| The first chapter focused on the adaptability, yield potential, and response to N fertilization of switchgrass (Panicum virgatum) under five N fertilization rates (0 to 300 kg N ha-1 yr-1) in four distinct ecoregions of California from 2008-2010. Switchgrass was not suitable in the temperate climate due to winter mortality. Yields ranged from 13 to 27.1 Mg ha-1 yr-1 across locations and years, with greatest yields in the Mediterranean, semi-arid, and Desert climates. Yields increased linearly in three of four locations, increasing by 9.7 and 13 Mg ha -1 and N use efficiency of 30 and 44 kg biomass kg-1 N applied in 2009 and 2010, respectively. The second chapter evaluated the yield potential and response to N fertilization of five perennial C4 [switchgrass, miscanthus (Miscanthus giganteus ), elephantgrass (Pennisetum purpureum), bermudagrass (Cynodon dactylon), and big bluestem (Andropogon gerardii)] and two C3 [tall fescue (Festuca arundinacea) and tall wheatgrass (Agropyron elongatum)] species as bioenergy crops in California's Central Valley from 2009-2011. Elephantgrass was excluded from the trial due to winter mortality. Highest yields were 33.9, 22.9, 17.2, 16.2, 15.6, and 12.0 Mg ha-1 yr-1 for miscanthus, switchgrass, tall wheatgrass, big bluestem, tall fescue, and bermudagrass, respectively. Significant responses to N were observed for all crops in all years. Switchgrass and miscanthus have greatest potential as bioenergy crops due to high yields, greatest response to N, and lowest biomass N concentration. The final chapter determined the sustainability of low-input (single-harvest irrigated until flowering) and high-input (two-harvest irrigated throughout the growing season) switchgrass systems as a function of yield, irrigation requirement, crop N removal, N translocation during senescence, and fertilizer 15N recovery in the crop and soil. The low-input was more sustainable than the high-input system. Average yields were 17.4 (low-input) and 21.2 Mg ha-1 (high-input). The low-input system required 47% less irrigation and had 49% of N translocation from AG to BG biomass during senescence, resulting in 46% lower crop N removal, 53% higher N stored in BG biomass, positive N balance, lower response to N fertilization, and 50% higher fertilizer N remaining in the plant-soil system at the end of 3 years. |
