Distribution and Function of Soil Thaumarchaeota

Despite the importance of ammonia-oxidizing archaea (AOA; Thaumarchaeota) to soil nitrification (Chapter 1), their biogeography in terrestrial environments and relative contributions to nitrification remain unclear. Leveraging the close proximity of forest, field, and agricultural plots at the rare Charitable Research Reserve (Cambridge, Ontario), thaumarchaeotal biogeography was examined at three different depths (0-15, 15-30, and 30-45 cm) from plots within areas of contrasting land usage (Chapter 2). High-throughput sequencing of thaumarchaeotal 16S rRNA gene sequences demonstrated that OTU richness was affected significantly by depth and land-use type. Specifically, thaumarchaeotal diversity was higher in soils from forest sites than from field sites, and lower within 0-15 cm soils than either 15-30 cm or 30-45 cm soils. Soil land use type influenced the relative abundance of the Soil Crenarchaeota Group (SCG), with a lower relative abundance of SCG in forest sites compared to field sites. At the OTU level, thaumarchaeotal communities changed with increasing soil depth for agricultural soils, in contrast to homogeneous depth profiles generated from forest site samples. Soil pH was the strongest factor impacting thaumarchaeotal community composition and, the evenness of archaeal taxa. Nitrogen, carbon, and soil texture shaped thaumarchaeotal community composition among field site samples. Selected sites within the rare Charitable Research investigateg for temperature- and depth-dependence of AOA and ammonia-oxidizing bacteria (AOB) activities (Chapter 3). This work applied the recently discovered AOB inhibitor, octyne, to soil microcosms incubated at different temperatures (20, 30, 40°C) in order to differentiate ammonia-oxidation potential and N2O production by AOA and AOB, in soils from different land uses and depth. The results showed that surface soils (0-15 cm) possessed significantly greater ammonia oxidation potential than subsurface soils (30-45 cm) at all temperatures tested, and that AOA-associated nitrification potential dominated at higher temperatures for both summer- and autumn-collected soils. The accumulation of N2O was only detected in surface agricultural soil at 30°C and positively correlated with nitrite accumulation within the incubation period. The detected N2O production, along with most nitrification potential activity, were attributed to AOB, implicating AOB as major producers of this greenhouse gas in the tested agricultural so