Landowners in two Western Iowa watersheds are applying nitrogen fertilizer — in the forms of commercial fertilizer and livestock manure — at more than double the recommended rate, causing higher nitrate levels in streams there, according to new research by the University of Iowa.
The paper, published Dec. 19 in “Ambio: A Journal of the Human Environment,” attempts to quantify on a large scale how animal feeding operations are affecting water quality.
“It’s pretty clear in these watersheds in Western Iowa, the ones with the highest concentration of livestock are the ones with the highest stream nitrate,” said Chris Jones, a UI research engineer and lead author of the paper.
The Floyd and Rock watersheds, thousands of square miles in northwest Iowa ultimately draining into the Missouri River, have the highest density of animal feeding operations in the state.
Sioux County, in the Floyd and Rock watersheds, had the largest number of medium and large feeding operations in the state, with 472 last spring, Iowa Department of Natural Resources data show. Lyon County, the next highest with 313 animal feeding operations, also is in the Floyd watershed.
To determine how much nitrogen is being applied to fields in these watersheds, Jones and his colleagues, including two DNR researchers, combined county-level commercial fertilizer sales data with the amount of nitrogen in manure generated by animals at feeding operations in those watersheds.
The total nitrogen input in the Floyd watershed is 337 pounds per corn acre, more than double Iowa State University Extension’s recommended application rates of an average 134 pounds per acre for corn, according to the paper. The Rock watershed nitrogen input is 328 pounds per acre, the paper states.
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So although farmers in these watersheds have vast resources for manure application, sales of commercial fertilizer remain high.
These numbers are important because, while some nitrogen applied to fields is taken up by crops and some evaporates, excess nitrogen washes into nearby streams and rivers, where it can pollute the water supply and hurt the environment.
Nitrate in drinking water has been linked to health problems, including colorectal cancer, thyroid disease and infant methemoglobinemia — often called blue baby syndrome — a life-threatening condition reducing the blood’s ability to carry oxygen.
The UI used its network of water sensors on Iowa streams and rivers to measure nitrate levels on the Floyd and Rock rivers.
Nitrate levels on the rivers in 2017 consistently were over the 10 milligram-per-liter standard for drinking water, with the Floyd River averaging 16 milligrams per liter and the Rock at 11.5 milligrams per liter, the paper reported.
No cities are drawing their water supply directly from the Floyd River, Jones said, but rising nitrate levels in some municipal wells in northwest Iowa have caused concern.
While these effects are more pronounced in northwest Iowa, researchers have seen similar correlations with livestock concentration in other parts of the state, Jones said.
Dan Andersen, an ISU associate professor of manure management and water quality, said the research overestimates nitrogen from manure by using the amount of manure an animal can excrete versus the amount of nitrogen left after storage and application. But he acknowledges the challenges of avoiding nutrient runoff from manure.
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“As with any fertilizer there are challenges with manure. Things like timing, consistency of the manure, and uniformity of our application can all be challenging when using manure as a fertilizer,” Andersen wrote in an email. “But we continue to see equipment development that leads to improvement in these areas.”
Greg McIsaac, a University of Illinois emeritus professor in environmental sciences, said the new UI paper refines previous research. “I think their earlier paper on nitrate trends only suggested that livestock might be an important source, but this paper provides a detailed quantitative assessment for the connection,” he said.
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