N remains in soil, leaks toward groundwater for decades
Nitrogen fertilizer applied to crops lingers in the soil and leaks out as nitrate for decades toward groundwater – "much longer than previously thought," scientists in France and at the University of Calgary say in a new study.
Thirty years after synthetic nitrogen (N) fertilizer had been applied to crops in 1982, about 15 percent of the fertilizer N still remained in soil organic matter, the scientists found.
After three decades, approximately 10 percent of the fertilizer N had seeped through the soil toward the groundwater and will continue to leak in low amounts for at least another 50 years.
The study was led by researcher Mathieu Sebilo at the Université Pierre et Marie Currie in Paris, France, and by Bernhard Mayer in the U of C's Department of Geoscience, and included several research organizations in France.
Their paper, "Long-term fate of nitrate fertilizer in agricultural soils," was published this week in the Proceedings of the National Academy of Sciences of the United States of America.
The findings show that losses of fertilizer N toward the groundwater occur at low rates but over many decades, says Mayer, U of C professor of geochemistry and head of the Applied Geochemistry Group.
That means it could take longer than previously thought to reduce nitrate contamination in groundwater, including in aquifers that supply drinking water in North America and elsewhere, he says.
"There's a lot of fertilizer nitrogen that has accumulated in agricultural soils over the last few decades which will continue to leak as nitrate toward groundwater," Mayer says.
Canada and the U.S. regulate the amount of nitrate allowed in drinking water. In the 1980s, surveys by the U.S. Environmental Protection Agency and the U.S. Geological Survey showed that nitrate contamination had probably impacted more public and domestic water supply wells in the U.S. than any other contaminant.
Mayer is an internationally recognized expert in the use of stable isotopes to track contaminants in the environment.
The French-U of C study is the first that tracks, using stable isotope "fingerprinting," the fate of fertilizer N remaining in the soil zone over several decades.
The research team used a stable isotope of nitrogen, N-15, as a tracer to track fertilizer nitrogen applied in 1982 to sugar beet and winter wheat crops on a pair of two-metre-square plots at a site in France.
Over the 30-year study, the researchers measured the amount of N-15 labelled fertilizer N taken up by plants and they quantified the amount of fertilizer N remaining in the soil.