Soil Carbon Dynamics of Transition to Pacific Northwest Cellulosic Ethanol Feedstock Production

Cellulosic ethanol production will remove plant biomass for the production of energy. Plant biomass maintains soil organic carbon (SOC) which is essential for soil function. Available total plant biomass for maintenance of SOC and residue removal depends on plant net primary productivity (NPP) and crop type. This study was conducted to compare the biomass produced by three crops under a range of NPP for the maintenance of SOC and residue removal for cellulosic ethanol production. Biomass yields, ethanol conversion efficiencies, and SOC dynamics were assessed for differences between residue removed (RR) and not removed (NR) treatments of corn (Zea mays L.), wheat (Triticum aestivum, L.), and switchgrass (SG, Panicum virgatum, Var; Blackwell) under High, Medium, and Low NPP regimes. In 2008, crops were assessed following one year of establishment when ethanol conversion efficiencies are accounted for, there was no significant difference between crops ethanol production. In 2009 the SG RR Low treatment ethanol yield of 5.7 kL ha-1 14 was not significantly different from all levels of other crops. In the same year SG RR Medium and High yielded 9.2 and 11.8 kL ha-1, significantly more than the third highest yield of 5.7 kL ha-1 16 for Wheat RR High NPP. Soil carbon dynamics revealed significant contributions active and resistant soil C dynamics by SG and negative impacts by other crops. Significantly greater biomass yield, ethanol yield, and long term contributions to soil C dynamics by SG over a broad range of NPP make it the superior candidate for this system.