N Cycling Dynamics in Perennial Bioenergy Crops.

Partitioning of C and N within perennial bioenergy crop tissues throughout the growing season impacts yield and survival. Our objective was to quantify seasonal N accumulation and partitioning among organs of Miscanthus x. giganteus, switchgrass (Panicum virgatum), and a big bluestem (Andropogon gerardii)-dominated prairie. Miscanthus and switchgrass plots received 56 kg ha^-1 yr^-1 of N while the prairie plots were unfertilized. Plants were sampled six times during the growing season and once in December. Plants were divided into roots, rhizomes, stem bases, and above-ground herbage, and were analyzed for yield and composition. In 2009 yield of the 2-yr-old Miscanthus stand in August (16.03 t ha^-1) was greater than that of the 3-yr-old switchgrass stand (8.96 t ha^-1) and the 28-yr-old prairie stand (3.47 t ha^-1). While Miscanthus produced the most above-ground growth, it produced the least amount of total below-ground tissue in August, yielding 2.93 kg m^-3 compared to the prairie (9.6 kg m^-3) and the switchgrass (6.8 kg m^-3). These below-ground tissues of switchgrass and Miscanthus had similar sugar (128 & 127 mg g^-1 dry wt.) and starch (335 & 320 mg g^-1 dry wt.) concentrations. Below-ground prairie tissues contained 123 mg g^-1 dry wt. sugar, but had low starch concentrations (185 mg g^-1 dry wt.). Data from 2010 will be reported. Quantities and above-/below-ground allocation patterns of total C, starches, and sugars suggest a comparative advantage for Miscanthus as a bioenergy crop. However, more research is needed to understand spatio-temporal production aspects including stand-age and soil fertility.