Nitrification Responses of Ammonia-Oxidizing Bacteria and Archaea to Ammonia Additions in Cultivated and Uncultivated Soils.

Ammonia (NH₃) oxidization is the first and rate-limiting step in nitrification, and is carried out by both NH₃-oxidizing archaea (AOA) and NH₃-oxidizing bacteria (AOB). While both organisms are present in most soils, it is unknown what controls their relative contributions to nitrification. Recently in our laboratory we have shown that the aliphatic 1-octyne can discriminate between the contributions of AOB (octyne-sensitive) and of AOA (octyne-resistant) to soil nitrification. We investigated the nitrification responses of AOA and AOB in paired cultivated and uncultivated soils from four different regions across Oregon.  We determined the nitrification responses of these soils to a range of additions of anhydrous NH₃ (1-40 µmol NH₃ g^-1 soil) in the presence and absence of 1-octyne.  Uncultivated soils generally needed lower NH₃ additions (2-5 µmol NH₃ g^-1 soil) to reach saturating nitrification rates than did cultivated soils (6-22 µmol NH₃ g^-1 soil).  In uncultivated soils, AOA were involved in NH₃ oxidation at all NH₃ additions, while AOB contributed to nitrification in only two of the four uncultivated soils and only after addition of > 2 µmol NH₃ g^-1 soil. Cultivated soils showed no measurable contribution from AOA at any NH₃ addition, and all activity was octyne sensitive and attributed to AOB. The octyne sensitive nitrification rates in the two uncultivated soils saturated at lower rates of NH₃ addition  (0.5-0.7 µmol NO₂^- + NO₃^- g^-1 soil day^-1) than the AOB nitrification rates in cultivated soils (0.6-1.5 µmol NO₂^- + NO₃ g^-1 soil day^-1). This whole soil microcosm approach with and without octyne treatment is providing useful insights into the effects of cultivation and fertilization on nitrification by soil-borne AOA and AOB.