Biomass Yield Response to Nitrogen Fertilizer Rate of Five Potential Bioenergy Grasses.

Perennial grasses are projected to contribute to biomass energy production goals. Our objectives were to evaluate biomass yield responses to nitrogen fertilizer rate for five perennial grasses grown on sandy soil in the southern Great Plains. These grasses were switchgrass (Panicum virgatum experimental line NF/GA993), giant reed (Arundo donax), weeping lovegrass (Eragrostis curvula cv. Ermelo), kleingrass (Panicum coloratum cv. Selection 75) and Johnsongrass (Sorghum halepense). Each species was established in monoculture plots arranged in a randomized complete block design with six replications in 2008. These grasses were then fertilized at 0, 56, 112, or 168 kg N ha^-1 in spring and harvested for biomass after a killing frost in winter 2009 and 2010. In general, biomass yields did not show consistent relationships with N fertilizer rate (P=0.08) and were affected by year and species interactions (P<0.01). Weeping lovegrass produced 29.0 Mg DM ha^-1in 2009 but only 13.0 Mg ha^-1 in 2010. Biomass yield of Kleingrass averaged 16.0 Mg ha^-1 in 2009 but only 9.8 Mg ha^-1 in 2010. Biomass yields were similar across years for giant reed, switchgrass and Johnsongrass, averaging 23.3, 17.8, and 6.0 Mg ha^-1, respectively. Giant reed, however, produced the highest yield among species, 33.2 Mg DM ha^-1, when fertilized at 168 kg N ha^-1. Giant reed and weeping lovegrass removed more than 100 kg N ha^-1 and more than 20 kg P ha^-1 while other species removed less than 100 kg N ha^-1 and less than 20 kg P ha^-1. Low amount of K was removed by all species in 2010 compared to 2009.

Giant reed, weeping lovegrass and Kleingrass established at 26900 seedling ha^-1 show good potential for biomass production under the climatic conditions prevalent in the Southern Great Plain. In site with sandy soils, first production year for weeping lovegrass and kleingrass may need to be two year after establishment to allow for better root anchorage, check on harvest damage, and ensure sustained  year to year yields all other factors constant. Although giant reed showed promise as an alternative to switchgrass for biomass production, its higher nutrient removal rates would increase fertilization requirements in time. More research on giant reed as an alternative to C-4 grasses for bioenergy is needed that focuses on reducing leaf-loss during senescence and with reduced tissue nutrient concentrations.  Retaining leaf will increase biomass at harvest during winter, allow nutrient relocation to belowground storage structures, and may increase next season growth vigor.