Dissertation Research: Linking N cycling bacterial community composition and function along a mycorrhizal gradient

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Soil bacteria play a central role in the cycling of nitrogen (N) in all ecosystems, where plant productivity is often limited by the availability of N. Certain types of bacteria are able to capture N from the atmosphere and make it available for plants through the process of N-fixation. Other bacterial groups use their metabolism to return N to the atmosphere. Plant roots and fungi in soil can influence bacteria, but the effect of these interactions on N cycling is unclear. The aims of this project are to (1) characterize relationships between N-cycle reactions and different groups of bacteria, and (2) to explore how these relationships are influenced by plant roots and fungi. Understanding how factors in soil control N-availability for plants is important for agriculture and forestry. This Doctoral Dissertation Improvement Grant project will enhance the graduate student's dissertation research, provide opportunities for undergraduate students to develop field and laboratory techniques in biogeochemistry, and will be integrated in outreach activities organized through the Boston University Biogeoscience program. Plant roots and mycorrhizal fungi can influence the form and availability of inorganic N in soil, potentially impacting the composition and activity of N-cycling bacterial functional groups. This research will test three hypotheses: (1) enhanced competition for N between plants and soil microbes in forest stands dominated by ectomycorrhizal (ECM) fungi relative to stands dominated by arbuscular mycorrhizal (AM) fungi will select for N-fixers and select against nitrifiers and denitrifiers; (2) high plant demand for inorganic N in rhizosphere soils relative to bulk soils will select for diazotrophs and select against nitrifiers and denitrifiers; and (3) the magnitude of the rhizosphere effect will increase as ECM abundance increases. To test these hypotheses, the abundance of gene copies (DNA) and gene transcripts (RNA) associated with N-fixers, nitrifiers, and denitrifiers will be quantified in rhizosphere soil samples across a mycorrhizal gradient established at the Harvard Forest LTER site in Petersham, MA. Stable isotope approaches will be used to measure rates of N-fixation, nitrification and denitrification. These paired analyses will elucidate relationships between microbial community composition and function and identify how these relationships vary based on biotic interactions between N-cycling bacteria, mycorrhizal fungi and plant roots.