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Bioreactors Form a Last Line of Defense against Nitrate Runoff

Posted by Kari Cohen, Natural Resources Conservation Service in Conservation
Feb 26, 2016
Illustration of how a denitrifying bioreactor fits in with drainage water management (DWM). Image by John Peterson.
Illustration of how a denitrifying bioreactor fits in with drainage water management

NOTE: This year, we’ll be highlighting some of 2015’s conservation practice innovations in a monthly series. NRCS supports science-based conservation by offering technical and financial assistance for nearly 170 conservation practice standards. As conservation science and technology advances, NRCS reevaluates each standard every five years and incorporates new advancements into conservation practice standards.

Walk to the edge of certain crop fields in Iowa and look down. You might not notice anything unusual, but just beneath the surface hordes of woodchip-dwelling microorganisms are busy removing excess nitrates from water before it leaves the field. By filtering nitrates, this organic gauntlet safeguards local streams and, eventually, the Gulf of Mexico.

Hungry microorganisms and woodchips are the key components of denitrifying bioreactors, a conservation practice that USDA’s Natural Resources Conservation Service (NRCS) made eligible for financial assistance last year with the publication of a Denitrifying Bioreactor conservation practice standard. Progressive farmers in the Midwest have been experimenting with bioreactors for about a decade, and NRCS has piloted the practice in a handful of states including Iowa, Minnesota and Illinois.

“We’re really excited about the potential to spread this technology across the Mississippi River Basin,” said Dr. Wayne Honeycutt, deputy chief for science and technology at NRCS. “When paired with nutrient management, cover crops and no-till practices, denitrifying bioreactors are a fantastic line of defense for subsurface nitrates.”

Water control structures (in the large white circles) route water running through tile lines into the denitrifying bioreactor
Water control structures (in the large white circles) route water running through tile lines into the denitrifying bioreactor, which lies underground at the edge of a field. Image by John Peterson.

A bioreactor is basically a buried trench filled with a carbon source – usually wood chips – installed at the edge of a field. Tile drains from the field carry excess water from the plant root zone, and divert a portion of the drainage water into the bioreactor. Microorganisms on the wood chips consume the nitrates in the water and expel it as nitrogen gas. Performance varies based on size, location, and a variety of other factors, but the average bioreactor can be expected to remove 35-50 percent of nitrates from the water flowing through it. Bioreactors have no adverse effects on crop production and do not restrict drainage.

In 2011, the Iowa Soybean Association was awarded a Conservation Innovation Grant from NRCS to increase farmer awareness and accelerate implementation of denitrifying bioreactors. Project leaders monitored and analyzed the performance of new and existing bioreactors, and explored ways to limit buildup of harmful contaminants in the woodchip pile.

“One thing we learned is that it’s important not to build bioreactors too large,” said Keegan Kult, environmental projects manager with the Iowa Soybeans Association. “If they’re too large, it becomes hard to control the flow of water through the bioreactor.”

The project also provided outreach and training to NRCS field staff and drainage contractors to build confidence and familiarity with the new practice. Kult said that the Iowa Soybean Association has been involved in the installation of 22 bioreactors in Iowa, and estimates that there are probably another 10-15 beyond that.

Installation costs of a bioreactor vary, but the average model costs $8,000-$12,000. NRCS, through the new conservation practice standard, may provide partial financial assistance for the cost of implementation. Nabbing Nitrates Before Water Leaves the Farm: Bioreactors, a short video by the Missouri & Mississippi Divide Resource Conservation and Development with support from NRCS, provides a nice overview of a bioreactor from start to finish.

NRCS promotes coordinated conservation practices that help producers avoid loss, and control and trap nutrients and sediment at the edge of farm fields. Denitrifying bioreactors hold great promise as a nutrient trap, reducing the flow of excess nitrates into local bodies of water, a significant water quality concern throughout the Mississippi River Basin.

Due to their potential for capturing nitrates, bioreactors are gaining popularity. Bioreactors are still a relatively new technology, however, and continued research and testing will lead to further refinement and performance improvement in the future.

Installation of a denitrifying bioreactor
Installation of a denitrifying bioreactor. Nitrates beware. Photograph by Jason Johnson, NRCS Iowa.
Category/Topic: Conservation