They’re called “denitrifying bioreactors” but in laymen’s terms, these “bark beds” are simply wide, shallow trenches filled with wood chips located at the end of field tiles lines.
These simple structures can be amazingly efficient at removing nitrogen from field sub-surface flow from tile lines. The wood chips themselves serve as the medium for microbes that use carbon in the wood chips as their food source. And as the naturally occurring microbes grow and multiply, they convert nitrate nitrogen from subsurface flow primarily di-nitrogen gas and release it into the atmosphere, explains Matt Helmers, an Iowa State University a specialist in agricultural and bioengineering.
The microbes will remove as much as 98% of the nitrogen from the runoff water. However, bark beds are designed to only take about 15% of the peak flow from the drainage system, and they treat about 60% of the long-term average annual flow. As a result, their over-all efficiency drops to about 25% to 40% nitrate-nitrogen removal, says Helmers.
Lee Kinnard first learned about bark beds when he was asked to speak at a Nature Conservancy meeting a few years ago. Kinnard milks 7,000 cows and farms more than 10,000 acres with his family on Wisconsin’s Door Peninsula that juts into Lake Michigan. The peninsula has one of the highest concentrations of cattle in the state and is environmentally sensitive because it naturally drains into Lake Michigan. Areas of the peninsula also can have very shallow depth to bedrock, which itself is fractured. That combination can lead to nitrogen contamination of wells.
Kinnard has been working with the Nature Conservancy to show agriculture can play a part in solving these environmental issues. So this past year, he decided to install a 25’ X 55’ x 5’ bark bed at the end of a tile line that drains 160 acres. It took four semi-loads of bark to fill the bed.
The bark bed was installed on land where his mother was born and raised, and it drains into the bay side of the peninsula. The site has anywhere from 200’ to 300’ of soil depth to bedrock, and it has a 4 ½ to 5% slope so that tile lines easily drain. “It’s a good way to sneak in a little wetland at the base of the tile line at very little loss of acreage,” he says.
Because this is a demonstration project with the Wisconsin Department of Agriculture, Trade and Consumer Protection and USDA’s Natural Resources Conservation Service, monitoring wells were also installed to gather data on water flow and nitrogen reduction. Since the bark bed has just recently been installed, Kinnard has no data to share. Typically, a bark bed this size would cost about $15,000 to install, but the monitoring equipment and a small building to house that equipment pushed the cost up to $21,000.
Kinnard is anxious for data to start rolling in. Because the drainage area is all one field, it is typically planted to one crop—either corn or alfalfa. “Alfalfa can put out a lot of nitrogen. We’ve been sampling tiles lines for the past decade for our own purposes, and we’ve seen spikes in nitrogen after alfalfa has been killed off at the end of its rotation,” he says. So he is curious to see what the actual levels of nitrogen are once the water is processed through the bark bed.
Scott Youse, a Caroline County, Md. dairy farmer, installed the first bark bed in his state in the fall of 2013. At that time, barks beds were still such a new concept bioengineers weren’t sure if they would work in Maryland’s more humid climate. Maryland is technically described as a humid, subtropical coastal plain.
Farmers along the Chesapeake Bay have been under pressure for years to reduce nitrogen field losses. When the Midshore Riverkeepers Conservancy came to Youse and asked if he would be willing to install a bark bed, he instantly agreed. “The Riverkeepers have been good to work with,” he says. They offered a potential solution, he says, and he was willing to try it.
Youse’s bed is 20’ X 100’ X 2’, and processes runoff from 180 acres and the main building site of the dairy. “The bed is located on a triangle of land where our center pivot doesn’t hit,” says Youse. “So it really cost us nothing in terms of taking land out of production.”
Tim Rosen, a watershed scientist with ShoreRivers (previously known as the Midshore Riverkeepers), now says the bark bed installed at the Youse dairy is under-sized for the amount of land it processes. “We would need to double the width of the bed for that volume of water,” he says. The goal is to have six to eight hours of water retention time in the bioreactor to allow the microbes to do their work.
The Delmarva Peninsula is also very flat, and most of the tile lines drain on the Peninsula into ditches. So bioengineers are now looking at ways to use bark beds to process water from ditches using ditch diversion bioreactors, sawdust denitrification walls and in-ditch bioreactors. While all three systems can be effective in reducing nitrogen, they typically have higher costs than conventional bark beds.
Some states offer cost-share programs that cover some of the costs of bark beds. Maryland, for example, offers 87.5% cost share on piping and structures, but not on the construction and installation of the bioreactors themselves. USDA’s Natural Resources Conservation Services also has some cost share available through EQIP, though these are usually available only through a bid process.
Research in Maryland and the Midwest has shown that bark beds, even in their many forms, can reduce nitrogen loads from agricultural drainage. “The biggest questions are cost, and do they take land out of production,” says Rosen. “We’re asking farmers to incur these costs when there might not be agronomic benefits. But if we can site them on the edge of fields or on buffer areas, we see it as a win-win when we put these practices in.”
Adds Lee Kinnard: “It is incredibly cool that such a simple device can be so effective at protecting our water resources. This shows that animal agriculture can absolutely be part of the solution.”