Impact of Net Primary Productivity Levels On Bioenergy Production in the Pacific Northwest.

Biofuels of cellulosic origin result from the production of dedicated energy crops and removal of agricultural residues, primarily cereal grains. We conducted a study in the low rainfall zone of the Columbia Basin to provide a quantitative assessment of herbaceous biomass production and crop residue removal over a range of net primary productivity (NPP). The experimental treatments consist of crop type, biomass removal, and NPP level, which were induced by varying irrigation, N fertilizer and seeding rates. Crops include continuous corn (Zea mays L.), continuous wheat (Triticum aestivum L.), a warm season grass monoculture of switchgrass (Panicum virgatum L.), and a switchgrass dominated warm season grass polyculture. Crop residues from wheat and corn are removed at harvest and warm season grasses are harvested similar to a forage crop twice a year. Each crop is managed under three production regimes intended to create low, intermediate and high NPP levels found in the United States. Total biomass production (Mg ha-1) in 2008 for corn, wheat, switchgrass and polyculture averaged 6.5, 6.0, 4.1 and 3.3, respectively for low NPP, 12.2, 8.0, 10.9 and 11.1, respectively, for intermediate NPP and 13.2, 8.6, 12.0, and 12.8, respectively, for high NPP levels. The ethanol equivalent (L kg-1) for the high NPP level in 2008 averaged 3739, 4260 and 4544 for corn, wheat and the switchgrass dominated grasses, respectively. The warm season grasses were in their first year of production and will not reach full production levels until the third year. Production data for 2009 will be summarized and compared to 2008.