Switchgrass Yield, Nutrient Uptake, And Rhizosphere Microbial Community Composition As Affected By Cultivar And Soil Fertility

Abstract

Switchgrass (Panicum virgatum L.) is a native warm-season perennial that can be grown as a bioenergy crop on fragile or low-productivity soils, reserving prime agricultural land for food crops. The objectives of this dissertation were to evaluate switchgrass and mixed native perennial production and nutrient removal as a function of nitrogen (N) and phosphorus (P) application, harvest regime, and cultivar selection at two contrasting locations in Minnesota. Because bioenergy crop management decisions may alter rhizosphere microbial populations and associated ecosystem functions such as carbon and N cycling, this research also incorporated next-generation sequencing technology to examine bacterial and fungal community composition as a function of switchgrass cultivar selection as well as N and P fertility. The first experiment evaluated biomass production and N removal in switchgrass and mixed native perennials as a function of harvest regime (anthesis and post-frost) and N application rate (0, 56 and 112 kg N ha-1) at two locations in Minnesota. One plot was located near Becker, Minnesota, on Hubbard-Mosford complex loamy sand (Sandy, mixed, frigid Entic (Hubbard) and Typic (Mosford) Hapludolls) and the other was located near Lamberton, Minnesota, on Amiret-Swanlake loams (fine-loamy, mixed, superactive, mesic Calcic Hapludolls (Amiret) and fine-loamy, mixed, superactive, mesic Typic Calciudolls (Swanlake)) with 3-6% slope. Two switchgrass forage varieties, ‘Shawnee’ and ‘Sunburst’, produced 8.1 and 28.2 Mg ha-1 over three post-establishment years at Becker and Lamberton, respectively, and a low-diversity grass mix was similar in production to ‘Shawnee’. ‘Liberty’ switchgrass, a new bioenergy variety, yielded less than ‘Sunburst’, ‘Shawnee’, and the low-diversity grass mix. Based on these results, recommendations for maximum biomass production include planting either a well-adapted switchgrass variety or low-diversity grass mix fertilized with 56 kg N ha-1 annually, post-establishment, on loam soil, or with 112 kg N ha-1 in split application on sandy loam soil, and utilizing a post-frost harvest regime to remove less N and promote stand longevity over time. The second experiment, conducted on the anthesis harvest plots described in the first experiment, examined the effects of cultivar and N fertilization on bacterial community composition in the rhizosphere of ‘Sunburst’, ‘Shawnee’, and ‘Liberty’ switchgrass. While rhizobacterial community structure was primarily shaped by the existing microbial inoculum in the soil, variability in community composition was evident in response to cultivar and N. Only N fertilization, however, resulted in differences at both locations that were consistent across bacterial orders, including orders containing genera involved in N dynamics in soil: Nitrosomonadales and Rhodocyclales. The third experiment was located near Lamberton, Minnesota, on Webster clay loam (Fine-loamy, mixed, superactive, mesic Typic Endoaquoll). This experiment examined the effects of cultivar and P fertilization (0, 19.6, 35.1, and 58.6 kg P ha-1) on biomass yield, phosphorus removal, and rhizosphere microbial community structure in switchgrass grown on a low to medium soil test P soil. Post-frost biomass yields on low to medium soil test P soils increased linearly in response to P applied prior to establishment. ‘Shawnee’ produced greater biomass than ‘Liberty’ or ‘Sunburst’ (11.3, 10.2, and 8.6 Mg ha yr-1, respectively) over three years. Bacterial and fungal community structure was influenced by cultivar, but not P, although existing soil characteristics explained a greater proportion of variability in the rhizosphere community composition than did treatment effects.