Impacts of Internal Nitrogen Translocation within Switchgrass On Biomass Feedstock Sustainability.

Immediate nitrogen concentration [N] reductions can be achieved in perennial feedstocks by exploiting natural seasonal nutrient cycling (translocation).  Nitrogen (N) is needed in greater quantities compared to other nutrients for biomass production, but it has consequences in thermochemical conversion to bioenergy or biofuels.  Harvesting before senescence will result in more N in the above-ground tissues and may impact biofuel quality and nutrient requirements for subsequent years.  The objectives of this study are to: (1) quantify [N] of above- and below-ground biomass of switchgrass over two years, (2) determine how much total N is removed at different switchgrass harvest dates, and (3) determine switchgrass yield differences at different harvest dates.  Field trials of switchgrass were established in spring 2008 in Madrid, IA as a split-plot, randomized complete block design with switchgrass sampling dates (April, June, July, September, and October) as subplots.  The [N] in above-ground tissues (plant components) decreased over the season. Whereas, [N] in below-ground tissues decreased from April to July when plant growth commences and increased from July to April as the plant senesces.  There was a significant above-ground biomass yield reduction from harvesting in July with 12.3 Mg ha^-1 in 2009 to 5.4 Mg ha^-1 in 2010, compared to no significant difference in yield with an April 2010 and 2011 harvest. Biomass yield declines of above-ground tissues over two-years may be attributed to insufficient nitrogen reserves in below-ground organs of switchgrass harvested before senescence and nutrient translocation. By allowing nitrogen to fully translocate to below-ground tissues and harvesting after senescence, N removal can be reduced and long term biomass yields can be improved.