A study led by researchers at Université de Montréal has quantified the maximum amount of phosphorus that can accumulate in a watershed before additional pollution is discharged into downriver ecosystems.

The calculated average threshold is 2.1 tonnes per square kilometre of land, the researchers estimated in their study, Low buffering capacity and slow recovery of anthropogenic phosphorus pollution in watersheds, published in Nature Geoscience. “Beyond this, further [phosphorus] inputs to watersheds cause a significant acceleration of [phosphorus] loss in runoff.”

This amount is shockingly low, said the researchers, given current nutrient application rates in most agricultural watersheds around the world, tipping points in some cases could be reached in less than a decade.

The study was led by Jean-Olivier Goyette, a doctoral student in biology at UdeM, and supervised by UdeM aquatic ecosystem ecologist Roxane Maranger, in collaboration with sustainability scientist Elena Bennett at McGill University.

Focusing on 23 watersheds feeding the St. Lawrence River in Quebec, the researchers reconstructed historic land-use practices in order to calculate how much phosphorus has accumulated on the land over the past century.

Using Quebec government data, the researchers matched the estimated accumulation with phosphorus concentrations measured in the water for the last 26 years. Since the watersheds they studied had different histories, some had been used intensively for agriculture for decades whereas others were forested and pristine, this method allowed the researchers to establish a gradient of different phosphorus accumulations among sites. In so doing, they were able to see at what point the watershed reached a threshold and began to leak considerably more phosphorus into the water.

“This is a very important finding,” said Bennett. “It takes our farm-scale knowledge of fertilizers and pollution and scales it up to understand how whole watersheds respond within a historical context.”

Agriculture on a mass scale began in Quebec only in the 1950s, but some of the province’s more historical agricultural watersheds had already passed the tipping point by the 1920s, the study found.

Even if phosphorus inputs ceased immediately, eliminating the accumulated phosphorus in saturated Quebec watersheds would take between 100 and 2,000 years, the researchers estimate.

“Are some of our more extreme (agricultural) watersheds impossible to repair?” Maranger asked.
“I can’t answer that. It’s a societal issue, and there are solutions. We should never despair, but it’s a wicked problem.”

In some countries, including China, Canada, and the US, phosphorus is so heavily used now that the saturation point is reached in as little as five years.

“Nutrient management strategies developed using novel creative approaches … are urgently required for the long-term sustainability of water resources,” the researchers urge in their study.

“One possible mitigating measure would be to do what is already being done in some European countries: instead of adding more and more to help plants grow, phosphorus already stored in soils can be accessed using new practices and approaches,” said Goyette. “Furthermore, phosphorus can be recycled and reused as fertilizer rather than accessing more of the raw mined material.”


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