Nitrates linger decades in cropland soil

A 30-year study of nitrogen fertilizer use in France suggests that nitrate from the fertilizer lingers in the soil for decades and is likely to continue both being taken up by plants and leaking into groundwater for decades to come.

The research was published Oct. 21 in the online edition of the Proceedings of the National Academy of Sciences. Read the research paper. Read a Los Angeles Times article about the study.

Nitrogen with an isotopic marker was applied from 1982 through 2012 to French farmland cropped in a rotation of sugar beets and winter wheat.

Over that time, the research found:

  •   61 percent to 65 percent of the nitrogen was taken up the crops.
  •  12 percent to 15 percent was still in the soil at the conclusion of the study.
  •   8 percent to 12 percent had leaked into groundwater.

The longevity of nitrogen fertilizer in the soil is an important issue in Minnesota as the state Pollution Control Agency attempts to reduce the flow of nitrates into rivers and streams and the state Department of Agriculture attempts to track and reduce nitrate pollution of groundwater.

An MPCA report last summer estimated that mineralization of nitrogen in the soil accounts for 1.73 billion pounds per year – 36 percent – of the nitrogen inputs to cropland in Minnesota. That was slightly less than the total nitrogen input from chemical fertilizers and manure.

The same report estimated that about 6 percent of all those nitrogen inputs to cropland end up flowing into surface waters – mostly through tile lines or the discharge of groundwater to streams – each year.

Another MPCA draft report, released Oct. 7, sets a goal of reducing the flow of  nitrogen down the Mississippi River to the Gulf of Mexico by 20 percent by 2025.