Title: Contributions of Ammonia-Oxidizing Bacteria and Archaea to Nitrification in

Prior to 2005, ammonia oxidation, the first step of nitrification, was thought mediated mainly by ammonia-oxidizing bacteria (AOB). However, the discovery of Thaumarchaeota carrying the genes coding for the ammonia monooxygenase (AMO) enzyme led to the discovery that ammonia-oxidizing archaea (AOA) also contribute to nitrification. Despite the uptick in studies on nitrification following the recognition of AOA, the relative importance and the contribution of AOA and AOB in different ecosystems, and the factors controlling their abundance and activity, have not been well understood. Because nitrogen (N) cycling is mediated by microbes and controls the net primary productivity in forests, and nitrification is the rate controlling step in N cycling, it is important to have a better understanding of nitrification in forest ecosystem, especially the abundance and contribution of AOA and AOB in forest soils. The objective of this study was to determine nitrification activity and nitrifier abundance in forest soils. I wanted to know: (i) if the inclusion of red alder influences the distribution, total nitrification activity, and relative contribution of AOA and AOB to nitrification; (ii) if pH and substrate concentration influence the distribution an