Nitrogen fertiliser remains in soils and leaks towards groundwater for decades – study

Nitrogen fertiliser applied to crops lingers in the soil and leaks out as nitrate for decades towards groundwater”much longer than previously thought,” according to scientists in France and at the University of Calgary.

They found that 30 years after synthetic nitrogen (N) fertiliser had been applied to crops in 1982, about 15 per cent of the fertiliser N still remained in soil organic matter.

After three decades, approximately 10 per cent of the fertiliser N had seeped through the soil towards 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 University of Calgary’s Department of Geoscience, and included several research organizations in France.

Their paper, “Long-term fate of nitrate fertiliser in agricultural soils,” was published this month in the Proceedings of the National Academy of Sciences of the United States of America.

The findings show that losses of fertiliser N towards the groundwater occur at low rates but over many decades, says Mayer, University of Calgary 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 fertiliser nitrogen that has accumulated in agricultural soils over the last few decades which will continue to leak as nitrate towards groundwater,” Mayer says.

Canada and the US regulate the amount of nitrate allowed in drinking water. In the 1980s, surveys by the US Environmental Protection Agency and the US Geological Survey showed that nitrate contamination had probably impacted more public and domestic water supply wells in the USA than any other contaminant.

Mayer is an internationally recognised expert in the use of stable isotopes to track contaminants in the environment.

The new study is the first that tracks, using stable isotope “fingerprinting,” the fate of fertiliser 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 fertiliser 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 fertiliser N taken up by plants and they quantified the amount of fertiliser N remaining in the soil.

The novel aspect of their study was that they subsequently determined the long-term fate of this fertiliser N ‘pool’ retained in the soil. Their measurements of seepage water from locations two metres deep in the soil revealed the amount of fertiliser nitrate leaking towards the groundwater.

The team found that 61-65 per cent of the N-15 fertiliser applied in 1982 was taken up by the sugar beet and wheat plants over the 30-year study.

However, 32-37 per cent of the fertiliser N remained in the soil organic matter in 1985 or three years after application, while 12-15 per cent still lingered in the soils after three decades.

Between 8-12 per cent of the fertiliser N applied in 1982 had leaked in the form of nitrate toward groundwater during the 30 years, and will continue to leak at low rates “for at least another five decades, much longer than previously thought,” the study says.

The scientists predict that about 15 per cent of the initially applied fertiliser N will be exported from the soils towards the groundwater over a time span of almost one century after the 1982 fertiliser application.

Mayer speculates that if the same research were done in Alberta, the findings would be similar in terms of fertiliser uptake by plants and nitrogen retention in the soils, although Alberta’s comparatively dry climate and different geology might slow the rate of nitrate seeping towards the groundwater.

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