The Argument for Perennial Biomass Crops. Many major agricultural crops today are annual plants propagated from seed or cuttings at the beginning of each growing season. By contrast, crops developed and grown specifically for biofuel production are expected to be based on perennial species grown from roots or rhizomes that remain in the soil after harvesting the above-ground biomass. Perennial species are considered advantageous for several reasons. First, input costs are lower than for annuals because costs of tillage are eliminated once a perennial crop is established. Additionally, long-lived roots of perennials may establish beneficial interactions with root symbionts that facilitate acquisition of mineral nutrients, thereby decreasing the amount of fertilizer needed. Some perennials also withdraw a substantial fraction of mineral nutrients from above-ground portions of the plant at the end of the season but before harvest. Perennial plants in temperate zones also may have significantly higher total biomass yield per unit of land area than comparable annual species. Perennials establish a photosynthetically active canopy more quickly in the spring and may persist longer in the fall (see next figure in set). Thus, their annual solar-energy conversion efficiency is higher than that of annual plants with similar capabilities. Perennial species have much lower fertilizer runoff than do annuals. For instance, comparing the native perennial switchgrass with corn indicates that switchgrass has about one-eighth the nitrogen runoff and one-hundredth the soil erosion. Perennial grasses harvested for biomass exhibit increased soil-carbon levels and provide habitat for up to five times as many species of birds. Finally, in contrast to annual row crops that typically are monocultures, increasing habitat diversity by growing several intermixed species of perennials may prove more feasible.
Credit or Source: S. Long, University of Illinois
U.S. DOE. 2006. Breaking the Biological Barriers to Cellulosic Ethanol: A Joint Research Agenda, DOE/SC/EE-0095, U.S. Department of Energy Office of Science and Office of Energy Efficiency and Renewable Energy. (p. 59) (website)