Row crops, field tiles causing water quality problems

In addition to causing widespread flooding, the rains drenching the Midwest this spring may exacerbate another environmental problem - phosphorus and nitrate pollution in the water supply that is causing a growing hypoxic zone in the Gulf of Mexico, presenting a danger to marine life and wildlife habitats, according to recent studies by a team of scientists from the University of Illinois and Cornell University.

The hypoxic zone, which forms every spring or summer in the Gulf, covered 7,000 square miles last summer. With high flow in the Mississippi this spring, the zone may be large again this summer.

The researchers found that tiled farm fields in the Corn Belt - the area spanning Illinois, Indiana, Iowa, Ohio and southwest Minnesota - and in the southeastern Missouri and northeastern Arkansas regions are the largest contributors of nitrate and phosphorus pollution to rivers, lakes and streams that empty into the Mississippi River and, ultimately, the Gulf of Mexico.

Ingesting phosphorus in water is not harmful to humans, as it is an essential nutrient, said Mark David, a biogeochemist in the department of natural resources and environmental sciences, which is a unit in the College of Agricultural, Consumer and Environmental Sciences at the U. of I. David also is a member of the research team and has studied the problem of nutrient losses from agricultural fields to rivers since 1993.
The danger is that, in combination with nitrate, phosphorus can impair water quality and lead to the formation of algae that depletes water of oxygen after it dies, harming both freshwater and marine life and destroying animal habitats, as is happening in the Gulf region, David said.

The research team analyzed 10 years of data on the 1,768 counties in the Mississippi River Basin, including information on crop acres and yields, livestock head counts and census data, fertilizer usage and tile drainage systems, nitrate and phosphorus concentrations in waterways and river flow measurements.

They found that the extensive row cropping in the Midwest, where land is flat and tile drainage systems and channelized ditches and streams are common, in combination with precipitation and soil erosion creates pathways that transport nitrate and phosphorus from the soil into the water supply. The largest losses of nitrate, which occur every January-June, are related to intensive, fertilized agriculture with tile drainage. Phosphorus loads in rivers, however, are determined by several factors - precipitation, the percentage of cropland and human consumption of phosphorus, which ends up in sewage effluent and is released into rivers after processing at sewage plants.

In some of the most concentrated agricultural areas, annual harvests of soybean and corn actually remove more of the nutrients from the soil than is being applied in the form of fertilizer, David said.

"This illustrates the complexity of the problem, as we have created an agricultural system with high yields, but one that is leaky with nutrients," David said. "Although we lose very little phosphorus in terms of how much we apply in fertilizer, it takes only a small amount to cause a water quality problem.

"In the 1970s, farmers were told to put a little extra phosphorus on and build up their soil - it was money in the bank. Phosphorus stays very well in the soil ... so basically we're using that old buildup .... However, the loss of phosphorus from soil to rivers has not changed and is still causing environmental problems."

Although animal manure is another potential source of nutrient pollution, the researchers didn't find it to be a big contributor in the Mississippi River Basin, since animal production has moved further west, where there's less precipitation and tile drainage - and less phosphorus and nitrogen entering the water supply than in Illinois, Indiana, Iowa and Ohio, which have extensive crop production.

"A watershed that's 95 percent row cropped is going to have a higher phosphorus load than a forested watershed or a watershed that has only a small percentage of agricultural drainage and typically less row crops, whether it has livestock or not," David said.

Mitigating the nutrient losses will require a broad range of measures - some of them costly, such as implementing technology to remove nitrogen and phosphorus from sewage effluent before it's released, David said.

Where nitrogen and phosphorus are entering waterways from agricultural sources, efforts would need to focus on reducing the amounts lost through tile systems and surface runoff or by removing the nutrients at the edges of the fields.

Incentives such as cost-sharing programs might encourage landowners to address nutrient removal by modifying tile systems with bioreactors such as trenches of wood chips, through drainage management that would reduce tile flow during winter and early spring before planting, or by establishing wetlands at tile outlets. Other possibilities: planting cover crops to retain nutrients in the field over the winter, growing biofuel crops such as Miscanthus or switchgrass that don't require the nitrogen and phosphorus fertilizers that corn and soybeans need, or improved timing and incorporation of fertilizers.

In a March 16 announcement, Nancy K. Stoner, acting assistant director of the U.S. Environmental Protection Agency, called upon officials in state agencies to partner with landowners and other stakeholders to reduce concentrations of nitrogen and phosphorus in the water supply and improve water quality. Nitrogen and phosphorus pollution has the potential to become one of the costliest and most challenging environmental problems facing the U.S., Stoner wrote.

"The EPA is pushing all the states, especially Illinois right now, to have a water quality standard for both nitrogen and phosphorus to reduce both local and downstream concentrations and amounts heading to the Gulf of Mexico," David said. "And if they do that, at almost any levels that would be chosen, we won't meet it in most waters."

The team's research was funded by the National Science Foundation Biocomplexity in the Environment/Coupled Natural-Human Cycles Program. Reports about their findings appear in the May-June 2011 and the September-October 2010 issues of the Journal of Environmental Quality.

Co-investigators on the studies were Laurie E. Drinkwater, Cornell University; and Linda M. Jacobson and Greg McIsaac, both at Illinois.