Seasonal Cycling and Partitioning of N, P, and K In Perennial Bioenergy Crops.

Life cycle assessment and understanding system efficiencies require data on yield and nutrient needs of candidate bioenergy species. Our objective was to determine yield, dry matter partitioning, and patterns of nitrogen (N), phosphorus (P), and potassium (K) accumulation/partitioning  among tissues of Miscanthus x giganteus (est. 2008), switchgrass (Panicum virgatum; est. 2007), and an unmanaged  big bluestem (Andropogon gerardii; est. 1992)-dominated prairie.  Plants were sampled monthly throughout the growing season and once during the winter in both 2009 and 2010. In both years Miscanthus had the most above ground biomass (16 Mg/ha and 38 Mg/ha, respectively) compared to switchgrass (9 Mg/ha and 13 Mg/ha, respectively) and the prairie (3 Mg/ha, and 5 Mg/ha, respectively). Miscanthus consistently had the highest shoot/root values over the two years (3 and 6, respectively) compared to switchgrass (0.8 and 1, respectively) and the prairie (0.2 and 0.3, respectively). In 2010 Miscanthus translocated 55 kg N/ha to storage organs (roots, rhizomes, and stem bases) between August and December, while switchgrass and prairie translocated 24 kg N/ha and 16 kg N/ha respectively to these storage organs during this interval. Miscanthus also translocated more P (14 kg/ha) belowground from August to December, while both switchgrass and prairie transported only 2 kg P/ha belowground during this time interval. Similarly, Miscanthus transported 66 kg K/ha belowground from August to December but no net shoot-root K transport was observed in switchgrass and prairie with storage organs losing 5 and 12 kg K/ha, respectively, to the environment from August to December. For Miscanthus these transfers to storage organs represent 24, 40, and 18% of the N, P, and K content at maximum biomass in August. For switchgrass and prairie, these changes represent 34, 10, and -14% and 56, 25, and -29%, respectively for N, P and K content at maximum biomass in August. These preliminary results suggest that Miscanthus is unique in its translocation abilities of inorganic nutrients to storage organs during fall senescence.