Nixing nitrate flow from the farm

Photo by Peggy Greb, ARSPreventing movement of nitrates from crop fields to streams is the focus of ARS soil microbiologist Tom Moorman’s research. Here, Moorman examines a farmer’s subsurface drain pipe. When early settlers arrived in the Midwest, they began constructing an underground network of tile drains to channel water away from the soggy prairies, which then became some of the most fertile crop fields in the country. But now when nitrate from soils and fertilizers leaches out of those flourishing fields, the subsoil engineering also facilitates the discharge of nitrates into nearby streams and rivers.

Because these local waterways are part of the vast Mississippi River Watershed, the nitrates are eventually transported into the Gulf of Mexico, where they can feed the development of oxygen-deficient “dead zones.” But nitrate management isn’t just an issue for the folks downstream. The U.S. Environmental Protection Agency has mandated that nitrate concentrations in drinking water—obtained either from surface water or ground water—cannot exceed 10 parts per million. Minimizing nitrate loss can also help producers obtain the greatest economic returns from the application of expensive fertilizers. So everyone benefits when nitrates are stopped from contaminating local water supplies.

Agricultural Research Service soil microbiologist Tom Moorman and others at the National Laboratory for Agriculture and the Environment in Ames, Iowa, have spent the last decade studying whether underground trenches filled with wood chips could help stem this nitrate flow. Microorganisms that live in the wood use a process called “denitrification” to convert nitrates in the field leachate into nitrogen gas or nitrous oxide, which then diffuse into the atmosphere.

Soils have some capacity to denitrify field leachate, but it generally decreases with soil depth,” Moorman says. “So we wanted to see how well wood chip ‘denitrification walls’ could protect nearby waterways from the nitrates that leach out of the soil. We also wanted to see how quickly the wood breaks down in the subsoil.”

Digging For Answers

Moorman and his team—technician Colin Greenan, microbiologist Timothy Parkin, plant physiologist Tom Kaspar, and soil scientist Dan Jaynes—set up experimental sites in a field north of Ames. They installed perforated plastic drainage pipes 4 feet below the soil surface and then dug trenches on either side of the pipe and filled the trenches with wood chips. They buried the trenches and the pipes, and then cropped the fields with a corn-soybean rotation for the next 9 years.

The results from this work were published in 2010 in a special issue of Ecological Engineering. In part because the benefits of using wood chip bioreactors for denitrification were so conclusive, Agriculture’s Clean Water Alliance—a group of leading farm retailers in west-central Iowa—and the Iowa Soybean Association, in partnership with Wisconsin-based Sand County Foundation, are now encouraging farmers to install the denitrification walls to help mitigate the nitrate pollution associated with regional agricultural production.

“This study helped us confirm that using wood chips to build denitrification walls will result in a significant level of denitrification in field leachate,” says Moorman. “We also understand much more about the different mechanics of denitrification itself, and now we have good numbers on how many denitrification bacteria live in wood and how long that wood can last in a trench under typical field conditions.”