Publication Date

8-2016

Date of Final Oral Examination (Defense)

4-22-2016

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Biology

Department

Biology

Supervisory Committee Chair

Marie-Anne de Graaff, Ph.D.

Supervisory Committee Member

Kevin Feris, Ph.D.

Supervisory Committee Member

Marcelo Serpe, Ph.D.

Abstract

Bioenergy feedstock production is an important component of the national renewable energy strategy, which is based on biomass supply. Biofuels for ethanol production may be produced in high-input crop production systems, but the efficacy of these systems for increasing net energy yields over its full life-cycle compared to traditional fuels is under debate, because it is now evident that the benefits of feedstock production are maximized only when biofuels are derived from feedstocks produced with much lower life-cycle greenhouse-gas emissions than traditional fossil fuels. To this end, the reduction of agricultural inputs is key to developing an effective biofuel feedstock crop. Native prairie grasses have low-input production requirements, and upon land conversion for biofuel production they have positive impacts on belowground carbon (C) sequestration, a measure of soil quality. Specifically, Panicum virgatum (hereafter switchgrass), a perennial C4 grass native to the mid-west of the United States, is a promising bioenergy crop. It has large root systems, which allow it to produce large amounts of biomass with less water and nutrient requirements than traditional bioenergy crops, such as corn.

To produce switchgrass feedstock in an environmentally sustainably manner (i.e., with the least amount of fertilizer inputs), we will need to adopt agricultural practices that promote N cycling efficiency in the system. Previous studies have found that different cultivars of switchgrass vary significantly in specific root length (SRL), and greater SRL may be linked to greater N acquisition owing to the root systems’ greater surface area. In addition, it has been found that growing switchgrass in genotypically diverse mixtures enhanced biomass production, which may result from belowground niche differentiation and complementarity effects that enhance N acquisition. With this study, I aimed to evaluate (1) whether differences in the architecture among root systems of switchgrass cultivars led to differences in the efficiency of nitrogen uptake, and (2) whether growing switchgrass cultivars in diverse mixtures would enhance the efficiency of nitrogen cycling though niche differentiation and complementarity effects.

Our experiment was conducted at the Sustainable Bioenergy Crop Research Facility at the Fermilab National Environmental Research Park, where experimental field plots consisted of seven switchgrass cultivars, planted either in monoculture or in diverse mixtures of 2, 4, or 6 randomly selected cultivars. To evaluate differences in nitrogen use efficiency (NUE) among cultivars in monocultures and among diversity treatments, I applied a stable isotope 15N tracer at the beginning of the growing season. Following senescence, the switchgrass was harvested and the percent of 15N recovered was measured in the aboveground biomass to determine NUE. I found that switchgrass cultivars differed in NUE and these differences could potentially be linked to germplasm origin in relation to the field site. I also found that NUE was not influenced by increases in cultivar diversity. Our results suggest that NUE is not the sole mechanism behind greater biomass production associated with enhanced diversity.

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