Study: Adding Wetlands in the Corn Belt Can Shrink Gulf of Mexico Dead Zone

 (February 2, 2015) Fertilizer efficiency measures and planting of cover crops can reduce nutrient pollution delivered to the Gulf of Mexico by 30 percent, finds a new Environmental Defense Fund (EDF) led study released today in the Journal of the American Water Resources Association (JAWRA). Yet when combined with filter practices such as wetlands, the study found that nitrogen pollution in the Upper Mississippi and Ohio River basins can be reduced by 45 percent – the Environmental Protection Agency (EPA) established target that will shrink the Gulf’s dead zone to a manageable size.

The JAWRA study, “Reducing Nitrogen Export from the Corn Belt to the Gulf of Mexico: Agricultural Strategies for Remediating Hypoxia,” is the first to quantify the impact of agricultural strategies for reducing hypoxia at such a large scale. Researchers used an advanced water-quality model developed by the U.S. Geological Survey (USGS) to quantify the benefits of agricultural practices on croplands.

 “The good news is that adopting soil health and fertilizer efficiency measures across the Corn Belt can get us two-thirds of the way to the tipping point,” said the study’s lead author and EDF senior scientist Eileen McLellan. “But by strategically placing wetlands on less than 1 percent of the region’s croplands, we’ll be able to reverse the trend of significant losses in aquatic life, and improve flood resiliency for downstream communities with minimal impact to crop production.”

Over 400 hypoxic or “dead” zones, ocean areas where low oxygen concentration causes marine life to suffocate and die, have been identified in estuaries and coastal waters worldwide. Studies have shown the number and extent of these zones is increasing, and experts agree that the majority of dead zones are caused by nutrient pollution.

Among the findings:

• Optimizing fertilizer use in the Upper Mississippi and Ohio River Basins can reduce nitrogen pollution by 12 percent, less than one-third of EPA’s goal.
• Combining cover crops with nitrogen efficiency practices on all cropland acres can reduce nitrogen loss by 30 percent, representing two-thirds of EPA’s target.
• Restoring and adding wetlands to the region’s croplands can further reduce the nitrogen loss to 45 percent, thereby reaching the tipping point.

“Over time, we’ve changed the plumbing of the Corn Belt landscape in such a way that some nitrogen loss is inevitable,” added McLellan. “But the chemical process that takes place in wetlands traps and treats nitrogen lost from farm fields, and turns nitrogen fertilizer into harmless nitrogen gas. Wetlands are a key part of the conservation equation.”

In addition to wetlands, other nitrogen removal practices include drainage ditch enhancements, stream channel restoration, and floodplain reconnection. These filter practices can be implemented on marginal land with low yields, thereby minimizing impacts to crop production. Furthermore, the authors highlight the economic benefits of fertilizer efficiency measures and planting cover crops, both of which are likely to offset the loss of any cropland. Other filter practices can be implemented within the footprint of existing ditches and streams, without any loss of productive land.

 “Results from this study help expand our understanding of the importance of considering a variety of conservation practices applied throughout the entire watershed,” said Dale Robertson, research hydrologist with the USGS and co-author of the paper. “Improving water quality is a community-wide effort that should save money, clean up local streams, and benefit the Gulf.”