The majority of N transformations in the soil are biologically mediated processes. Physical and chemical processes related to soil type also affect the rate of N transformations such as nitrification and ammonia volatilization. Ammonium and ammonia held on exchange sites or fixed by clays can reduce ammonia volatilization and nitrification rates by influencing substrate availability. Different species of nitrifiers have varying nitrification rates and ability to scavenge NH4+. Our research was designed to determine the effect of soil type on NH4+ transformations and availability to nitrifiers. The availability of NH4+ is expected to influence the dynamics and structure of nitrifier communities. Soil samples were collected at USDA-ARS sites across a variety of ecoregions and soil types. Each soil series was used in a 30 d incubation containing a dairy slurry (300 kg N ha-1) amended soil and a soil control. Sub samples were removed at 5 time intervals for analyses of nitrification potentials and community structure of beta-ammonia oxidizers via terminal restriction fragment length polymorphism. Separating out the effect of microbial community structure and soil type on N transformations will improve our understanding of nutrient cycling, as well as, bridge the gap between soil chemistry and microbiology.