Does Nitrogen Fertilization Elicit a Productivity Boost In Mature Stands of Miscanthus (Miscanthus x giganteus) and Switchgrass (Panicum virgatum) Under Midwestern USA Growing Conditions?.

Highly productive rhizomatous perennial grasses Miscanthus (Miscanthus x giganteus) and Switchgrass (Panicum vigatum) are potential bioenergy feedstocks. High initial establishment cost of these perennial grasses necessitates high sustained productivity for them to be economically feasible.  Miscanthus and switchgrass trials were established in replicated 10x10 meter plots in 2002 and 2004 at seven locations in Illinois in a completely randomized design (n=4).  A split-plot nitrogen fertility treatment was initiated at three locations in 2007 and at all locations in 2008. Each plot was sub-divided into four 5x5 meter sub-plots which were randomly assigned a nitrogen treatment of 0, 67, 134, or 202 kg [N]ha^-1.  Each year, nitrogen treatments were applied in the spring at each location.  Switchgrass end of season biomass was determined between August and October and in December for Miscanthus, which is later to mature.  Harvestable shoot dry weight  per unit ground area was determined by drying to constant weight at 60ºC. Statistical analysis used a split-plot mixed model ANOVA with N-treatment and site as fixed effects and plot replication as random.  Statistical significance is reported at α=0.05 and α=0.10 for marginal significance.  The nitrogen yield response of Miscanthus is variable with location and year. For 2008, a significant effect of N-treatment in Dekalb (northernmost) drove the overall response while in 2009, the N response at the individual sites of Dixon Springs (southernmost) and Havana (very sandy soils) were significant.  Switchgrass showed no significant nitrogen end-of-season yield response in 2008 or 2009 at any of the sites within this study.  In 2008, pooled over all locations Miscanthus end-or season yield (December) also showed a positive increase (p=0.0003) while Switchgrass end of season (August) yields showed only a marginally significant increase (p=0.0786).  In 2009 when pooled across all locations, both Miscanthus and switchgrass yields were not significantly effected (p=0.0802 and p=0.5161).  However, when pooled across years and locations Miscanthus end of season yield (December) significantly increased with nitrogen fertilization (p=0.0016) from 23.35 t ha^-1 with zero fertilization to 28.04 t ha^-1 with 134 [N] ha^-1 fertilization and then no further significant increase within the levels tested in this study. Consistent with studies in Europe, Miscanthus can sustain high yields that are more than double those of switchgrass after several years and in the absence of nitrogen fertilization.  However, as sites which may be more stressful such as on a very sandy soil or at the northern end of its range, fertilization does significantly increase yield.  One interpretation may be that the N-fixing association detected in Miscanthus, is less effective under these more marginal conditions.

This work was funded by the Illinois Council for Agricultural Research (CFAR).