Quantifying Cradle-to-Farm Gate Life Cycle Impacts Associated with Fertilizer used for Corn, Soybean, and Stover Production

Rebecca Thomason and Susan E. Powers, PhD, P.E.

Department of Civil and Environmental Engineering

As our country attempts to shift energy dependence away from foreign fossil fuel resources, it has become necessary to investigate alternative domestic energy sources. The use of biomass such as corn, soybeans and corn stover for bio-based products and fuels is one alternative to satisfy growing energy needs. It is important to identify potential environmental impacts related to this technology. The purpose of this study was to answer the question: Does harvesting stover for biofuel contribute to nutrient related environmental degradation? Eutrophication, a result of non-point source nutrient pollution from agricultural practices, has deleterious effects on the health of natural ecosystems as well as humans.

The area of study encompasses three watersheds in Eastern Iowa spanning 33 counties. Nutrient flows through soil, water and the air were modeled using a cradle-to-farm gate life cycle impact assessment. The key objective of this study was to perform sensitivity analysis of parameters used to calculate the inventory of nitrogen flows in the system.

The work presented is an extension of the contributions relating to eutrophication potential made by Susan Powers, PhD. of Clarkson University to the life cycle assessment for stover harvest and conversion to ethanol for transportation fuels conducted at the National Renewable Energy Laboratory. Nitrate models for three scenarios were analyzed: base case (current practice of corn-soy rotation with no stover removal), corn-soy rotation with stover removal, and continuous corn with stover removal. Aided by the analytical tool, Crystal Ball ®, Monte Carlo simulations were run to quantify parameters with uncertainty. Further understanding of the sensitivity of the model was attained by calculating standard deviations for the parameters analyzed in the base case scenario.

The graph below represents three components of the model that were analyzed: mineralization, fertilizer, and leaching. The two alternative scenarios as shown as percentages of the base case scenario (base case equals 100%). Mineralization of ammonia nitrogen present in soil organic matter and remaining crop residue introduces leachable nitrate into the system. Switching to stover removal would reduce mineralization in both alternative scenarios. Stover removal requires increased fertilizer application resulting in increased leaching (rates are rainfall dependent).

By applying statistically determined sensitivity to the parameters in each modeled scenario, it is clear that continuous corn production with stover removal contributes most significantly to eutrophication. Based on the model, there is a much less significant increase in nutrient leached from agricultural lands to surface water bodies for a corn-soy crop rotation system with stover removal as compared to the base case, current practice of corn-soy rotation with no stover removal.