In a recent analysis, scientitsts Simon Donner and Christopher Kucharik demonstrates the quantitative link between biofuels and dead zones (Corn-based ethanol production compromises goal of reducing nitrogen export by the Mississippi River, Proceedings of the National Academy of Sciences, March 10, 2008). Their analysis highlights the value of ecosystem modeling as a tool for analyzing the conflicts, consequences and trade-offs associate with competing environmental policies.
The Conflict Between Growing Corn for Ethanol and Reducing Nitrogen Pollution and Dead Zones
Recent U.S. Energy Policy calls for large increases in the domestic production of renewable energy, including biofuels, setting targets for the production of corn-ethanol that range from 15 to possibly 36 billion gallons per year by 2022. Much of the increased corn production required to achieve these goals would be in the Mississippi River watershed, increasing nitrogen runoff to the Mississippi river and eventually the Gulf of Mexico.
However, nitrogen export by the Mississippi River is already a major driver of the northern Gulf of Mexico dead zone, which the U.S. government has a goal of shrinking by reducing river nitrogen export by 45%. Thus, a policy that increases corn-ethanol production is in conflict with a policy to reduce river nitrogen export.
Using an Ecosystem Model to Link Corn Production to Nitrogen Export by the Mississippi River
To analyse this conflict, Donner and Kucharik used a sophisticated ecosystem model capable of tracking virtual nitrogen from its application as fertilizer on the corn fields of the American Midwest, to its entry into the waterways of the Mississippi River watershed as agricultural runoff, and then down through the watershed to the Gulf of Mexico.
They modeled several ethanol production scenarios, each representing a different pattern of land-use across the Midwest from the year 2007 to 2022, including: a ‘2007’ scenario representing increased corn production observed in the year 2007; two ‘15 billion gallon’ scenarios representing different ways of achieving that corn-ethanol target; an ‘Extreme’ scenario under which the full 36 billion gallon target is met with corn-ethanol; and a ‘Mitigation’ scenario.
The ‘Mitigation’ scenario assumed a radical shift in diet away from grain-fed meats, freeing up farmland to grow corn for ethanol instead of cattle, and the large-scale construction of nutrient-absorbing wetlands adjacent to all corn fields to minimise nitrogen runoff. It represented the largest reduction in nitrogen export possible without changing on-farm crop and nutrient management practices.
Quantifying the Trade-off Between Growing Biofuels and Shrinking the Gulf of Mexico Dead Zone
The model results showed that under all ethanol-scenarios except ‘Mitigation’, nitrogen export levels increased relative to a ‘baseline’ scenario representing pre-2007 conditions that the 45% reduction target is based upon. Looking only at the mean nitrogen export values for their results, the ‘2007’ scenario was 7% above baseline, the ’15 billion’ scenarios were 10-18% above ‘baseline’ and the ’36 billion’ was 34% above 'baseline'.
The 'Mitigation' scenario performed the best with mean nitrogen export levels 34% lower than the 'baseline'. However, they still did not meet the 45% reduction target. These results make the trade-off very clear: corn-ethanol production may increase energy security and reduce greenhouse gas emissions, but it will very likely be at the cost of a more polluted coastal ecosystem.
Ecosystem Analysis Identifies both Environmenal Problems and Potential Management Solutions
These results are sobering, but Donner and Kucharik’s analysis shows the value of ecosystem modeling for analysing and refining environmental policy. Not only did they confirm the conflict between increasing corn-ethanol production and reducing nitrogen pollution, they quantified the consequences, making the trade-off between the two policies clear.
Clear trade-offs provide clear direction for future policy options. Recall that only the ‘Mitigation’ scenario came close to meeting both the ethanol and nitrogen reduction goals to be met. While the assumptions of this scenario were likely unrealistic, the results highlight the need for intensive application of on-farm nutrient management and cultivation practices. This is a more realistic mitigation approach, consistent with the government nitrogen reduction efforts, that could be explored in future model analyses.
 |
