Nutrient Management and Seasonal Dynamics in Switchgrass (Panicum virgatum L.) Production.

The evaluation of switchgrass (Panicum virgatum L.) as a biomass fuel crop has shown great potential in the US, because of its ability for growth on marginal soils and use in cellulosic ethanol production.  Research results have shown biomass yield response to fertilization varies widely throughout the US.  The objectives of this study are to determine the optimum nitrogen (N) fertilization rates for biomass production of switchgrass and to investigate nutrient cycling and dynamics. Two locations, Stillwater and Haskell, OK, were used for the N response studies.  Eight N treatments (0, 30, 60, 90, 120,150, 180 and 210 lbs N acre^-1 as urea) with four replications in a randomized complete block design were used in both locations.  At Stillwater, whole plant samples within 120 N treatment plots (2008) and 90 N treatment plots (2009) were hand harvested at six week intervals from June to the following February to monitor above and below ground nutrient changes.  In 2008, switchgrass yields responded well with N rates at the Stillwater site (R² = 0.69), but little response was found at the Haskell site, probably due to high soil residual N (R² = 0.13).  In both 2008 and 2009 at Stillwater, the concentrations of N, phosphorus and potassium in the aboveground biomass tended to decrease with time, and increased with time in the belowground biomass.  The mechanism of nutrient concentration changes in different directions is unclear.  New approaches of study are needed to accurately evaluate nutrient dynamics within switchgrass.