Effect of Different Winter Legumes As Nitrogen Sources for Switchgrass Grown for Cellulosic Ethanol.

Switchgrass (Panicum virgatum L.) is a perennial warm-season low-maintenance grass that produces high biomass yields. Because of its high cellulose content researchers are focusing on this grass species as a renewable energy solution to the energy crisis. To produce high quality forage, switchgrass requires large amount of nitrogen. Winter annual legumes may provide a considerable amount of nitrogen requirement which could be an alternative for inorganic fertilizer. A two-year field study was initiated in fall 2009 at Perkins, Oklahoma to determine the effect of six winter legumes on yield, nitrogen fertilizer requirement and soil properties in a switchgrass stand. Six winter legume treatments (Arrowleaf Clover, Austrian Winter Pea, Button Medic, Crimson Clover, Hairy Vetch, and Red Clover) were no-till planted between switchgrass rows. The legumes were allowed to mature assuming that the plant residues also add nitrogen for the switchgrass and improve soil properties. Also, four inorganic nitrogen rates (0, 56, 112, and 168KgN/ha) were broadcasted in spring 2010 to compare switchgrass yields with the legume treatments. The experimental design was a Complete Randomized Block Design. Red Clover and Button Medic were very poorly established. Probably these two legumes were unable to compete with giant switchgrass. Crimson Clover, Arrowleaf clover and Austrian Winter Pea were moderately established comparing other three legume species. Only Hairy Vetch had a good stand with the switchgrass on the basis of dry matter weight.  Switchgrass yields were not satisfactory from the six winter legumes treatments. Based on the dry biomass production of switchgrass an average of 3272.23 Kg/ha yield was obtained from the plots of Austrian Winter Pea. When 168 Kg/ha inorganic fertilizer rate produced highest yield which was 4757.65 Kg/ha. Other two nitrogen rates 56 KgN/ha and 112 KgN/ha produced 2820.14 Kg/ha and 3896.53 Kg/ha yield respectively. Although Hairy Vetch had a good stand with switchgrass only 2992.36 Kg/ha switchgrass biomass was obtained. Probably dense vegetative growth of Hairy Vetch influenced the normal growth of switchgrass. Crimson Clover, Button Medic, Arrowleaf Clover and, Red Clover plots produced 3100.008 Kg/ha, 3056.952 Kg/ha, 2863.2 Kg/ha and 2561.81 Kg/ha switchgrass yield respectively. No measurable changes were found in soil organic matter and nitrogen content in the top six inches soil. After one year of observation it was difficult to determine the advantage of the legumes over inorganic nitrogen. The legumes did not supply an economical nitrogen source for switchgrass.