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Cover crops and bioreactors: can they reduce tile nitrate loads?


covercropimageMP.jpgKeeping nutrients in the fields – and out of the rivers and reservoirs – is a vital part of successful harvests and environmental sustainability. NREC funds several projects that explore ways to reduce tile nitrate loads while maintaining high crop yields. Factors such as cover crops, nitrate application timing, and bioreactors (and combinations) offer promising methods of sustainable practices.

The IL Nutrient Loss Reduction Strategy’s goal is to reduce the export of N and P by 45 percent by the year 2035. There is an interim target of a 15 percent reduction for N and 25 percent reduction in P by 2025.

The following is a summary of results from two recent NREC supported studies around the effectiveness of cover crops, nitrogen application timing, and bioreactors on nitrate loading via tile drainage and crop production.

Cover crops and nitrogen availability

How has nitrogen management changed since 1996? How do cover crops affect nitrate availability and fate within common nitrate management systems in Illinois farms? Does the timing of application affect yield in fields that use cover crops? To answer the first question, Dr. Shalamar Armstrong of Purdue and Dr. Aslihan Spaulding of Illinois State University conducted a sequential survey in a central IL watershed. The data suggest that since the late 90s there had been a decrease in fall-applied nitrogen and an increase in spring applications. Also, there was a large increase in the percentage of farmers who made two or more nitrogen applications throughout the corn growing season, a systematic nitrogen approach.

To determine the impact of nitrogen application timing and cover crops on nitrate loss and crop yield, Dr. Armstrong and his collaborative research team designed a system that tested three Nutrient Loss Reduction scenarios: change nitrogen application from fall to spring; change application from fall to spring and add cover crops; and add cover crop to fall application of nitrogen.

The study was conducted on 15 individually tile drained fields in central, IL. The cover crop treatment was a mixture of cereal rye (92 percent) and daikon radish (8 percent).

“This particular blend was chosen because the daikon radish decomposes after winter terminating, and the cereal rye over winters and continue to scavenge nitrogen until it is terminated, which cause a slow release of nitrate back to the soil,” said Dr. Armstrong, citing his previous research experience.

Cover crop planting occurred early to mid-September using a retrofitted high clearance sprayer, with only a one percent loss in corn crop due to cover crop planting.

Seven key findings of the study:

  • The in-filed practice of cover cropping is significantly more effective in reducing nitrate losses relative to only adjusting nitrogen application timing.
  • On average, the cover crops took up 25 percent (50 lb A-1) of the total nitrate applied.
  • There was a 32-42 percent reduction in drainage during the cover crop growing season.
  • Despite nitrogen application timing, cover crops reduced nitrate loss from tile by 41-49 percent.
  • Cover crop reduced residual nitrate loss via tile in the soybean year by 57 to 67 percent.
  • Cover crops slightly reduced corn yield in the spring system, but not in the fall nitrate system that was not different from spring N without cover crops. Cover crops did not affect soybean yield.


Combining cover crops with bioreactors

A project focused on cover crops was successful in minimizing nutrient loss in tile drained fields.

“Our studies support the idea that nitrate loads can be greatly reduced with a longer rotation and cover crops, and nearly be eliminated when cover crops are combined with a bioreactor,” said Lowell Gentry, principal research specialist at the University of Illinois Urbana-Champaign.

The question remains how to use cover crops and bioreactors in a manner that does not sacrifice yield.  The project used three tile-drained fields that alternated corn-soybean-wheat. Crop rotation with cover crops was: corn, cereal rye, soybean, winter wheat, radish/turnip blend.

Results showed that while cover crops were very successful in reducing tile nitrate loads, there was a loss of yield for corn but not for soybean. Gentry explains, however, that there are several possible reasons for the yield loss in corn:

  • Cooler soil due to cover crop residue
  • Less N availablity due to N immobilization from the poor biomass quality (i.e. wide C/N ratio)
  • Allelopathy from volunteer wheat

“While more research needs to be done, we are finding that the nutrient loss reduction strategies have the potential to work,” Gentry said.

He also noted that N immobilization occurred after the cover crop, and that the two N rate studies (corn after wheat with and without a cover crop) show the economic optimum N rate (EONR) was 60 lbs/A greater for corn after wheat with the cover crop.

Detailed findings from these projects, and additional research about cover crops and nitrate loss, are available at