Keeping nitrogen from running away

A bioreactor trench 20 percent the size of a CFL end zone can filter up to 70 percent of the nitrates from an 80 acre tile drained field

Scientists around the globe are on a quest to keep cropland nitrogen out of waterways.

A simple bioreactor trench filled with woodchips is one promising idea already being implemented.

A great deal of research focuses on vegetated riparian buffer zones to absorb nitrogen and phosphorous, but these zones require a lot of land, which farmers are understandably reluctant to take out of crop production.

However, Laura Christianson, water quality professor at the University of Illinois, said farmers can filter excess nitrogen with small trenches rather than large vegetated buffer zones.

Christianson is involved in researching bioreactors, which have a small footprint and carries out a similar function to a riparian buffer zone. They divert water through a wood chip-filled trench to remove nitrogen as water moves off a tile drained field.

“If placed smartly, no land is generally removed from production,” said Christianson.

“And a bypass line that is part of every bioreactor design means there will not be any significant reduction in drainage capacity.”

This type of conservation drainage is growing in popularity across the U.S. Midwest. Trenches are usually 30 metres long, six metres wide, one metre deep and covered with 30 centimetres of topsoil.

The wood chips in the poly-lined trench provide a home and carbon-rich food source for microbes to colonize and thrive. These microbes take on the process of denitrification, where they convert nitrates into nitrogen gas.

Plants take up nitrogen from the soil when it’s in nitrate form, but excess nitrates pollute water sources when they are drained away from fields.

The situation is especially pronounced in irrigated fields with tile drainage. These fields typically grow crops requiring a lot of nitrogen to be profitable. Excess water exiting through the tile system is loaded with nitrates, and water leaving the drain wants to take shortcuts to nearby waterways.

However, the presence of drain water in the pipe is a major positive factor because it can be captured and managed more effectively than surface runoff.

Tile drain water is diverted into the trench, where microbes consume the nitrates and carbon from the wood chips before “exhaling” the harmless nitrogen gas into the atmosphere. After the microbes have feasted on the nitrogen, clean water drains out of the trench and into the waterways.

According to the Iowa Nutrient Reduction Strategy, these bioreactors remove an average of 43 percent of the nitrates from the tile drained water. Studies indicate that 30 to 70 percent of nitrate is removed.

A bioreactor that can drain 80 acres costs around $8,000. It will last up to 20 years with minimal maintenance before the woodchips need to be replaced.

The bioreactors have been proven to be an effective, low maintenance option for preserving water quality while taking up little space, but they are not a stand-alone solution.

The microbes move slower and less efficiently in the spring and cold weather, and the installation cost can be prohibitive if not subsidized.

Christianson said the bioreactor does have drawbacks.

“Specific focus on nitrate in drainage water means this practice does not typically provide other benefits like some other practices,” he said.

“For example, wetlands and cover crops can help keep nitrate out of drainage water, and wetlands additionally provide wildlife habitat and flood retention, while cover crops can also help improve soil health.”

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