Nitrogen Rate and Cultivar Effects on Switchgrass Biomass Production and Nutrient Removal.

Nitrogen fertility is an important determinant of crop productivity, quality and profitability; therefore it is critical to understand how N and other management factors affect switchgrass (Panicum virgatum L.), an important warm-season biofuel feedstock crop.  The objective of this study was to quantify the effects of N rates and cultivars on biomass production and nutrient removal capacities of switchgrass.  A 2-yr study was conducted at the Brown Loam Branch Experiment Station, Raymond, MS during 2008 and 2009 using four N application rates (0, 80, 160 and 240 kg ha^-1) and four cultivars (‘Alamo’, ‘Cave-in-Rock’, NF/GA001 and NF/GA992).     There was a N rate × year interaction effect on dry matter (DM) yield (P = 0.01), with a linear response to N rate in 2008 and a quadratic response in 2009.  Average DM yields across N rate for Alamo, Cave-in-Rock, NF/GA001 and NF/GA992 were 23.07, 12.11, 26.72, and 27.80 kg ha^-1, respectively.  There was a N rate effect on removal rates of Ca, Mg, Fe, and Mn while year × N rate interactions were observed for Zn and Cu removal.  There was no effect of N rate on tissue concentration of the reported elements, however, year × cultivar interactions were observed for P, K, Ca, and Mg tissue concentrations.  Nitrogen use efficiency, apparent N recovery, and partial factor productivity all declined with increase in N rate.  Positive correlations (P < 0.05) were found between yield and N, P, Ca, Mg, S, Fe, Mn, Zn, Cu, and B removal while negative correlations (P < 0.05) were found between yield and K and Ca tissue nutrient concentration.  Phosphorus, K, Mg, S, Fe, Zn, and Cu removal correlated (P < 0.0001) with the respective tissue concentration of these elements.  These results indicate that there is opportunity to increase switchgrass biomass yields and productivity through cultivar selection and N management.