

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 nitrification 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 AOA-supported 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.