Intraseasonal changes in switchgrass nitrogen distribution compared with corn

Incorporating perennial, bioenergy crops like switchgrass (Panicum virgatum L.) into agricultural landscapes can provide harvestable biomass while improving ecosystem functions, but clear plant N management recommendations for switchgrass remain elusive. Delaying harvest until spring can reduce plant N concentration ([N]) but also harvestable yield, creating a tradeoffbetween feedstock quality and quantity. Th is study investigated internal changes in the temporal and spatial distribution of N in switchgrass tissues. Above- and belowground switchgrass biomass was harvested at five dates annually from field plots in central Iowa for 2 yr. The [N] of aboveground tissues decreased during the growing season, with no further change over winter. The [N] of belowground biomass tissues found within the upper strata (0-45 cm) of the soil profile increased significantly from July to October (P = 0.0050), suggesting seasonal N translocation. Plots harvested at peak biomass, before senescence, yielded significantly less the following season (P < 0.0001), suggesting that this harvest strategy would not be sustainable in the long term. Although delaying harvest to after frost reduced the harvested biomass, less N was removed with the harvest, more was stored in belowground tissues, and yields were more similar between years. A postfrost switchgrass harvest would remove much less N during a 2-yr period compared with continuous corn (Zea mays L.) with 50% stover removal (57.7 vs. 432.2 kg N ha^-1). Results provide values of N stocks and fluxes in switchgrass tissues, showing the tradeoffs between feedstock quality and quantity.