Aboveground and Belowground Biomass and Nitrogen Retranslocation in Switchgrass (Panicum virgatum).

Switchgrass is a promising biomass energy source that has a broad native geographic distribution, and is perennial, highly productive, and nutrient use efficient.  To better understand nitrogen dynamics in switchgrass and to assist managers in optimizing soil nitrogen (N) retention,  biomass production efficiency, and the timing of harvest, we examined N retranslocation and aboveground and belowground biomass distribution at three switchgrass sites: ‘Shawnee’ in Nebraska and Pennsylvania, and ‘Cave-in-Rock’ in Pennsylvania.  At monthly intervals from June until November, four 15x15x15 cm cubes of soil and associated aboveground and belowground biomass were collected from each field, separated into seven tissue groups, and analyzed for N.  Additionally, aboveground biomass was harvested from three one square meter plots in 2008 in Pennsylvania; AP horizon soil and roots were excavated with a backhoe in 2008, and B-horizon soil and roots were excavated with an auger in 2009.  Total living plant mass was 14.9 MT/ha + SE 2.5 for Cave-in-Rock-PA and 10.3 MT/ha + SE 2.4 for Shawnee-PA.  Belowground mass comprised 52% of total biomass for Cave-in-Rock-PA and 57% for Shawnee-PA; most belowground biomass was located in the AP horizon.  Generally blades had the highest N concentration of the tissue groups during summer, ranging from 2.2% (Cave-in-Rock-PA in July) to 0.62% (Shawnee-NE in August).  From September to senescence, N concentrations in aboveground tissues decreased, whereas concentrations in belowground tissues increased from August to senescence.  Shawnee-PA aboveground tissues decreased from 1.65% N in June to 0.50% N in October while N concentration in belowground tissues increased from 0.43% N in June to 0.87% N in October.  For all three sites, total N retranslocated between September and October resulted in an average movement of 17 kg N/ha from aboveground to belowground biomass components.  Based on current N fertilizer costs, retranslocation results in an average savings of approximately $4/ha, indicating that delayed harvest would result in N and slight financial savings if harvested after October but before overwinter biomass loss.  However, the timing of harvest is based not only on N conservation, but also on biofuel moisture and ash content, over-winter loss rates, and the means by which the material is harvested.