Nitrogen From Farm Fertilizers Remains in Soil and Could Pollute Drinking Water for Decades

Corn Field in Iowa
Corn fields spanning nearly 14 million acres in Iowa require plenty of fertilizer that provides nitrogen to increase crop yields. Over time, excess nitrogen is mineralized as nitrate, which then leaches into water. Reuters/Jason Reed

Des Moines, Iowa’s water utility is suing three rural counties in federal court, claiming they violated the Clean Water Act by polluting the river from which the city gets its drinking water with nitrates present in agricultural runoff that have been linked to cancer and birth defects.

It’s possible Des Moines Water Works is just playing the clean water card to get out of spending millions a year to filter the nitrates. After all, currently, agricultural runoff is exempt from federal clean water laws. But if the lawsuit is successful, the agriculture industry will have to limit the runoff before it enters the river as would any other industry.

A study published this week in a special issue of the journal Environmental Research Letters indicates a win could be more relevant to public health than ever. The researchers from Canada’s University of Waterloo found that even if farmers completely stop applying nitrogen fertilizers to crops today, nitrate levels in rivers and lakes would remain high and pose a threat to drinking water for dozens of years to come.

“The major implication is that nitrogen in the soil will continue to be a source of contamination to drinking water as well as surface water bodies like the Mississippi River and Gulf of Mexico for decades,” says Kimberly Van Meter, the study’s lead author and a doctoral student at the University of Waterloo.

Nitrates in drinking water have been linked to thyroid cancer, blue baby syndrome in infants and birth defects. Ecosystems are at risk too, according to Nandita Basu, a water sustainability and ecohydrology expert at the University of Waterloo. “Too much concentration of nitrates in coastal areas deprives them of oxygen. This causes hypoxia which creates a dead zone with large areas void of animal or plant life,” she says.

While scientists generally acknowledge the presence of nitrates in groundwater, this new study identifies nitrogen buildup in agricultural soils above the groundwater table. That’s what spells danger, according to Basu: “The fact that nitrogen is being stored in the soil means it can still be a source of elevated nitrate levels long after fertilizers are no longer being applied.”

The team believes their data presents the first direct evidence of a large-scale nitrogen legacy across the United States' Mississippi River Basin. The team compared data on nitrogen levels in soil that had been sampled in Iowa in the 1940s with levels in soil sampled between 2007 and 2008. It learned that nitrogen had diminished in the top 10 inches of soil but that it had accumulated from a point about 10 inches beneath ground level extending down to a depth of 3.2 feet. The team then turned to data from the National Cooperative Soil Survey, a partnership that investigates and documents information about soil across the U.S. They analyzed long-term data from over 2,000 soil samples taken throughout the Mississippi River Basin and again concluded nitrogen to be accumulating as much as three feet beneath ground level.

Once the research team identified nitrate below groundwater levels, they set out to predict how long it would remain there. They developed a model to assess how the landscape is likely to process nitrogen under different historic agricultural scenarios based on what they know about how nitrogen is mineralized. When fertilizer is applied to farm fields, some gets taken up by crops, some gets dissolved in water and some gets converted to nitrogen gas by microorganisms. The excess that becomes part of organic matter in the soil is not harmful in and of itself, according to Basu. In fact, it makes the soil more fertile; but over time, excess nitrogen is mineralized as nitrate, which then leaches into water.

The researchers used Rooks County, Kansas as a case study to investigate nitrogen depletion and accumulation when different land management techniques were present and before and after cultivation. The researchers’ modeling indicates that excess nitrogen could still be leaching into waterways 35 years after the fertilizer was applied to fields. A previous study led by Van Meter and Basu found that nitrate levels never returned to normal in a small Iowa watershed that was converted back to native prairie, even when fertilizer application had completely ceased. These results led them to consider more seriously the idea of nitrogen legacies in agricultural watersheds.

“A large portion of the nitrogen applied as fertilizer has remained unaccounted for over the last decades,” says Basu. “The presence of this ‘legacy nitrogen’ means it will take even longer for best management practices to have a measurable benefit. If we're going to set policy goals, it's critical we quantify nitrogen legacies and time lags in human impacted landscapes.”