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 soil. If consistent for other sites and land usages, higher ammonia-oxidation activity and N2O production within surface agricultural soil reinforces the importance of agricultural surface soils as sources of nitrification and N2O production, with potential implications for land management practices and responses to climate change.In order to explore other functions of soil Thaumarchaeota besides ammonia oxidation, thaumarchaeotal cobalamin producing potential was targeted, and expanded to a broader range of cobalamin-producing and consuming microorganisms in soils (Chapter 4). Vitamin B12 (cobalamin) is the most structurally complex coenzyme known and its availability is thought to influence microbial diversity and community composition. Although previous studies have investigated marine cobalamin synthesis, the producers, remodelers, and consumers of cobalamin in terrestrial habitats are unknown. Here 155 globally distributed soil metagenomes were surveyed for cobalamin-producing microorganisms by quantifying and classifying cobalamin biosynthesis marker genes (cob/cbi) with profile hidden Markov models (HMMs). Complementing this sequence-based analysis, different forms of cobalamin (CN-, Me-, OH-, Ado-B12) were measured, as well as the cobalamin lower ligand (5,6-dimethylbenzimidazole; DMB), in an independent set of 40 diverse soil samples. Metagenomic analysis revealed that less than 10% of soil microbial taxa are capable of complete cobalamin biosynthesis, predominantly encoded by taxa affiliated with Proteobacteria, Actinobacteria, Firmicutes, Nitrospirae, and Thaumarchaeota. Consistent with vitamin production being a keystone community function, a larger proportion of soil genera possessed genes for cobalamin transport and lacked biosynthesis genes. In addition, cobalamin-dependent genes outnumbered cobalamin synthesis genes in all tested soil metagenomes. A significant positive correlation between cobalamin concentration and microbial biomass was observed, consistent with the metagenomic results showing that cobalamin-producing potential (cob/cbi) correlated positively with microbial community size (rpoB). Chemical measurements demonstrated that free water-leachable cobalamin was a relatively small portion of total cobalamin, compared to non-water-leachable cobalamin (associated with microbial biomass or tightly bound to minerals). The cobalamin lower ligand, DMB, was more abundant than intact cobalamin, in agreement with metagenome data showing a higher relative abundance of DMB synthesis cob/cbi genes than corrin ring synthesis or final assembly cob/cbi genes, suggesting an important role for cobalamin remodeling in terrestrial habitats. With broad implications for soil nutrient cycling and primary productivity (Chapter 5), these combined metagenomic and biochemical data implicate microbial cobalamin production as a keystone function that may influence total microbial community size, diversity, and associated biogeochemistry of terrestrial ecosystems.
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