Ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing thaumarchaea (AOA) are commonly found together in soils. The factors that influence their relative contributions to soil nitrification remain unclear. To address this question we have investigated different types of soils under different land uses with a combination of molecular and analytical techniques that interrogate community composition, community size, and that discriminate between ammonia oxidizing activities by AOA and AOB. Study sites covered a range of land-uses (forest, range, pasture, cropped, and long term (~20y) fallowed cropland) throughout the state of Oregon. AOA dominated the nitrification potentials of unfertilized rangeland, and pasture soils, whereas AOB dominated nitrification of cropped soils heavily fertilized with N. In the case of wheat cropped/summer fallow soils, both AOB and AOA contributed to nitrification potentials, with AOA dominating nitrification potentials during fall and winter, and AOB dominant during late spring.  In soil samples taken at monthly intervals for one year from adjacent cropped/summer fallow sites, both the archaeal and bacterial amoA gene community compositions were heterogeneous across the landscape, yet differed in their patterns of variability, suggesting the drivers of their community compositions are different. Quantitative-PCR estimates of AOB population densities correlated with their contribution to activity measurements. Populations of AOB increased in spring in both cropped and summer fallowed soils coincident with increases in soil temperature and NH₄⁺ levels, declined in mid-summer and remained low through fall and winter. Furthermore, there was evidence of community compositional changes across seasons. In contrast, AOA population density remained relatively constant throughout the year despite seasonal shifts in their relative contribution to soil nitrification potential. Further studies are underway to identify which members of the soil communities are actively oxidizing ammonia in different soils, at different seasons within a soil, and to determine if the properties of nitrification differ in soils where activities are dominated by either AOA or AOB.