Corn and soybean farmers in the upper Mississippi River are growing more than they intend: a thriving crop of nitrogen-fertilized algae blooms in the Gulf of Mexico.
A study recently published in the Journal of Environmental Quality by researchers from Cornell U. and U. Illinois at Urbana-Champaign found that tile drainage systems — which help to drain excess water from the soil subsurface — in upper Mississippi farmlands are the biggest contributors of nitrogen runoff into the Gulf of Mexico.
Runoff has been identified as a major contributor to “seasonal hypoxia,” also known as dead zones. Each summer, nitrogen-fertilized algae blooms deplete oxygen and suffocate other life forms over thousands of square miles, an area in the Gulf of Mexico that often rivals the size of New Jersey.
Currently, there are between 400 and 500 identified dead zones throughout the world, most of which are associated with waters near a shoreline, where there is usually a major river draining the continent.
“The near-shore waters in the ocean are among the most productive,” said Prof. Laurie Drinkwater, horticulture, a co-author of the paper. “We rely on those areas for a lot of important fisheries, particularly shellfish . . . But this runoff also has many consequences for biodiversity and the ecosystem function as well.”
To estimate nitrogen inputs and outputs, the researchers constructed a database that spanned from 1997 to 2006 and included data on crop yields, livestock, fertilizer, human populations and other information from 1,768 counties in the area.
The database also included nitrate concentrations and their flow into streams and rivers from 153 of these counties. 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.
The Mississippi River basin produces more corn and soybeans than anywhere else in the world and covers 40 percent of the continental United States.
“Over time, with a lot of research and looking at the patterns in the loading of nitrate in the streams and rivers that drain to the basin, we’ve been able to pinpoint the locations that contribute most of the nitrogen and also identify the sources, which are primarily agriculture,” Drinkwater said.
The tile drainage system is an important engineering aspect of the basin, according to Drinkwater, because the soils in the area are naturally very poorly drained. However the system causes water to move more quickly out of the field and into streams and rivers — a major problem when associated with the heavy use of nitrogen containing fertilizer.
Drinkwater said solutions to the problem include modifying practices in areas that rely on tile drainage, as well as “promoting practices such as cover cropping and diversifying crop rotations, as well as other strategies that occur outside the field such as buffers along waterways to try to catch the nitrogen before it gets into the rivers and streams.”
Prof. Mark David, natural resources and environmental sciences, University of Illinois, is the paper’s lead author. Prof. Gregory McIsaac, natural resources and environmental sciences, University of Illinois, was also a co-author. The paper was funded by the National Science Foundation.