Nutrient export from the Corn Belt of the United States is impacting rivers at local and regional scales and is contributing to the development of hypoxic conditions in the Gulf of Mexico. The extent of Gulf hypoxia is largely driven by nitrate-nitrogen loads delivered by the Mississippi River that is sourced from row crop agriculture in the U.S. Midwest. Soluble nitrate is typically leached from cropped and fertilized fields and is transported to streams through groundwater discharge and artificial subsurface drainage systems.
Watersheds containing subsurface tile drainage systems are very susceptible to increased nitrate losses, and agricultural conservation practices are needed that reduce the loss of nitrate from tile drainage to streams and rivers.
Oxbow lakes are natural floodplain features that are formed when a river cuts off a meander loop as it migrates laterally within its floodplain. Over the course of years and decades, natural oxbows often fill with accumulated sediment. Removing the oxbow fill material allows the lake to fill with water and improves habitat for fishes, including the federally endangered Topeka shiner (Notropis topeka) (Bakevich et al., 2013) and waterfowl (LaGrange and Dinsmore, 1989). New work in Iowa is quantifying the nitrate reduction benefits of oxbow restorations (Schilling et al., 2017, 2918a, 2918b).
A new study quantified the nitrate mass load reductions of a restored oxbow in north-central Iowa that was reconstructed to receive flow and nutrients from two subsurface field tiles (Schilling et al., 2018b). The oxbow site was constructed in the floodplain of White Fox Creek (Figure 1) by excavating approximately 2000 m3 of post-settlement alluvium to a depth of approximately 2 m (Figure 2).
The oxbow outlet is connected to the channel of White Fox Creek during high flow events. Monitoring at the site included a geophysical survey, a network of shallow monitoring wells, water monitoring of tiles, wells and surface water and deployment of a continuously-reading nitrate sensor in the oxbow. From the monitoring data, a nitrate mass balance was developed to estimate annual and monthly load reductions occurring in the oxbow.
Based on monitoring conducted in 2017, inflow nitrate loads to the oxbow were dominated by tile drainage contributions and mass balance quantification indicated that the oxbow reduced total inflow NO3-N loads by 35% over the course of the year. Greatest nitrate removal efficiency was found to occur in the late summer and fall. Overall, nitrate retention and construction costs (~$10,000) for a restored oxbow are similar to other practices designed for treating tile drainage, including bioreactors and saturated buffers. However, the benefits of oxbow restorations compared to other strategies extend beyond simply reducing nitrate. Bioreactors and saturated buffers are installed below-ground to treat tile drainage effluent whereas oxbows provide holistic ecosystem benefits for wildlife and recreational benefits for the landowner (Figure 3). Although more work is needed to quantify ecosystem services at a wider range of sites, oxbow restorations should be considered a viable nitrate reduction practice in agricultural regions.
These findings are described in the article entitled Nitrate-N load reduction measured in a central Iowa restored oxbow, recently published in the journal Ecological Engineering. This work was conducted by Keith E. Schilling and Christopher S. Jones from the University of Iowa, Keegan Kult and Anthony Seemon from the Iowa Soybean Association, and Karen Wilke from the Nature Conservancy.
The oxbow water quality research is a product of a collaboration among the University of Iowa (Schilling, Dr. Chris Jones), The Nature Conservancy (Karen Wilke) and the Iowa Soybean Association (Keegan Kult, Anthony Seeman). Funding for the project was provided, in part, by TNC, ISA, NRCS and Iowa State University Nutrient Research Center.
- Bakevich, B.D., Pierce, C.L., and Quist, M.C. 2013. Habitat fish species, and fish assemblage associations of the Topeka Shiner in west-central Iowa. N. Am. J. Fish. Manage. 33:1258-1268.
- LaGrange, T. G., and Dinsmore, J. J. 1989. Plant and animal community responses to restored Iowa wetlands. Prairie Naturalist, 21:39-48.
- Schilling, K.E., Haines, B.J., Jones, C.S. and St Clair, M. 2018a. Effectiveness of a newly reconstructed floodplain oxbow to reduce NO 3-N loads from a spring flood. J. Environ. Man. 215:385-393.
- Schilling, K.E., Kult, K., Wilke, K., Streeter, M. and Vogelgesang, J. 2017. Nitrate reduction in a reconstructed floodplain oxbow fed by tile drainage. Ecol. Eng. 102:98-107.
- Schilling, K.E., Kult, K., Seemon, A., Wilke, and Jones, C.J. 2018b. Nitrate-N Load Reduction Measured in a Central Iowa Restored Oxbow. Ecol. Eng. 124:19-22.