Fertilizer applied today could still be there in 2093, scientists report October 21 in the Proceedings of the National Academy of Sciences. The study is the first to measure how long nitrogen-based fertilizers persist in the field.
In 1982, a team led by Mathieu Sebilo of the Université Pierre et Marie Curie in Paris infused soil with fertilizer containing nitrogen-15, a form of the element uncommon in nature. The next year, the researchers found that crops had absorbed around half of the fertilizer, with most of the rest taken up by microorganisms and released back into the soil.
For the next 30 years, the scientists grew sugar beets and winter wheat and fertilized them with compounds containing the more common nitrogen-14. They found that nitrogen-15 in the soil decreased slowly, with small amounts taken up by plants or leached into groundwater each year. In 2010, 12 to 15 percent of the original nitrogen-15 remained in the soil; the researchers calculate that it will take at least another 50 years for it to disappear entirely.
The finding helps explain why nitrogen pollution can continue in rivers even when farmers in the area reduce their fertilizer use.