Quantifying the Timing and Impact of Nitrogen Cycling within Switchgrass On Crop Sustainability.

Low nitrogen concentration [N] is desirable in biomass feedstocks used for thermochemical conversion to bioenergy or biofuels. Immediate [N] reductions can be achieved in perennial feedstocks by exploiting natural seasonal nutrient cycling (translocation). Knowing when switchgrass moves N to different tissues, and how much N is moved will enable management that: reduces the need for N fertilizer, increases stand longevity, produces a cleaner energy (less NO_x), produces higher quality bio-oil from pyrolysis, and reduces conversion costs. The objective of this study was to quantify the [N] of above- and below-ground plant biomass throughout the growing season of switchgrass (Panicum virgatum L).  Switchgrass and corn plots were established in 2008 and field measurements commenced in 2009 at an Iowa State University research farm in Madrid, Iowa. The experiment was designed as a split-plot, randomized complete block design with switchgrass sampling dates (April, June, July, September, and October) as sub-plots. Switchgrass biomass was measured in two, 1 m² quadrats within each sub-plot, and then the sub-plot was cleared with a commercial forage harvester. In 2009 yields peaked in July at 12.3 Mg ha^-1, decreased to 9.6 Mg ha^-1 by October then dropped to 5.5 Mg ha^-1 by April 2010. There was a 56% yield decrease between 2009 and 2010 in the July-sampled sub-plot (12.3 Mg ha^-1 and 5.4 Mg ha^-1, respectively).  This may be attributed to insufficient nitrogen reserves in belowground organs of switchgrass harvested prior to senescence and nutrient translocation.