Background/Question/Methods
Ammonia oxidation is the first step in nitrification, the microbially mediated biogeochemical process that determines the balance between nitrate and ammonium in soils and waterbodies.  For many years, bacteria such as Nitrosospira and Nitrosomonas spp. have been studied as the dominant nitrifiers in soils.  However, recent molecular studies have revealed a high abundance and wide distribution of archaeal ammonia monooxygenase (amoA) genes in the environment.  Although ammonia-oxidizing bacteria (AOB) perform this reaction obligately, as their only source of energy, the relative contribution of ammonia-oxidizing archaea (AOA) to oxidation of ammonia in the environment is unknown.  We examined the abundance and composition of archaeal and bacterial amoA genes and nitrification potential in high and low nitrogen soils along a catena and land use gradient in the Willamette Valley, Oregon.  If AOA are more general in their habitat requirements, they might show less variety in community structure across different soil types, and may be relatively more abundant in higher nutrient soils where they can better compete with heterotrophs for ammonium.

Results/Conclusions

The ratio of archaeal to bacterial amoA genes was lowest in forested upland soils (approximately 1) and highest in agricultural lowland soils (3 – 40).  Both AOA and AOB amoA abundance was higher in high versus low nutrient upland soil (1 x 10⁷ vs. 1 x 10⁶ amoA gene copies g^-1 dry soil for both bacteria and archaea), but only AOA amoA abundance was higher in high versus low nutrient lowland soil (4 x 10⁷ vs. 2 x 10⁶ amoA gene copies g^-1 dry soil).  Nitrification potential ranged from 0.3 to 1.5 μmol nitrate plus nitrite g^-1 dry soil h^-1, and was higher in all high nitrogen soils relative to low nitrogen soils.  Terminal restriction fragment length polymorphism (T-RFLP) fingerprints of amoA showed different communities of both AOA and AOB in the different soil types.  In addition, T-RLFP profiles showed that high and low nitrogen lowland soils harbored distinct bacterial amoA communities, but similar archaeal amoA communities.  Using data from both field distribution and lab manipulation of soil conditions, we are investigating further whether and how environmental differences affect the relative abundance and activity of AOA and AOB in these soils.  This study provides a template for learning more about the niche preferences of ammonia-oxidizing archaea and bacteria, and about the environmental conditions that influence nitrification activity by both microbial groups.