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New Technology Filters Nitrogen Leaving Field

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With nitrate levels unusually high for this time of year in both the Des Moines and Raccoon rivers, Iowa's water quality is back in the news.

Conservation practices like seeding cover crops in the fall can hold nutrients in place on a field and make them available to the next year's crop. However, they're expensive, and take years to get established.

Keegan Kult with the Iowa Soybean Association says there are a few other options for treating runoff as it leaves the field; for one, a bioreactor.

It's simpler than it sounds; using a network of underground drainage pipes called a tile line, a portion of the outgoing flow is routed through a bed of wood chips, where bacteria process nitrates from the water and expel it as gas.

Kult points out that there is a catch to edge-of-field practices like bioreactors, however: installing them is a hard cost, with little direct benefit for the farmer footing the bill.

"A bioreactor will cost on average about $8,000, and when you look at it in terms of cost per pound of nitrogen removed, it's very competitive with the other denitrification practices that are out there, and it's actually cheaper than what a cover crop would be, in terms of cost-per-pound removed, but we're not going to see that in-field benefit from a bioreactor." He says, "There's no direct agronomic benefit of these edge-of-field practices to the producer, so finding cost-share for producers to be able to take advantage of is very important."

That cost-share is primarily available from USDA's Natural Resources Conservation Service.

Kult says another treatment option, called a saturated buffer, works much the same way, "What we do is we redirect that tile flow and let it drain through the soil profile of the buffer, whereas right now there's a pipe that goes through the buffer, so it bypasses that buffer system. And instead of using wood chips for the carbon source, we actually use that organic material that's in that buffer's soil profile, and that's what spurs denitrification in that system."