Bioreactors, Wetlands, and Managed Drainage to Reduce Tile Nitrate Losses in East-Central Illinois Fields.

Reducing nitrate losses to rivers from tile drainage is a predominant theme of the many state level nutrient loss reduction strategies that have been developed for the Mississippi River basin, with end of tile conservation practices heavily featured. These include constructed wetlands, wood chip bioreactors, and managed drainage. Each of these techniques allows for denitrification to remove nitrate, although this is difficult to quantify where and when in managed drainage systems. We have studied each of these techniques on tiled fields with a corn and soybean rotation on poorly drained Mollisols in east-central Illinois. Wetlands consistently performed best (about 60% N removal), removing nitrate throughout the drainage period of January through early July; however, little removal occurs during high flow periods where the berms were overtopped by the river. Bioreactor performance was variable, depending on age of the wood chips, tile water temperature, and nitrate load. On a percentage basis, nitrate removal varied greatly from < 5 to > 80%. However, on a g/m³/d basis, rates typically were controlled by water temperature and were similar across the bioreactors after following higher rates from fresh wood chips. We found that managed drainage didn’t work well in our paired fields that were retrofitted for drainage water management, as water moved from tile system to tile system, or simply came out when the tile was opened. There are substantial costs for wetlands and bioreactors as well as limitations on where they can be located, and retrofitting managed drainage onto existing tile systems can be difficult. Large scale implementation of these methods to reduce nitrate losses is not likely, but on some fields these approaches may work well.