EVALUATING ENVIRONMENTAL CONSEQUENCES OF PRODUCING HERBACEOUS CROPS FOR BIOENERGY

The environmental costs and benefits of producing bioenergy crops can be measured both in terms of the relative effects on soil, water and w ildlife habitat quality of replacing alternate cropping systems with t he designated bioenergy system, and in terms of the quality and amount of energy that is produced per unit of energy expended. While many fo rms of herbaceous and woody energy crops will likely contribute to fut ure biofuels systems, The Department of Energy's Bioenegy Feedstock De velopment Program (BFDP), has chosen to focus its primary herbaceous c rops research emphasis on a perennial grass species, switchgrass (Pani cum virgatum). The choice of switchgrass as a model bioenergy species was based on its high yields, high nutrient use efficiency and wide ge ographic distribution. Another important consideration was its positiv e environmental attributes. The latter include its positive effects on soil quality and stability, its cover value for wildlife, and relativ ely low inputs of energy, water and agrochemicals required per unit of energy produced. A comparison of the energy budgets for corn, which i s the primary current source of bioethanol, and switchgrass reveals th at the efficiency of energy production for a perennial grass system ca n exceed that for an energy intensive annual row crop by as much as 15 times. In addition potential reductions in CO2 emissions, tied to the energetic efficiency of producing transportation fuels and replacing non-renewable petrochemical fuels with ethanol derived from grasses ar e very promising. Calculated carbon sequestration rates may exceed tho se of annual crops by as much as 20-30 times, due in part to carbon st orage in the soil. These differences have major implications for both the rate and efficiency with which fossil energy sources can be replac ed with cleaner burning biofuels. Current research is emphasizing quan tification of changes in soil nutrients and soil organic matter to pro vide improved understanding of the long term changes in soil quality a ssociated with annual removal of high yields of herbaceous energy crop s. (C) 1998 Elsevier Science Ltd. All rights reserved.