Concerns over the potential effects of climate change on energy and food production in the last ten years have created a new market for alternative fuels. In the United States, corn-based ethanol, likely due to the political clout of the US corn lobby, has dominated biofuels research to date. However, corn may be ill-suited for ethanol production because oil is used in the production, transport, and application of the large amounts of nitrogen fertilizer necessary to boost corn yields. The nitrogen fertilizers have a detrimental effect on the environment by decreasing soil productivity and leaching into neighboring soils and water tables. With the advent of the US Energy Independence Act of 2007, the US government created demand for up to 15 billion gallons of corn derived ethanol per year, mandating any amount beyond that be produced from other feedstock sources. Although high in energy yield, corn’s dependence on oil makes it less efficient overall if environmental damage and GHG emissions are considered. The cap on corn ethanol production initially stimulated research into alternative feedstock, with preliminary research showing great promise from perennial grasses like switchgrass and miscanthus. The initial studies on the relative energy yield efficiency of corn and alternative feedstock prompted Parton et al. to develop a model capable of estimating the benefits of switching ethanol feedstock from corn to perennials. By using regression analysis of the DAYCENT model, they found that by substituting miscanthus and switchgrass for corn on lands already designated for ethanol production food productivity would increase by 4% and available feedstock for ethanol by 82% all while avoiding the GHG releases associated with the conversion of uncultivated land for agricultural production (known as ILUC, indirect land-use change).—Michael Gazeley-Romney
Davis, S., Parton, W., Del Grosso, S., Keough, C., Marx, E., Adler, P., DeLucia, E., 2011. “Impact of second-generation biofuel agriculture on greenhouse-gas emissions in the corn-growing regions of the US”. Frontiers in Ecology and the Environment; doi:10.1890/110003
Davis et al. simulated the effects of substituting 30% of the central US regional corn crop with three alternative biofuel crops: (1) switchgrass, (2) switchgrass with fertilizer treatments, and (3) miscanthus. Using the model they were able to calculate the feedstock production potential in each case as well as related GHG emissions, soil carbon sequestration, and nitrogen leaching over a ten year period. Their model is version of the CENTURY model that operates on a daily time step simulating exchanges between soil, plants, and the atmosphere as well as the affects of management practices like prescribed burning, grazing, and fertilizer use. To verify the accuracy of the model, the simulations were compared to test results from biofuel feedstock test plots already present in the study region finding close correlations for crop yields, GHG emissions, and nitrogen leaching. Using a simulation of ethanol corn production as a baseline, the researchers were able to calculate the differences in outputs between the growth scenarios.
Possible imperfections in the model stem from the unknown effects of ILUC on GHG emissions. Although it is generally understood that large amounts of GHGs are produced in the tilling of virgin soil for agriculture, the amounts are hard to predict and can vary greatly. In order to compensate for this, Davis et al. calculated the effects of converting 30% of central US corn acreage to ethanol production using ILUC accounting from the California Air Resources Board finding emissions of 4.7–5.3 Tg C. When computed with the model results, the ILUC emissions did not have a significant effect on the large net differences between emissions for corn and those for the perennials. This is significant because the ILUC calculations should not apply to the test scenarios as no new land is being converted; the crops are simply being rotated. Controversy over outcrossing and slow investment realization (three years needed to establish perennial grass crops) are also omitted from the model analysis. However, both switchgrass and miscanthus present a low outcrossing risk because switchgrass is native to the US and miscanthus is a sterile hybrid.
The results of the modeling showed significant environmental benefits from switching to perennial cellulose feedstock. These findings are more significant because the model was constructed to only consider the conversion of land already being used in ethanol production. In this way, the environmental benefits realized by switching feedstock crops comes absent the usual concerns about ethanol land use competing with food production. By limiting the model to a 30% corn-to-perennial land-use switch, the researchers hoped to simulate the complete transition from corn to perennial feedstock in US ethanol production (30% of all corn grown in the US is used in ethanol production). By avoiding the effects of ILUC entirely and only substituting feedstock within the existing production capacity of 30%, the study reduces foreseeable land use pressure from ethanol production on the 8% of US corn grown for food.
Modeling of soil organic carbon (SOC) showed increases under fertilized switchgrass and miscanthus cultivation of 27 and 173 Tg Ceq yr-1 respectively. Compared to corn, fertilized switchgrass increased SOC by 1.9% and miscanthus by 19%. The conversion from corn to perennial feedstock changed also altered the regional output of GHGs–in terms of the re-appropriated cropland–from 27 Tg Ceq yr–1 for corn to 17 Tg Ceq yr–1 for switchgrass, –0.05 Tg Ceq yr–1 for fertilized switchgrass, and –97 Tg Ceq yr–1 for miscanthus. While the switchgrass succeeded in reducing agricultural GHG emissions, the substitution of miscanthus effectively transformed the entire region into a massive carbon sink. Davis et al. attributed the reduction in GHG emissions to less fertilizer use and increased carbon sequestration in the perennial crops compared to corn. Davis et al. reinforce the magnitude of this finding by citing a recent study that shows the reduction in GHGs from ethanol use (in the place of fossil fuel) are wholly offset by the heavy application of nitrogen fertilizers on corn.
Using other research to interpret the significance of the modeling results, Davis et al. found that switching to perennial grasses would reduce nitrogen use overall, resulting in 0.7–0.8 Tg N yr–1 less nitrogen leaching through the soil. With 52% of the nitrogen polluting the Gulf of Mexico stemming from US corn and soybean production, the savings on environmental mitigation measures in the Gulf alone would be significant. To further demonstrate the relative benefits of switching feedstock, the researchers calculated the CO2 emissions from harvests of the corn and grasses, with the grasses producing 74% less CO2 during the harvest cycle.
In a second run, Davis et al. altered the model constraints to substitute the perennials for corn on only the least productive 30% of the ethanol corn grown in the region. In this model the differences in efficiencies between corn and the perennials were even more pronounced with miscanthus producing 82% more biomass for ethanol feedstock than the corn baseline scenario, the equivalent of about twelve billion gallons of ethanol.
As we continue to wean ourselves from foreign oil, energy efficiency within national production systems will take on a higher priority for policy makers. Choosing a low N-input, high energy-output feedstock over traditional corn has been shown under comprehensive modeling by Davis et al. to be much more efficient. When considering biofuels, it cannot be forgotten that they are meant to be a low-impact replacement for fossil fuels. With its dependence on oil for growth in the current agricultural system, corn has become an unsuitable and highly inefficient ethanol feedstock compared to perennial grasses. The findings of the study with regard to the yield efficiency and environmental benefits of miscanthus make it the clear choice for future feedstock use. In light of the findings of Davis et al., national production capacity for cellulose feedstock like miscanthus needs to be addressed in order to realize its benefits. Replacing corn ethanol feedstock in the central US region could increase the regional productivity of food by 4% and feedstock biomass 82% all while avoiding additional ILUC, making ethanol a truly environmentally friendly substitute to fossil fuel.