Measurements of the ecosystem impacts of four biofuel crops on the cycles of carbon, water, and nitrogen in Central Illinois

Background/Question/Methods   Second-generation biofuels, or fuels derived from the decomposition of the cellulosic material in the plant biomass, have a great potential regarding energy security and sustainability. However, the choice of species should be based on their productivity per hectare and low environmental impacts. Maize has the advantage of being already extensively planted in the Midwest. Miscanthus is a fast growing perennial grass that has a great potential for biofuel production due to its high productivity and no requirement of fertilizers or irrigation. Nevertheless, the use of maize residue for biofuels might impact the carbon cycle through the reduction of soil organic matter. The total water used for Miscanthus and other grasses during the growing season, and their water use efficiency (WUE, the ratio of carbon accumulated over total water used), might impose limits on the regions where these biofuel crops can be sustainably planted on a large scale. Four potential candidates are being monitored since 2008 in a side-by-side study that aims to compare the ecosystem impacts of these species on the fluxes of carbon, water, and nitrogen. The site is located at a University of Illinois research facility in Urbana, Illinois, and the species chosen are maize (Zea mays), miscanthus (Miscanthus x giganteus), switchgrass (Panicum virgatum), and prairie (a mixture of native prairie varieties). The measurements are performed at micrometeorological towers placed at the center of 4 ha plots. The towers are equipped with eddy covariance systems (for the fluxes of carbon dioxide and water vapor), consisting of fast response 3D sonic anemometers and infrared gas analyzers; fluxes of nitrous oxide are measured using a tunable diode laser system, installed in June of 2009.

Results/Conclusions   Results for the first year (2008) show a higher carbon uptake for maize (including the grain), when compared to the other grasses. This result was expected because the grasses were still in the establishment phase. The carbon balance for the grasses was affected by mowing, resulting in increased soil emissions and a consequent reduced net uptake of carbon. The amount of water used by switchgrass was comparable to the amount used by maize, but the latter had a higher WUE due to the higher biomass accumulated. Fluxes of nitrous oxide were higher for maize, due to the use of fertilizer, and were correlated with soil temperature.