Dairy farmers may not have to dig too deep, learn to maintain new equipment or radically change production practices in order to reduce nitrogen runoff from agricultural fields. Good manure management, along with help from a few friendly microbes, may be the next big thing in cleaning up our waterways.

Freelance Writer
Tamara Scully, a freelance writer based in northwestern New Jersey, specializes in agricultural a...

“Estimates of nitrogen runoff from agricultural fields are around 120,000 Mg per year for the Chesapeake Bay watershed alone,” says William Pluer of the Soil and Water Lab at Cornell University. “Because this nitrogen disrupts aquatic ecosystems through eutrophication, mitigation efforts are necessary.”

Reducing nitrate runoff from agricultural operations may be as simple as creating conditions where naturally occurring microbes, already existing in the soil and water, can feed off of the nitrates in runoff, rendering them into a harmless gas. A denitrifying woodchip bioreactor does exactly that.

While these bioreactors are more common on farms in the Midwest, they are just recently being installed on farms in the Northeast, in part due to the Chesapeake Bay Watershed Initiative, which is a targeted effort to reduce nitrogen – as well as phosphorus and sediment – concentrations in the Chesapeake Bay.

Through the combined efforts of the NRCS and a variety of partners, including university researchers, regional environmental organizations, the U.S. Fish and Wildlife Service and willing farmers, these bioreactors are emerging as key players in reducing nitrate runoff from agricultural lands.


Keeping it simple

“The biggest benefits of woodchip bioreactors include the fact that they typically don’t require cropland to be removed from production, they are low maintenance over their estimated design life of 10 to 15 years, and they require no external power source such as electricity or diesel,” says Laura Christianson, Ph.D., of The Conservation Fund. “Woodchip bioreactors hold a lot of potential for reducing nitrogen loads in many kinds of farm effluents, drainage and wastewaters.”

Denitrifying woodchip bioreactors are relatively simple and shallow trench-like structures. Typically connected to a tile drainage system, the bioreactor itself is approximately 5 or 6 feet deep and can vary in length. Woodchips are placed in the bottom of the trench.

The woodchips act as a growing medium, supporting a large population of naturally occurring microbes. These microbes require anaerobic conditions, a carbon food source and oxidized nitrogen in order to survive. When runoff from the fields reaches the bioreactor, the nitrates it contains are converted by the microbes into harmless dinitrogen gas.

“These conditions occur in small amounts in agricultural fields, but the bioreactors create a large ideal environment for this process. By passing nutrient-rich agricultural runoff through the reactors, we can capitalize on a natural process to reduce the impacts of agriculture on aquatic ecosystems,” Pluer says.

Wastewater from the dairy barn can also be directed into the bioreactor. While the use of bioreactors specifically on dairy farms is relatively new, a handful have been installed on East Coast dairy farms over the past several years.

Cornell research

In the Cornell study, two independent New York dairy farms, as well as Cornell’s Homer C. Thompson Vegetable Research Farm, each have a set of identical paired denitrifying woodchip bioreactors. There are two bioreactors on each farm in order to provide replication of results, as well as comparison of reactor fills, Pluer explains.

On each farm, one bioreactor has also been amended with biochar, in addition to the woodchips, in an attempt to mitigate phosphorous levels. Water going into and going out of the bioreactors has been sampled regularly, with reduction of nitrate rates in water leaving the bioreactors ranging from 55 to 85 percent, although phosphorous reductions have been minimal.

Denitrifying woodchip bioreactors are placed underground, on marginal land along field edges. Once covered, they can be driven over and mowed. Farmers can adjust the groundwater levels in the field via control structures, and crop yields will not be impacted. While most of the Cornell research has been conducted on reactors of about 650 cubic feet, reactors can vary widely in size, Pluer says.

The Cornell researchers are studying different designs and controls that might affect reactor efficiency and hope to improve upon the positive results they’ve seen thus far in the study. They are comparing field results with laboratory results.

Because incomplete denitrification – when the nitrate is not completely converted by the microbes – could result in the formation of nitrous oxide, an undesirable greenhouse gas, researchers are also monitoring this potential complication. Cornell’s continuing research will also compare the denitrification abilities of these bioreactors with that of natural riparian zones.

“We are doing lab-scale experiments, field experiments and continuous monitoring,” Pluer says. “We are also looking at potential greenhouse gas emission from the reactors and controls to decrease this possible drawback to a great technology.”

woodchip bioreactor

The future

“It is very important for us to continue our research, particularly at the field scale with more great farmer-partners, so we can better understand how to design woodchip bioreactors to remove as much nitrate as possible from water flowing at different flow rates, at varying water temperatures and with varying levels of nitrogen,” Christianson emphasizes.

The exact size, design and woodchip volume needed for the bioreactors to perform optimally under a variety of circumstances continues to be studied. Bioreactors do have limitations, just like any technology, she says.

“Woodchip bioreactors are not a silver bullet. As research progresses to optimize the design of woodchip bioreactors for different applications and different types of farms, this technology will be an important, practical on-farm option for addressing pollution in the Chesapeake Bay watershed.”

Ongoing field studies, where the denitrifying bioreactors are monitored under real-use farm conditions, will help expand the efficiency and optimize the technology. These bioreactors may not be the answer for every farm, and other nitrate reduction practices – whether combined with bioreactor use or not – will remain relevant.

So far, the simplicity of the denitrifying woodchip bioreactors, along with their relative ease and low cost of construction, anticipated maintenance-free life span and encouraging initial results, all point to denitrifying bioreactors becoming an important part of the solution to reducing nitrate pollution from agricultural operations.

In the Midwest, the number of woodchip bioreactors has increased from less than a half dozen in 2008 to more than four dozen today, according to Christianson, who anticipates that the numbers will similarly increase along the Eastern seaboard as farmers adopt this user-friendly environmental practice.

“It has been extremely important for us to couch this technology within a ‘farmer-friendly’ framework. Water quality improvement strategies for agricultural operations need to minimize potential negative yield impacts but also need to fit easily with the many demands on a producer’s time,” Christianson says.

“As this technology expands to the dairy industry, making sure we have good research to inform the next bioreactor designs will be a vital step for increasing expansion of the technology.” PD

Tamara Scully, a freelance writer based in northwestern New Jersey, specializes in agricultural and food systems topics.

TOP: Illustration of how a bioreactor ties into tile drainage. Illustration courtesy of Christianson and Helmers, 2011.

BOTTOM: Woodchip bioreactor one year after installation at the Northeast Iowa Research Farm (Iowa State University); only the monitoring wells are visible because the bioreactor is “out of sight, out of mind.” Photo courtesy of Laura Christianson.