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 C 8 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 NO 2 − + NO 3 − ) to different concentrations of extractable NH 4 + 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 NH 4 + 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 NH 4 + concentration and raises the possibility that AOA and AOB nitrification activities might be differentially managed to improve N use efficiency.