Decreased Tile Nitrate Loss Using a Longer Rotation with Cover Crops and Bioreactor.

The current Gulf of Mexico hypoxia action plan calls for a 45% reduction in riverine nitrogen (N) and phosphorus (P) exiting each state in the Mississippi River Basin by 2035. In response to this challenge, the Illinois Nutrient Loss Reduction Strategy was created, which details a suite of agricultural best management practices designed to reduce nutrient runoff from fields to surface waters. Currently, we are evaluating a combination of several suggested practices (i.e., cover crops and bioreactors) on nutrient losses from fields with artificial drainage (pattern tile drainage). Using 3 adjacent fields (22-28 ha), each with an individual tile drainage system, we planted either corn, soybean, or winter wheat to accommodate each phase of the 3-yr rotation annually. For cover crops, we planted cereal rye after corn and a mixture of radish, turnip, and red clover after wheat, while winter wheat occupied the soil in place of a cover crop following soybean. During the 2015 drainage season (January to August), the greatest tile nitrate concentrations from corn, soybean, and wheat were 10.6, 5.4, and 13.3 mg N L^-1, respectively. Tile flow resumed in late November of 2015, and nitrate concentration in the tile draining the mixture of cover crops had decreased below 2 mg N L^-1 and remained below 1 mg N L^-1 for most of the winter of 2016. Planting cover crops in mid-August following wheat harvest provided favorable growing conditions, produced large total biomass (>6 Mg ha^-1 by mid-November, 2015), and decreased soil nitrate levels to 1 mg N kg^-1. Nitrate concentrations and loads were further decreased by passage through a woodchip bioreactor at the end of each tile system. These data show on a field scale that tile nitrate loss can be nearly eliminated within one growing season using a combination of in-field and end-of-pipe techniques.