July 29, 2020
A recent paper by a group of University of Minnesota scientists provides an excellent summary of factors contributing to variability in nitrogen loads from the Upper Midwest to the Mississippi River and ultimately the Gulf of Mexico.
Focusing on three rivers in Iowa, including the Raccoon River, which was in the news often during the Des Moines waterworks conversation, the paper explains very clearly the effect precipitation has on river flows and corresponding nitrogen movement relative to other factors.
Titled, “Precipitation Drives Nitrogen Load Variability in Three Iowa Rivers,” the paper serves as a reminder that while good nitrogen management practices are important, year-to-year variability in rainfall influences nitrogen transport so significantly that the effects of improved nitrogen practices may not be seen via river monitoring.
Building on a large body of previous research — the paper includes a list of more than 50 published papers as references — provides insights that ought to help direct future efforts to reduce nitrogen losses while tempering expectations.
Main takeaways for me included a reminder and caution regarding good nitrogen management, which I often write about for The Farmer. The reminder is, “N application substantially over and above the agronomic N rates will increase flow weighted nitrogen concentration as well as N loads from tile-drained plots.”
We’ve known this for quite a while, but also know the challenge in getting to a “right rate” that is so heavily influenced by specific field conditions.
The caution is that “there were larger differences in N losses from tile drained plots between years [precipitation effect] than between the N rate, timing of N applications or even the use of nitrification inhibitors in a corn-soybean cropping system.”
In other words, even at the plot level, Mother Nature plays a big role. No surprise then that state agencies have a hard time identifying trends in river N levels, let alone giving credit to farmers for their efforts.
The authors conclude that, “Under the current cropping systems with agronomic N rates, it appears that major reduction in river nitrogen loads to the Gulf of Mexico are only likely through water remediation strategies.”
Some will be inclined to focus on the first words of that passage calling for changes to cropping systems, and indeed millions of dollars are being spent in search of new crops and markets. I am sure that if we wait long enough, new crops and markets will emerge, as they have in the past. It will just depend on what people want and are willing to pay for.
The latter part of the passage refers to water remediation strategies with the recommendation that “expanding floodplain interactions along the Mississippi River and other smaller rivers may be a way to increase residence time for denitrification and thus help reduce river N loads to the Gulf of Mexico.”
This makes a lot of sense and is supported by a large body of research showing the effectiveness of wetlands in nitrate removal, while suggesting the wetland efforts be focused on floodplain.
Appearing in the Journal of Hydrology: Regional Studies, the paper was authored by Kari Wolf, Satish Gupta and Carl Rosen as part of Wolf’s master’s degree thesis. Readers may also recognize Gupta from his groundbreaking work that helped turn attention to stream beds and banks as significant sediment sources, and more recently, the effects of climate on soil and water.
Supported in part by the Minnesota Soybean Research and Promotion Council, the paper is also another great example of farmers investing in useful, practical research. Read the full article online.
Formo is executive director of the Minnesota Agricultural Water Resource Center.
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