Functional Group but Not Fertilization Affects the Structural Composition of Prairie Plants.

Prairies composed of diverse mixtures of native species have the potential to be both environmentally beneficial and economically viable biofuel feedstocks.  Little is known concerning the conversion of heterogeneous herbaceous feedstocks into biofuels, however, because much of the research on feedstock conversion has been conducted using corn stover, switchgrass, and other single-species materials.  In order for diverse prairies to be used efficaciously as biofuel feedstocks, more needs to be known about their structural composition and potential biofuel yield. The four main functional groups in prairies are cool-season (C3) grasses, warm-season (C4) grasses, legumes, and non-legume broadleaf herbs (forbs).  In 2008 and 2009, we determined the structural composition (hemicellulose, cellulose, and lignin concentrations) and estimated ethanol yield of C3 grasses, C4 grasses, legumes, and mixtures of these three functional groups with and without nitrogen fertilization, as well as continuous corn with nitrogen fertilization.  Structural composition of the prairie plants differed by functional group and between years, but was not affected by nitrogen fertilization.  Hemicellulose concentrations were greatest in the C4 grasses; intermediate in the C3 grasses and mixtures, which were dominated by C3 grasses; and least in the legumes.  The reverse was true of lignin concentrations.  Cellulose concentrations were greatest in the legumes, intermediate in the C4 grasses, and least in the C3 grasses and mixtures.  Lignin concentrations were greater in 2008 than 2009, whereas hemicellulose and cellulose concentrations were greater in 2009 than 2008.  The structural composition of corn was most similar to the prairie C4 grasses, but had greater interannual variability than the prairie plants.  Estimated ethanol yields were highest in the corn and C4 grasses, intermediate in the C3 grasses and mixtures, and least in the legumes.  Results of this experiment indicate that ethanol yields from prairie species can be optimized by developing management strategies for controlling functional group composition.