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Study probes sources of Mississippi phosphorus


May 10, 2011
By American Society of Agronomy

mississippiriverMay 9, 2011, Madison, WI –
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.

May 9, 2011, Madison, WI –
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.

mississippiriver  
   

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 percent 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.”

View the abstract at
https://www.agronomy.org/publications/jeq/abstracts/40/3/931.


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