Overwintering cover crops stop 40% of nitrates from washing down tile lines — a finding that’s been duplicated in field trials by scientists such as Lowell Gentry at the University of Illinois and Pete Fandel at Illinois Central College. But how can farmers reach nearly zero loss?
A portfolio of practices ranging from in-field (cover crops) to edge-of-field (bioreactors and wetlands) holds potential to nearly zero out losses from tile drainage outlets. That’s why ICC in East Peoria, Ill., has undertaken a large-scale experiment: rolling all three practices into their on-campus farm.
Earthmovers from the Illinois Land Improvement Contractors Association finished a constructed wetland on the site in late July. It takes in water that’s already gone through a bioreactor, and then some. The bioreactor was built almost three years ago, and much of the acreage has been in a cover crop experiment for five years.
“We’re trying to see if we can have zero discharge of nitrogen off of campus,” says Fandel, an agriculture professor at ICC and a Metamora farmer. “I don’t know if we can achieve the goal, but we’re going to try to get as close to it as we can, and the new wetland will certainly help.”
He says the bioreactor effectively cleans 80% to 90% of the nitrates in the water that flows through the mulch pit, where denitrifying microbes feed on the wood’s carbon and breathe the nitrogen content of the water. However, only 50% of total rainfall makes its way through the bioreactor, because during heavy rain events, a portion of water is designed to bypass it. As more wood gets eaten, the pit becomes less effective, and the wood chips will need to be replaced.
“Those big rain events are where a lot of nitrates wind up leaving your property, and the wetland is going to address that whole issue because all the water is going to end up in the lowest spot. It will be held in this wetland. I think it’ll be a great addition,” Fandel says, adding the wetland was designed to be big enough to clean water following the heaviest rain events and has a spillway for when it gets too full.
The wetland was designed by Jill Kostel, senior environmental engineer with the Wetlands Initiative. This wetland is the group’s fourth constructed wetland and was built on a site with patches of clay, sand and gravel — a far cry from the rich hydric topsoil that will eventually be formed after years of biological activity.
Contractors removed the “undesirables” and compacted a clay layer with a sheep foot roller towed behind a tractor. They will add topsoil, and students and volunteers will seed native plants in the spring. Grassy berms will tower out of the water and cause it to flow between emergent plants, which provide the carbon for the microbes to remove the nitrate-nitrogen from the water.
“Bioreactors and constructed wetlands work by the same principles, the carbon sources are just different,” Kostel says, concluding the wetland “is going to turn out really beautiful. Give it a few years to get itself up and running with all the native vegetation. It will get there eventually.”
Check out how the tile-drainage treatment wetland was constructed in the slideshow.