Fertilizer Then, Equals Water Pollution Now

A study led by researchers at the Université de Montréal determined for the first time what maximum amount of nutrients - phosphorus more accurately - can accumulate in a watershed before reaching aquatic ecosystems.

Phosphorus, a component of fertilizers, is essential for plant growth. But the mineral is also harmful when overused. Once in surface waters, it can cause excessive growth of plants in lakes and rivers, as well as a proliferation of toxic algae.

The average critical threshold established by researchers in their study published today in Nature Geoscience is 2.1 tonnes per square kilometer of land. Beyond this limit, additional inflows into watersheds cause a marked acceleration of the nutrient in the runoff.

This amount is extremely low, according to the researchers. Indeed, given the current rate of nutrient use around the world, this saturation threshold could be reached in some cases in less than a decade.

"You have to look at the earth like a sponge," says Roxane Maranger, a professor in UdeM's Department of Biological Sciences. After a while, a sponge that absorbs too much water starts to leak. In the case of phosphorus, the earth absorbs it year after year and, in the long term, its absorption capacity is reduced. That's when historical phosphorus inputs contribute more to what reaches our waters. "

The study was conducted by Jean-Olivier Goyette, PhD student in biology at UdeM supervised by Ms.  Maranger and Elena Bennett, professor at McGill University.

Twenty-three watersheds examined

In order to calculate the amount of phosphorus accumulated over the last century, the researchers reconstructed the historical land use practices of 23 watersheds that feed the St. Lawrence River.

The two main sources of phosphorus in the lands drained by tributaries of the St. Lawrence come from agriculture (fertilizers and animal manure) and other human activities (including wastewater).

Using Québec government data, the researchers compared the estimated accumulation with the levels of phosphorus found in the water for 26 years. Since the watersheds studied had diverse historical backgrounds - some of which have been intensively exploited for decades and others are relatively undisturbed by human activities - this methodology has allowed researchers to establish a gradient of phosphorus accumulation in different watersheds. places examined. Thus, they have been able to observe the accumulation threshold beyond which phosphorus is more easily transferred to surface water.

"A very important contribution"

"This is a very important discovery," says Elena Bennett. We use our knowledge of fertilizer and farm-scale pollution at the scale of the farm and apply it on a large scale to understand the historical evolution of watersheds. "

The study also reveals that even though intensive agriculture began only in the 1950s in Quebec, some of the watersheds had passed the critical threshold 30 years earlier, in the 1920s.

The researchers believe that, even if the surplus of phosphorus supply ceases immediately, it would take 100 to 2000 years to eliminate the phosphorus accumulated in the saturated watersheds of Quebec.

In some countries, such as China or the United States, and elsewhere in Canada, the current use of phosphorus is so massive that the saturation point could be reached within 5 years. "Nutrient management strategies incorporating new creative approaches [...] are urgently needed to ensure the sustainability of our water resources," urge researchers in their study.

Recycle and reuse

"Some European countries have already adopted mitigation measures," says Jean-Olivier Goyette. So, instead of adding more phosphorus to help plant growth, we can access the phosphorus already stored in the soil by using new approaches. In addition, we can recycle and reuse phosphorus as a fertilizer instead of producing more. "

The dilemma is that food production requires phosphorus, but an excess of this mineral pollutes water when it leaves the watershed and contaminates adjacent aquatic ecosystems.

"Can some of our poorer agricultural watersheds be restored? asks Roxane Maranger. Difficult to answer, since it is a problem of society. There are solutions and we must not despair, but it is a major issue. "

This article has been republished from materials provided by the University of Montreal. Note: material may have been edited for length and content. For further information, please contact the cited source.

Reference
"Low buffering capacity and slow recovery of anthropogenic phosphorus pollution in watersheds", by J.-O. Goyette, E. M. Bennett and R. Maranger, was published on October 8, 2018 in Nature Geoscience. doi: 10.1038 / s41561-018-0238-x.