Seasonal Accumulation and Partitioning of Carbon- and Nitrogen-Containing Compounds in Perennial Bioenergy Crops.

Transitioning bioenergy feedstocks  from first- (corn grain-ethanol) to second-generation (cellulosic-ethanol) crops will be enhanced by understanding growth constraints, biomass yield, and composition of perennial grasses. Our objective was to measure seasonal accumulation and partitioning of metabolic pools (i.e. carbon (C)- and nitrogen (N)-containing compounds) within perennial grasses as well as forage quality of aboveground biomass for ethanol conversion.  Seasonal trends in sugar, starch, buffer-soluble protein, soluble amino-N, and biomass fiber were measured in above- and below-ground tissues of Miscanthus (Miscanthus x giganteus; est. 2008), switchgrass (Panicum virgatum; 'Shawnee' est. 2007), and a big bluestem- (Andropogon girardii; est. 1992) dominated prairie. Whole-plant samples were collected monthly from April through September and December 2009, monthly from March through September and December 2010, and March, August, October, and December of 2011 while harvested aboveground biomass was collected August 2008 and October 2009 through 2011. Rhizomes from the unfertilized prairie consistently had low concentrations of sugar, starch, protein, and amino-N. Nitrogen fertilization of Miscanthus and switchgrass significantly increased root and rhizome amino-N concentrations, while the prairie did not consistently respond to N fertilization. Averaged across years, predicted ethanol yield of Miscanthus (5006 ± 344 L ha^-1) was greater than that of  switchgrass (2430 ± 176 L ha^-1), which in turn, exceeded that of the prairie (1031 ± 170 L ha^-1). These results reveal C and N partitioning differences between these candidate bioenergy crops that ultimately impact yield, composition, and economic return as illustrated by differences in predicted ethanol yields.