In their eagerness to cut nitrogen and phosphorus pollution in the Mississippi River and Gulf of Mexico, people have often sought simple explanations for the problem: too many large animal operations, for instance, or farmers who apply too much fertilizer, which then flows into waterways.
But according to new modeling research that examined phosphorus loading from all 1768 counties in the Mississippi River Basin (MRB), the true causes aren't nearly so straightforward. Livestock manure is widespread in many MRB counties, for example, but it shows little relationship to water quality, say researchers at the University of Illinois at Urbana-Champaign and Cornell University in the May-June 2011 issue of the Journal of Environmental Quality.
Moreover, areas that load the most phosphorus into the Mississippi are also places where farmers add less phosphorus to the soil than they remove each year in crop harvests, suggesting that overzealous fertilizer use is not the issue.
"If it were that, it would be easy to solve. But it's not that," says Mark David, a University of Illinois biogeochemist who led the research. "It's much more complex. So I think in that sense addressing the problem is harder."
Soil erosion and tile drainage contribute large amounts of phosphorus to the Mississippi and Gulf of Mexico each year, helping fuel a "dead zone" of oxygen-starved water in the Gulf that reached near-record size last summer. Local water quality may also decline due to phosphorus-driven algal blooms.
In an effort to pinpoint the most important sources of phosphorus across the entire MRB, David's team calculated the yearly phosphorus inputs and outputs for every county in the basin from 1997 to 2006. After aggregating these and other data within 113 watersheds throughout the MRB, they then estimated the river load of phosphorus from every county between January and June for the same time period.
Not surprisingly, counties with intensive row crop agriculture, such as those in the Upper Midwest Corn Belt states of Iowa, Illinois and Ohio, contributed the most phosphorus to rivers. However, these same counties often showed negative phosphorus balances, meaning that phosphorus outputs in crops exceeded inputs by farmers.
In other words, farmers in these regions are actually mining stored phosphorus from the soil, rather than putting more into the system, David says. "But that negative balance doesn't have much to do with the phosphorus that gets in the river." Instead, the overall intensity of agriculture seems to matter most. "When I'm sitting here in Illinois in a watershed that's 95% corn and soybeans, it's going to lose some phosphorus," he says, "whether the balance is negative or positive."
In addition, although animal manure is considered a major phosphorus source to streams and rivers, it was relatively unimportant to phosphorus loading across the entire MRB. David suspects the reason is that most large-scale animal farms have moved to western states in the basin, such as Colorado, where there's less precipitation to carry manure nutrients into the Mississippi.
Phosphorus from human waste did prove significant. Counties encompassing Chicago and other major metropolitan areas "showed up as hot spots," David says, because most municipalities don't remove phosphorus from the otherwise clean sewage effluent they discharge into streams. The team further found that about half of the variation in phosphorus loadings was not explained by their models, suggesting that other factors also contribute, such as stream bank erosion and phosphorus deposits in river sediments.
Overall, the findings suggest that reducing phosphorus pollution will require broad adoption of practices that limit nutrient runoff, such as cover crops, buffer strips, and incorporation of fertilizers. It will also require limits on phosphorus discharge from cities.
Achieving these objectives across the entire MRB won't be easy, but David hopes the study helps people move beyond common assumptions about causes to focus on the real issues.
"To me the value of the study is that it helps shift the debate," David says. "The problem is not as simple as two things. It's not as simple as too much fertilizer or manure."
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More information: www.agronomy.org/publications/jeq/abstracts/40/3/931