IDENTIFYING AND MANIPULATING DETERMINANTS OF PHOTOSYNTHATE PRODUCTION AND PARTITIONING
Location: Global Change and Photosynthesis Research Unit
Title: A regional comparison of water use efficiency for miscanthus, switchgrass and maize
Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 13, 2012
Publication Date: October 15, 2012
Citation: VanLoocke, A., Twine, T.E., Zeri, M., Bernacchi, C.J. 2012. A regional comparison of water use efficiency for miscanthus, switchgrass and maize. Agricultural and Forest Meteorology. 164:82-95.
Interpretive Summary: The US government has set in place mandates that require increased production of crops, other than maize grain (corn), dedicated specifically to the production of ethanol; these crops are called cellulosic feedstocks. It is not known however, which cellulosic feedstocks are best suited for production, and where they will be produced. Growing crops and refining biomass into useable energy requires water. Therefore it may be considered a benefit to choose a feedstock that maximizes the amount of productivity while minimizing the amount of water used. This trade-off between productivity and water use is called water use efficiency (WUE). Based primarily on high productivity, two perennial grasses, miscanthus and switchgrass, have been proposed as ideal candidate cellulosic feedstocks for production in the Midwest US. The WUE of these feedstocks could be an important factor in determining which feedstock to produce, however, it is not known for this region. The goal of this study was to compare the WUE of miscanthus, switchgrass and maize, the current dominant feedstock, over the Midwest US. We also compared maize with and without removing 25% of the non-grain (stem and leaf; i.e. residue) biomass, as this has also been proposed as a possible source of cellulosic biomass. To accomplish this goal we developed and tested a vegetation model (Agro-IBIS) to simulate the growth and water use of each feedstock using data collected at the University of Illinois. We ran model simulations over the Midwest US for a period of thirty-two years, and calculated productivity based on three commonly used metrics, harvested yield, net ecosystem productivity, and net biome productivity. The amount of water used to achieve productivity (i.e. evapotranspiration ET) was then compared across the region for each feedstock to obtain Harvest Water Use Efficiency (HWUE), Ecosystem Water Use Efficiency (EWUE), and Biome Water Use Efficiency (BWUE). Our analysis indicated that for most of the Midwest US (study domain), miscanthus had a higher HWUE, and EWUE than both switchgrass and maize, which had similar HWUE and EWUE. Miscanthus and switchgrass had similar BWUE, and were both higher than maize for most of the study domain. When 25% maize residue is simulated, HWUE increases by 10 to 20% while BWUE decreases by a much greater extent, 20 to 60%. The western portion (e.g. Nebraska, Kansas, and Colorado) of the Midwest US receives significantly less rain than the eastern portion (Illinois, Missouri, and Indiana). In these drier portions, maize had higher BWUE and HWUE than miscanthus, and higher EWUE than switchgrass. However, it is important to note that at no point in the study domain was maize higher than both perennials for any of the WUE metrics compared.
The production of cellulosic feedstock is likely to increase significantly over the coming decades. However, it is uncertain which feedstocks are best suited for production. A key factor
dictating feedstock selection for a given region is water use efficiency (WUE), the trade off between water use (evapotranspiration, ET) and biomass production or carbon uptake. Using the Agro-IBIS ecosystem model, two of the top candidate cellulosic feedstocks, Miscanthus X giganteus (miscanthus) and Panicum virgatum (switchgrass) are compared to the existing dominant bioenergy feedstock, maize for the Midwest US. We quantified productivity based on harvestable yield (HY), net ecosystem productivity (NEP) and net biome productivity (NBP; NBP = NEP-HY) and used these measures of productivity to calculate Harvest WUE (HWUE), Ecosystem WUE (EWUE), and Biome WUE (BWUE),respectively. Simulations indicate that over the Midwest US, miscanthus has a significantly higher HWUE compared to switchgrass and maize, while maize and switchgrass are similar. When EWUE is compared miscanthus is higher than both maize and switchgrass, which are similar for most of the region. Biome WUE is similarly higher for both perennials relative to maize for most of the Midwest with the exception of the driest portions of the domain where maize is the highest. Removing 25% of maize stover slightly increased HWUE and greatly decreased BWUE throughout the domain, however only HWUE changes were statistically significant. These results indicate that, of the species studied miscanthus will likely have the highest WUE except in the driest portions of the Midwest.