Due to government mandates in repsponse to climate change, ethanol production has steeply increased since 2009, and there are now for 79 billion liters of cellulosic biofuels yearly by 2022. Cellulosic crops such as maize, switch grass, and Miscanthus have been determined to be viable biofuel sources. In order to meet the biofuel target in 2022, cellulosic crop cultivation needs to be expanded and intensified. The impact on land and water use needs to be considered as well. Zhuang et al. (2013) present a data-model assimilation analysis assuming that maize, switchgrass, and Miscanthus can be grown on available U.S. croplands.—Christina Whalen
Zhuang, Q. Qin, Z. Chen, M. 2013. Biofuel, land, and water: maize, switchgrass, or Miscanthus. Environ. Res. Lett. 8, 015020.
The current production levels of maize are not enough to be simultaneously used as biofuels and as a food source. The cellulosic crops switchgrass and Miscanthus have been identified as viable alternatives to maize in producing second-generation biofuel. This is staged to work especially well in temperate regions because of their higher biomass productivity and available crop-producing land. Other studies have shown that bioenergy crops have higher land and water efficiencies than food crops do, but the increasing demand of land and water to cultivate these crops hasn’t been researched using ecosystem models. The study uses the terrestrial ecosystem model (TEM) to predict the demand of land and water for growing various biofuel crops so that enough ethanol can be produced to hit the 2022 target. The goal of the study is to analyze the demand for resources rather than to analyze the environmental impact of growing biofuel crops.
The TEM ecosystem model uses gross primary production (GPP) as the core algorithm, which describes the rate at which a plant produces usable chemical energy. The Net primary production (NPP) is the difference between GPP and plant respiration. In order to analyze the productivity of feedstocks and biofuels, the researchers estimated the biomass and biofuel production in terms of harvestable biomass (HBIO) and bioethanol yield. Current and future biofuel production was estimated using conversion efficiencies and currently available and potentially advanced technologies. TEM was run several times at each site in order to achieve model equilibrium. Analyses were conducted on biomass and biofuel yield, water balance, and water use efficiency and were estimated based on simulations.
The results of the model demonstrate that in order to produce 79 billion liters of ethanol from maize grain, there would be a need for 190 million tons of conventional grain and 26.5 million hectares of land, which is equivalent to 20% of total US cropland. The water loss of this production would be 92 km3, but if the maize stover were also used, water would be saved. Because switchgrass has lower conversion efficiency, using this crop would result in a higher demand of biomass. More land and water would need to be used to produce the same amount of ethanol than using maize. Alternatively, Miscanthuswould only require half the amount of land and two-thirds the amount of water used for maize grain in order to produce the same amount of ethanol. Furthermore, with the advanced technologies predicted for future years, even less water and land will need to be used in converting biomass to biofuel. The model experiments demonstrate that switching from maize to Miscanthus will save land and water, but that switching from maize to switchgrass will require more land and water.
This study only predicts ethanol production using available croplands, but recent studies have illustrated that marginal lands could also be a source for cultivating cellulosic crops. Experiments have also shown that switchgrass may be more productive on marginal lands than on traditional croplands. The model may produce some bias because it does not consider the effects of fertilization, irrigation, rotation, and tillage. To strengthen the study, analysis of economic viability, food security, nutritional and ethical concerns, and other environmental consequences and benefits need to be conducted.