Corn-based ethanol production compromises goal of reducing nitrogen export by the Mississippi River

Abstract

Corn cultivation in the United States is expected to increase to meet demand for ethanol. Nitrogen leaching from fertilized corn fields to the Mississippi-Atchafalaya River system is a primary cause of the bottom-water hypoxia that develops on the continental shelf of the northern Gulf of Mexico each summer. In this study, we combine agricultural land use scenarios with physically based models of terrestrial and aquatic nitrogen to examine the effect of present and future expansion of corn-based ethanol production on nitrogen export by the Mississippi and Atchafalaya Rivers to the Gulf of Mexico. The results show that the increase in corn cultivation required to meet the goal of 15-36 billion gallons of renewable fuels by the year 2022 suggested by a recent U.S. Senate energy policy would increase the annual average flux of dissolved inorganic nitrogen (DIN) export by the Mississippi and Atchafalaya Rivers by 10-34%. Generating 15 billion gallons of corn-based ethanol by the year 2022 will increase the odds that annual DIN export exceeds the target set for reducing hypoxia in the Gulf of Mexico to >95%. Examination of extreme mitigation options shows that expanding corn-based ethanol production would make the already difficult challenges of reducing nitrogen export to the Gulf of Mexico and the extent of hypoxia practically impossible without large shifts in food production and agricultural management.

Fig. 1.

Area of the Mississippi–Atchafalaya Basin planted in corn and soybeans in each land use scenario.

Fig. 2.

Fractional area of each 5′ × 5′ grid cell planted in corn in the 2004–2006 control (A), the 2007 projections (B), and the 36 billion gallon goal scenario (C). The area is expressed as a fraction of the total grid cell area.

Fig. 3.

Observed and simulated (control scenario) monthly DIN export to the Gulf of Mexico from 1981 to 2000. The export to the Gulf of Mexico is the sum of the Mississippi at St. Francisville, LA, and the Atchafalaya at Melville, LA.

Fig. 4.

Simulated annual DIN export by the Mississippi and Atchafalaya Rivers to the Gulf of Mexico in the 2004–2006 control and the five ethanol production scenarios. The blue box represents the 5–95% confidence interval range of annual DIN export based on 1981–2000 climate variability (the horizontal line represents the annual mean). In red is the estimated level of DIN export required to achieve the federal goal of reducing the hypoxic zone to <5,000 km²; the upper bound is the federally recommended 30% reduction in mean DIN flux (7), and the lower bound is the 55% reduction in mean DIN flux thought necessary to account for variability in climate and ocean dynamics (8, 9).

Fig. 5.

Simulated dissolved inorganic nitrogen leaching (kg·ha⁻¹yr⁻¹) from land across the Mississippi–Atchafalaya River Basin in the 2004–2006 control (A), the 2007 projections (B), and the 36 billion gallon goal scenario (C).