Farm fertilizers can accumulate in soil for up to 30 years or more, according to new University of Waterloo research. Scientists warned of a “biochemical legacy” that sees commercial fertilizers leach into groundwater, lakes and streams.
“You are growing crops and applying fertilizers and the nitrogen is accumulating in the soil,” said Kim Van Meter, a Waterloo doctoral student and co-author of the research Nitrogen Legacy: Emerging Evidence Of Nitrogen Accumulation In Anthropogenic Landscapes. “When you get rain or mineralization of the soil over time, nitrogen begins leaching from fields in the form of nitrates and can move into groundwater, into the rivers, the lakes and coastal areas.”
Van Meter noted Canadian drinking water standards set a safe level of 10 milligrams of nitrogen per litre: There are definitely places in Canada that go beyond that level,” she said. “Everything flows within a watershed.”
Farmers nationwide doubled their use of commercial nitrogen in the period from 1981 to 2011, according to Statistics Canada data, though the number of individual farms fell by half to fewer than 100,000 over the same period. StatsCan attributed the increased usage to gains in seeded acreage. Some 61.5 million acres of farmland are treated with fertilizers each year.
“Our general understanding had been there was some lag time as nitrates are dissolved into groundwater, but this study basically says it is going to be a lot longer than we’d assumed,” said Prof. Nandita Basu of Waterloo’s Department of Earth and Environmental Sciences. “It is tough to say how long – we don’t have a good handle of the science yet – but it could be thirty, forty or even fifty years.”
The findings were published in the periodical Environmental Research Letters. “Our study for the first time links multiple lines of evidence to show convincingly that nitrate, like phosphorus, has a bio-geochemical legacy, a legacy that complicates our previous understanding of the fate of this nutrient in anthropogenic landscapes and that must be accounted for in intervention efforts to improve water quality,” Nitrogen Legacy said.
Researchers used more than 2,000 soil samples from farm fields along the U.S. Mississippi River basin. Scientists found trace nitrates in soil to a depth of up to 39 inches.
The Commons environment committee in a 2014 study Great Lakes Water Quality identified farm runoff as a pollution source for the St. Lawrence River and lake waters in southwest Ontario. “What we really need to do is start managing the Great Lakes as ecosystems and manage them more holistically,” Dr. William Taylor, University of Waterloo professor emeritus, earlier told committee hearings; “It really takes a much more complex approach to the problem than just more or less phosphorus than what we are currently allowing.”
By Kaven Baker-Voakes