Tile drainage reduces soil moisture levels for optimal crop growth, but there is concern about nitrate loss from these systems. Because the water quality of regional streams, rivers, and lakes can be negatively impacted by nitrate in drainage, researchers at Iowa State University are studying several practices that can be done to reduce the amount of nitrate in drainage water.
In 2009, Iowa State University graduate student, Laura Christianson, applied for an NCR-SARE Graduate Student Program grant, and was awarded $9,953 to provide information to producers about nitrate reduction technologies, and to provide researchers information about producer acceptance of various nitrate reduction approaches, including an innovative approach using woodchip bioreactors.
“When I started my Ph.D., I was drawn to studying bioreactors because they combined an engineered approach to water quality improvement with a natural treatment system,” said Christianson. “I liked the idea that just by routing drainage water through a trench filled with woodchips, you can create a ‘vacation resort’ environment for denitrifying microbes that convert nitrate in drainage water to nitrogen gas. Of course, the further I got into bioreactors, the more I realized we really needed to put this technology within the context of other drainage water quality improvement strategies like wetlands and cover crops.”
Christianson’s NCR-SARE grant project compared the economic cost efficiencies and ecosystem services among seven nitrate reduction methods (wetlands, controlled drainage, cover crops, crop rotation, fertilizer rate reduction, fertilizer timing modification, and denitrification bioreactors). She used these comparisons to develop an educational program with an associated survey to gage the social acceptance of these seven nitrate reduction methods. To further study the perceived adoption potential of these practices in the context of ecosystem service provisioning, a small discussion group was held with farmers. A hand-out comparing the seven practices was distributed at events.
“Each nitrate reduction strategy provides landowners an additional distinct option for drainage water quality improvement and different strategies or combinations of such will be applicable in different locations,” said Christianson. “While the nitrate management practices were very cost effective and had high interest and compatibility, they offered few additional ecosystem services. Conversely, the practices that had high ecosystem service provisioning generally had lower compatibility (wetlands) or interest (crop rotation).”
The newest in the slate of technologies that Christianson researched and presented were woodchip bioreactors (also known as denitrification bioreactors), which are made by routing drainage water through a buried trench filled with woodchips. Woodchip bioreactors, installed at the edge of agricultural fields, can help remove nitrate in tile-drained water.
According to Christianson, bioreactors are well-suited for buffer strips or grassy areas, which typically means that little land is taken out of production. They are specifically
designed to treat subsurface drainage water that contains high amounts of nitrogen as nitrate and that has relatively little sediment. They work best in drainage systems that have few surface intakes, and are not intended to treat runoff or water collected along terraces. Most current bioreactor designs have been successful at reducing the amount of nitrate in drainage from 30 to 80 acres. Because this is an edge-of-field practice, other conservation practices such as no-till, cover crops, and improved nutrient management can be done in the field, and the bioreactor will treat the remaining nitrate that is lost in drainage.
“I think better understanding of the costs of conservation practices, like bioreactors, helps contribute to sustainable agriculture, but a better understanding of what farmers perceptions are of certain practices is important, too,” said Christianson. “We tried to get at both these things with this project. Bioreactors won’t get us to ‘sustainable agriculture’ by themselves, but they can be a new way to talk about water quality and may provide an option for some individuals.”