Corn-based ethanol production compromises goalof reducing nitrogen export by the Mississippi RiverSimon D. Donner*†and Christopher J. Kucharik‡*Department of Geography, University of British Columbia, 1984 West Mall, Vancouver, BC, Canada V6T 1Z2; and‡Center for Sustainability and the GlobalEnvironment, Nelson Institute for Environmental Studies, University of Wisconsin, 1710 University Avenue, Madison, WI 53726Edited by Robert Howarth, Cornell University, Ithaca, NY, and accepted by the Editorial Board January 21, 2008 (received for review September 1, 2007)Corn cultivation in the United States is expected to increase to meetdemand for ethanol. Nitrogen leaching from fertilized corn fieldsto the Mississippi–Atchafalaya River system is a primary cause ofthe bottom-water hypoxia that develops on the continental shelfof the northern Gulf of Mexico each summer. In this study, wecombine agricultural land use scenarios with physically basedmodels of terrestrial and aquatic nitrogen to examine the effect ofpresent and future expansion of corn-based ethanol production onnitrogen export by the Mississippi and Atchafalaya Rivers to theGulf of Mexico. The results show that the increase in corn cultiva-tion required to meet the goal of 15–36 billion gallons of renewablefuels by the year 2022 suggested by a recent U.S. Senate energypolicy would increase the annual average flux of dissolved inor-ganic nitrogen (DIN) export by the Mississippi and AtchafalayaRivers by 10–34%. Generating 15 billion gallons of corn-basedethanol by the year 2022 will increase the odds that annual DINexport exceeds the target set for reducing hypoxia in the Gulf ofMexico to >95%. Examination of extreme mitigation optionsshows that expanding corn-based ethanol production would makethe already difficult challenges of reducing nitrogen export to theGulf of Mexico and the extent of hypoxia practically impossiblewithout large shifts in food production and agricultural management.Gulf of Mexico 兩 hypoxia 兩 nitrogen cycling 兩 biofuels 兩 agricultureLast year, U.S. farmers planted ⬎90 million acres of corn for thefirst time in 60 years because of rising corn prices and thedemand for ethanol (1). Corn-based ethanol production has risenfaster than recent U.S. Department of Agriculture (USDA) pro-jections and has already surpassed the year 2012 target of 7.5 billiongallons per year set in the 2005 Energy Policy Act (2). The mostrecent U.S. Energy Bill set a target of 36 billion gallons of renewablefuels by the year 2022, of which 15 billion gallons can be producedfrom corn starch (3). U.S. corn cultivation may continue to increasein the coming years to meet such fuel production targets (2).Fertilizer applied to corn in the U.S. Midwest is a primary sourceof nitrogen exported to the Gulf of Mexico by the Mississippi andAtchafalaya Rivers (4). The flux of nitrogen, largely in the form ofthe nitrate, and fresh water promote the development of extensiveseasonal hypoxia on the continental shelf each summer (5, 6). Inrecent years, this ‘‘Dead Zone’’ has reached ⬎20, 000 km2in sizeand has contributed to benthic mortality and the risk of fisheriesdecline (6). The Mississippi Basin/Gulf of Mexico Task Force set agoal of reducing nitrogen export by the Mississippi and AtchafalayaRivers by 30% in hope s of reducing the annual spatial extent ofhypoxia to ⬍5,000 km2(7). Recent research suggests that nitrogenexport may need to be reduced by up to 55% to achieve thehypoxia-reduction goal because of annual climate-driven variabilityin nitrogen flux and annual variability in ocean dynamics (8, 9).In this study, we use an agricultural version of the IntegratedBiosphere Simulator (IBIS), a process-based dynamic ecosystemmodel, and the Terrestrial Hydrology Model with Biogeochemistry(THMB) to investigate how increasing corn cultivation to meetethanol production goals will affect nitrogen export to the Gulf ofMexico. The models have been thoroughly tested and appliedtogether to simulate the sensitivity of terrestrial nitrogen, carbon,and water cycling and downstream transport of nitrogen and wateracross the Mississippi–Atchafalaya River Basin to agricultural landuse practice s and climate variability (4, 10–16). First, we usedUSDA data to generate a series of spatially explicit land usescenarios including a control case (based on 2004–2006 mean landuse and land cover); a representation of 2007 land managementbased on the projected plantings from the spring (1); three scenariosdesigned to meet the ethanol production goals in the recent EnergyBill (3); and an extreme mitigation scenario (Table 1). Second, weused the control scenario to validate the ability of the models tosimulate nitrogen cycling across the Mississippi–Atchafalaya RiverBasin and nitrogen export to the Gulf of Mexico. Third, weevaluated the effect of the alternative land cover scenarios onnitrogen export to the Gulf of Mexico and the goal of reducing theextent of the seasonal hypoxic zone.ResultsThe study focuses on the 3.2 million-km2Mississippi–AtchafalayaRiver Basin, where ⬎80% of the total U.S. corn and soybeanacreage is cultivated. The smaller Atchafalaya Basin is included inthis analysis because the Old Water Control Structure north of NewOrleans redistributes the combined flow of the Mississippi Riverand the Atchafalaya River. Because the ethanol production goalsare national, the land use scenarios were developed for the entireU.S. The land-cover maps displayed are restricted to the Missis-sippi–Atchafalaya Basin.The land use scenarios were generated by simulating decisions byfarmers in each county to plant corn on other croplands depictedin the control case or on conservation reserve lands, where yieldsare expected to be lower (Table 1). The models were driven withthe different scenarios to simulate the effect of altered cultivationon terrestrial nitrogen leaching and downstream nitrogen export.Crop management is assumed to be the same in each scenario; forexample, the state-level fertilizer application rates for each of thesimulated crops (corn, soybean, and three varieties wheat) are thesame across all scenarios.Nitrogen export by the Mississippi and Atchafalaya Rivers hasbeen shown to vary not just with land use and land cover but withannual variability in rainfall and river discharge (8). To account forthe role of climate variability, the models were forced with the same1981–2000 monthly mean 0.5°⫻ 0.5° transient