Plowing, Drainage and Nitrogen Fertilizer and Illinois Rivers
Published: Oct. 17, 2001
October 19, 2001
Urbana - Speculation about the source of nitrogen in Illinois rivers prompted University of Illinois scientists to probe into the history of agriculture to come up with answers. The results suggest that plowing and draining prairie soils in the 1800s led to significant surpluses of nitrogen in agricultural fields, some of which entered surface waters during that time period, but since the early 1960s nitrogen fertilizer application has been the primary cause of excess nitrogen in streams and rivers.
"We found a wealth of information in agricultural statistics that go back to 1867 as well as the agricultural censuses from that time," said Mark David, U of I professor of biogeochemistry.
"We were able to piece together when the prairies were first plowed, when field drainage systems were first constructed and when farmers began widespread nitrogen fertilizer use."
The scientists matched this information with data from research fields that helped create a picture of nitrogen loss from soils and its subsequent movement into streams and rivers.
"We think during pre-settlement nitrogen levels in rivers were very low. Then settlers plowed prairie soils that were known to be rich in organic nitrogen, which is relatively immobile. Plowing the soil enhanced the conversion of soil organic nitrogen to nitrate, which is highly mobile. By the turn of the century, most of the land in Illinois was plowed. Because so much land was plowed during a relatively short time frame, we hypothesize that the nitrate load in the rivers was large," David said.
From other soils research, it is known that, when soils are continuously cultivated, it takes about 60 years to deplete soil nitrogen by about 40 percent. After that 60-year period the nitrogen loss appears to be minimal. This knowledge helps pinpoint 1880 as the peak period of nitrogen in Illinois rivers because of when plowing of the prairies occurred. But then Illinois farmers began to drain fields with tile systems. This, too, helps nitrogen move from field to streams. However, by the early 1900s nitrogen loss leveled off and declined into the 1940 and '50s as soil organic N approached a new, lower, steady state value.
Another team researcher, Gregory McIsaac, U of I assistant professor of environmental sciences, speculates that this is a benchmark period with the lowest levels of nitrates leaving fields and entering streams and rivers since intensive plowing of the prairies began. Monitoring, since the 1950s, has shown an increase in nitrates in many Illinois rivers.
"Farmers apply nitrogen fertilizers to fields to achieve high corn yields. A lot of this nitrogen is harvested in the grain, but there's a significant amount that remains in the field as unharvested crop residues and as unutilized fertilizer. For the state as a whole, 50 percent of the surplus nitrogen in Illinois soils appears to end up in rivers," McIsaac said.
That's high compared to much of the rest of the United States, where an average of 20 percent is more common. Illinois has a high susceptibility to nitrate finding its way to in surface water because so much of the farmland in Illinois has subsurface drainage systems. Drainage tiles are a major pathway for nitrates to move from fields to rivers.
In total, Illinois represents 6 percent of the drainage area of the Mississippi River, but David estimates that the state is responsible for about 15% of all the nitrates in the River.
"The interesting thing about soils is that they release nitrate, but they also immobilize it. Historically, plowing soils and draining fields caused significant movement of nitrate from fields to rivers. Today, however, the largest single factor affecting nitrate loads of Illinois rivers are nitrogen fertilizer applications," David said.
David presented this research on October 16, 2001 at the "N2001 Conference" sponsored by the Ecological Society of America, Fertilizer Institute, US EPA, and a number of other interested organizations.
The research was funded, in part, by an Illinois Council for Food and Agricultural Research grant. The project team also included Todd Royer, Postdoctoral Associate in aquatic ecology, Robert Darmody, U of I professor of soils, and Lowell Gentry, former U of I scientist. ? 30 ?