Wood-Chip Trenches Keep Nitrate Runoff in Check

Agricultural Research Service scientists have found that trenches filled with wood chips are a promising line of defense against nitrate seeping from fertilized cropland into waterways.

Dan Jaynes, a soil scientist and research leader at the ARS Soil and Water Quality Unit in Ames, Iowa, is leading studies on how to curb the flow of nitrate from farm fields into streams and rivers. The study, which is concentrating on fields planted with corn and soybeans, found that wood-chip-filled trenches can cut nitrate losses to surface water by 70 percent.

The wood chips create a carbon-based barrier that helps change the nitrate into nitrogen gas, a common atmospheric component. The system doesn’t require any management by the farmer. In the study, the 6-foot-deep trenches were filled with chips up to 1 foot below the surface and covered with soil.

Drainage tiles speed the draining of cropland and release the excess water into waterways. They have played a significant role in the development of U.S. agriculture and are used on 30 percent of Midwest cropland, according to Jaynes.

However, rapid draining allows nitrate to bypass the soil root zone, making it unavailable to plant roots and other natural processes that can remove it from soil. Once excess nitrate gets into waterways, it feeds the growth of aquatic plants. As the plants die and decompose, they use up all the oxygen in the body of water. This scenario has been cited as a cause of hypoxia--a deficiency of oxygen--in parts of the Gulf of Mexico. High nitrate levels have also caused problems for communities that use rivers for drinking water.

The trenches in the study are 2 feet wide, laid out parallel to tile drains, and placed 10 feet apart. The wood-chip trenches are 2 feet deeper than the tile drains. Jaynes says the trench method works equally well for small and large farms.

The study’s next phase will explore how long the wood chips in the trenches will filter nitrate before decomposing.

ARS is the chief scientific research agency of the U.S. Department of Agriculture.