Seeds of Gulf Dead Zones Are Draining from U.S. Farms

A study recently published in the Journal of Environmental Quality by a team from Cornell University and the University of Illinois-Urbana found that tile drainage systems in upper Mississippi farmlands – from southwest Minnesota to Iowa, Illinois, Indiana and Ohio – are the biggest contributors of nitrogen runoff into the Gulf.

That runoff has been identified as a major contributor to “seasonal hypoxia” or dead zones in which nitrogen fertilized algae blooms, depletes oxygen and suffocates other life forms over thousands of square miles each summer.

“Given the pivotal role of tile drainage in transporting fertilizer nitrogen from agricultural fields to streams and rivers, we need to consider some form of regulation if we expect to reverse hypoxia in the Gulf of Mexico,” said Laurie Drinkwater, associate professor of horticulture and co-author of the paper.

To estimate nitrogen inputs and outputs, the researchers constructed a database that spanned from 1997 to 2006 and included data on crops, livestock, fertilizer, human populations and other information for 1,768 counties. The database also included nitrate concentrations and their flow into streams and rivers from 153. Computer modeling revealed that the dominant source of nitrogen loss into the Mississippi came from fertilized cornfields on tile-drained watersheds in the upper Mississippi River basin.

Drinkwater said solutions include installing wetlands in areas where tiles drain to filter the water and fertilizing fields in the spring instead of the fall. Also, she added, “we know that we are losing nitrogen in the period between cash crops when nothing is growing in the field. If we plant winter cover crops and diversify crop rotations, nitrogen losses could be reduced quite a lot.”

The Mississippi River basin covers 40 percent of the continental United States and is the largest producer of corn and soybeans in the world.

Mark David, a biogeochemist at the University of Illinois, is the paper's lead author. It was also co-authored by Gregory McIsaac at the University of Illinois, and was funded by the National Science Foundation.