Leopold Center for Sustainable Agriculture

Student project looks at grassed waterway impacts on water quality

Back to Leopold Letter Spring 2008

By HENRY WILSON, special to the Leopold Letter

While working on my undergraduate degree in Agricultural Science from Western Illinois University, I learned about the effects that agriculture can have on the environment. I became particularly interested in taking relatively small, sensitive areas out of production to mitigate the impacts of agriculture. The work that has been conducted by the ISU Riparian Buffer Research Team (initiated with Leopold Center funding) is a great example of how this approach can offer options to substantially improve environmental quality.

When it came time to select a graduate research topic, I knew that minimizing the effects of agriculture on the environment would be high on my list. I also saw this as an opportunity to extend what was known about reducing the amount of nitrogen (N) and phosphorus (P) in agricultural runoff.

Grassed waterways interested me as a conservation practice with the potential to improve water quality. According to the USDA, there are more than 30,000 acres of USDA-funded grassed waterways in Iowa, yet fewer than 10 published studies worldwide on their effective management. (In comparison with grassed waterways, Iowa has more than twice as many acres of riparian buffers and eight times as many acres of vegetative filter strips enrolled in the Conservation Reserve Program.)

Grassed waterways, areas of potential
A grassed waterway is defined by the Natural Resources Conservation Service as “a natural or constructed channel that is shaped or graded to required dimensions and established in suitable vegetation for the stable conveyance of runoff.” Research conducted on vegetative filter strips suggests that various grass species have different impacts N and P exports from agricultural fields. The grass in grassed waterways also may serve as feedstock for future biofuels. The objective of this study is to determine how different grass species and biomass removal affect N and P export from grassed waterways.

In June 2006, I created 24 small grassed waterways for this study on my family’s farm in Hancock County, Illinois, just across the Mississippi River near Fort Madison. I planted two native warm-season grasses (big bluestem and switchgrass), a cool-season grass traditionally used in waterways (smooth bromegrass) and corn in plots with three replications. On half of the plots I also removed aboveground biomass.

For the experiment, a load of “runoff” is created in a tank with sediment and N and P concentrations similar to those cited in the literature. The “runoff” is applied to the head and sides of each grassed waterway using a pump sprayer, then collected at the bottom of each waterway where it is weighed and analyzed for sediment, N and P concentration. I conducted the experiment in June, August and October 2007, and will repeat it in April, June, August and October 2008.

The differences between the N and P levels in the runoff when it enters and leaves each plot will show how various treatments act as N and P sinks in grassed waterways. I also hope to determine where the N and P that does not exit the grassed waterways is stored.

By fall 2007, the native grasses only comprised about 20 percent of their respective plots, with the rest dominated by foxtail. However, preliminary results suggest that these native grasses (still being established) decreased the amount of N and P that left the waterways in runoff by an average of 15 percent, compared to corn and smooth bromegrass. Also, removing aboveground biomass increased N and P export by an average of 5 percent. These differences occurred because the volume of runoff was decreased, not because of an increase in concentration of N and P in the runoff.
When we removed aboveground biomass, we harvested 56 percent more from the native grass plots than from the corn and bromegrass plots. Based on these results, I hypothesize that the N and P that did not leave the grassed waterways was stored in the aboveground biomass. In this year’s experiments, I would like to find out why the native grasses decrease runoff more than other treatments.

Project generates local interest
I found that our neighbors and other farmers were very interested in using grassed waterways not only to improve water quality, but also to improve things such as wildlife habitat and biodiversity. After I explained the simplicity of my experiment, most people offered very good suggestions and considerations with respect to real life issues concerning grassed waterways and environmental issues in general. Community assistance was critical to the success of the first year of this project and several people offered services such as a source for water, equipment, trucks, surveying and expert advice.

Based on these early results, native warm-season grasses have the potential to decrease the amount of nitrogen and phosphorus that leaves grassed waterways. Even though these grasses may be difficult to establish, they were doing a better job than commonly planted species. I have learned a lot from this experience, and my results do not necessarily represent what might happen over the 10- to 20-year life of a grassed waterway. I hope, however, that my project will inspire more research on the long-term viability of using native, warm-season grasses in grassed waterways.

Back to Leopold Letter Spring 2008