Ecology of nitrogen-fixing bacteria associated with Miscanthus

Sustainability of is one of the biggest concerns for bioenergy crops, such as Miscanthus. Nitrogen is the most expensive and most limiting nutrient in agricultural systems, so it is necessary to develop strategies to reduce anthropogenic inputs of nitrogen fertilizer. Diazotrophs associated with non-leguminous grasses have great potential to contribute biologically fixed nitrogen as a sustainable N source for bioenergy grasses. There are previous few studies investigating diazotroph community composition associated with Miscanthus. The following thesis investigated two important aspects of Miscanthus-associated N-fixing bacteria with both culture-based methods and molecular approaches: i. confirming the presence diazotrophs associated with Miscanthus and evaluating their potential to provide nitrogen to the plant; ii. examining how environmental factors may influence the diazotroph communities. In order to address the objectives, both rhizomes and rhizosphere samples were collected from native Miscanthus in Taiwan and agricultural samples from Illinois. Thirty-two diazotroph strains were isolated from native Miscanthus rhizomes, and most of them were Gamma-proteobacteria. The taxonomic classification of the strains was similar to diazotroph strains previously isolated from agricultural Miscanthus. In addition, the nitrogen fixation potential was confirmed for these strains through detection of nitrogenase genes and assays to detect nitrogenase activity. Molecular approaches were introduced to investigate if the biological nitrogen fixation (BNF) process could be carried out by diverse bacterial groups besides the isolated taxa. Diazotrophs from 57 genera were found in native Miscanthus rhizomes, while diazotrophs from 73 genera were found in the rhizosphere. Molecular approaches also enable us to compare diazotrophs from different conditions. I first compared the endorhizosphere diazotrophs of native and agricultural Miscanthus, and the results indicated that the native Miscanthus rhizomes harbored more diverse diazotroph communities. I then investigated both total bacterial communities and diazotroph communities from the rhizosphere and endorhizosphere of native Miscanthus with deep-sequencing methods. The result showed that total bacteria communities showed strong response to plant niches, while diazotroph bacteria did not. For both total bacterial and diazotroph communities, the Miscanthus rhizosphere soil harbored communities with higher richness and biodiversity than Miscanthus rhizome tissues. Other factors, such as geographic distribution and soil edaphic factors also influenced the diversity and community structure of both total bacterial and diazotroph communities. Understanding community composition of diazotrophs associated with Miscanthus and the environmental factors that govern the diazotroph community may facilitate the agricultural management to effectively utilize beneficial microbes for sustainable crop production

Wide-angle energy-momentum spectroscopy

Light emission is defined by its distribution in energy, momentum, and polarization. Here, we demonstrate a method that resolves these distributions by means of wide-angle energy-momentum spectroscopy. Specifically, we image the back focal plane of a microscope objective through a Wollaston prism to obtain polarized Fourier-space momentum distributions, and disperse these two-dimensional radiation patterns through an imaging spectrograph without an entrance slit. The resulting measurements represent a convolution of individual radiation patterns at adjacent wavelengths, which can be readily deconvolved using any well-defined basis for light emission. As an illustrative example, we use this technique with the multipole basis to quantify the intrinsic emission rates for electric and magnetic dipole transitions in europium-doped yttrium oxide (Eu$^{3+}$:Y$_{2}$O$_{3}$) and chromium-doped magnesium oxide (Cr$^{3+}$:MgO). Once extracted, these rates allow us to reconstruct the full, polarized, two-dimensional radiation patterns at each wavelength.Comment: 4 pages, 4 figure