It’s easy to offer simplistic explanations for nitrogen and phosphorus pollution in the Mississippi River and Gulf of Mexico.
Two common theories as to why an excess of these nutrients gets into waterways suggest:
- Too many large animal operations (and the manure generated by them).
- Or, farmers apply too much fertilizer.
Several previous studies dating back to 1991 seem to support these hypotheses. They show increased phosphorus and nitrogen loads in the Mississippi River following the expanded use of fertilizer by U.S. farmers that began in the 1930s.
But Science Daily reports that 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 excessive 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."
After calculating the estimated river load of phosphorus from basin watersheds, the team reached some interesting conclusions as to the most important sources of this nutrient.
For example, although animal manure is considered a major phosphorus source to streams and rivers, it was relatively unimportant to phosphorus loading across the entire basin. Researchers suspect 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.
Counties with intensive row-crop agriculture, like those in 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.
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 prudent manure and fertilizer applications — including coordination with weather events — cover crops, buffer strips, and incorporation of fertilizers. It will also require limits on phosphorus discharge from cities.
"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."