Farm Progress

Iowa Learning Farms: Start the conservation conversation between landlords and tenants on using denitrifying practices to protect water quality.

April 11, 2017

4 Min Read
BIOREACTOR: Tile drainage water is routed through a trench filled with woodchips. Bioreactors can reduce nitrates by 15% to 60% and last for 10 to 15 years before the woodchips may need to be replaced.

By Liz Juchems

Editor’s note: This is the second of a four-part series on encouraging conservation conversations between landowners and tenants.

A new publication series by the Iowa Learning Farms can help start the conversation. The four-part “Talking With Your Tenant” publication series, along with other print and video resources, are available online. Copies also are available at ILF field days and workshops, or mailed upon request.

Start the dialogue about denitrifying practices like wetlands during May, which is National Wetlands Month.

Just what are denitrifying practices?
Practices like wetlands, bioreactors, and saturated buffers remove nitrates from tile drainage water through a process called denitrification where microbes breathe in nitrate (NO3) and exhale inert N2 gas back into the atmosphere. These microbes require an anaerobic environment to ensure that they use the nitrate in the water, rather than oxygen, as part of their respiration process.

In addition to improved water quality, what other benefits do these practices provide?
After installation, these practices require minimal cost and maintenance, create habitat for pollinators and wildlife, establish migration corridors, generate income from hunting leases, and as edge-of-field practices, have no impact on yield.

How do wetlands remove nitrate?
Wetlands are characterized as having water at or near the soil surface during at least part of the year, containing hydric soils, and containing plants that are adapted to wet conditions providing a great environment for denitrification. Wetlands are shallow in depth and allow the water to slow down and deposit sediment. On average, wetlands can remove 40% to 90% of nitrate.

Where should wetlands be installed?
Constructed wetlands are strategically located and designed to treat drainage areas of 500 to 3,600 acres. The typical wetland is 0.5% to 2% of the drainage area, and an easement area is established as a buffer to prevent sedimentation. For example, for a 1,000-acre watershed, the wetland area would be 5 to 20 acres, and the easement buffer area would be 10 to 40 acres.

So how does a bioreactor work?
Tile-drained water is routed to a woodchip-filled trench located in a grassed buffer, where the tile leaves the field. Bacteria use the carbon from the woodchips as a food source, and the incoming nitrate for their respiration process. Bioreactors can reduce nitrate levels by 15% to 60%.



WETLANDS: On average, constructed wetlands can remove 40% to 90% of nitrate from the water. A typical wetland occupies only 0.5% to 2% of the drainage area, along with a buffer established to prevent sedimentation.

Where can I install a bioreactor?
Because they are long and narrow, they fit well in edge-of-field buffer strips and grassed areas, and typically no land is taken out of production. Current designs can treat 30 to 80 acres of tile-drained fields. Bioreactors can last for 10 to 15 years, after which the woodchips may need to be replaced.

Can you tell me more about saturated buffers?
In a saturated buffer, tile-drained water is directed into a lateral tile that runs parallel to a riparian buffer. As the water moves across the buffer, microbes living in the saturated zone use the nitrate in the water as part of their respiration process. Plants in the buffer also remove nitrate from drainage water through root uptake. Saturated buffers can remove, on average, 50% of the nitrate in subsurface flow.

How do I know if I can install a saturated buffer?
Existing buffers and existing tile drainage systems can be retrofitted for this practice if they meet specific site requirements. A good candidate site has the following: a minimum 30-foot-wide buffer area, well-established vegetation, a flat area at least 300 feet along the waterway, and no more than a 2-foot elevation change along the length of the buffer. Check with your local Natural Resource Conservation Service to see if you have a qualifying site.

How do the costs of these edge-of-field practices compare to other practices addressing nitrate?
Wetlands, bioreactors and saturated buffers are the most cost-effective practices for reducing nitrate. Over the lifetime of the practices, wetlands cost approximately $1.38 per pound of N removed, bioreactors cost about 92 cents per pound, and saturated buffers cost roughly $1.91 per pound. By comparison, for cover crops, the cost per pound of N removed is about $6. There are several funding and technical sources available to support the installation of these practices through the NRCS, Iowa Department of Agriculture and Land Stewardship, and Iowa Soybean Association.

Final advice for someone interested in denitrifying practices?
Edge-of-field practices do not impact yield and require minimal management to make sure they are working effectively. It’s recommended the landowner and tenant work together to gather information about the practices and address management concerns before installation. Your local NRCS staff and ISU Extension field specialists are available to meet with you and your tenants to help answer questions, and to provide resources and technical assistance.

Juchems is the events coordinator for Iowa Learning Farms.


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