American Soil Losing More Nutrients for Crops Due to Heavier Rainstorms, Study Shows

Phosphorus, a nutrient in soil essential for sustaining most forms of life, is increasingly disappearing from land as it is washed into waterways throughout the United States, according to a new study led by researchers at Penn State.

The study, published Nov. 18 in the Proceedings of the National Academy of Sciences, analyzed data from 430 rivers across the U.S. and found that phosphorus loss from agricultural lands has increased over the past four decades, despite efforts to reduce it. This loss of phosphorus can potentially lead to decreased crop yields, which could drive up the cost of food, the researchers explained.

"We've seen from recent weather events that water quantity, the amount of water that storms and waterways carry, can lead to dangerous flooding and mudslides," said Li Li, professor of civil and environmental engineering at Penn State and corresponding author on the study.

"What we wanted to understand is what happens to the land when these storms pull the subsurface of the soil into rivers and streams. What we found is an alarming loss of this finite element that lets soil sustain life."

Phosphorus plays an essential role in various biological processes, like creating DNA structure and facilitating the energy transfer between cells, Li explained. But unlike nitrogen, another critical soil nutrient, phosphorus is a non-renewable resource with limited geological deposits, meaning that once it moves from land to water, it can't get back into the land.

"This is a problem for many reasons," Li said.

"When it's lost from farmland, it can reduce crop yields, potentially leading to higher food prices. Then, when phosphorus enters our waterways, it can cause harmful algal blooms that can make water unsafe for swimming and fishing—and can even reduce oxygen levels in water and kill fish and other aquatic life. It also makes it more expensive to treat drinking water, which means higher water bills for all of us."

The study used a sophisticated deep learning model to analyze four decades of data, from 1980 to 2019, from 430 rivers throughout the contiguous United States.

It revealed that while 60% of the rivers studied showed declining levels of phosphorus, the overall amount of phosphorus flowing into rivers has increased. Agricultural areas are the biggest contributors to the problem, with phosphorus levels increasing in most rivers near agricultural areas, even as they decline in rivers near urban areas.

"This suggests that efforts to control phosphorus pollution from sources like agricultural runoff are not as effective as we thought," Li said.

"But declining levels of phosphorus, particularly in rivers flowing through urban areas, suggests that efforts to control phosphorus pollution from point sources such as wastewater treatment plants are working."

Despite these targeted efforts, Li said the overall increased amount of phosphorus flowing into rivers is due to the increased frequency of extreme weather events that lead to heavier rainfall and higher river flows—with that increased flow comes more phosphorus.

"This means that even though we're doing a better job at limiting phosphorus pollution from urban areas, the problem is getting worse overall, due to factors largely beyond the control of just one region," Li said. "This is a problem that is tied to climate change."

Going forward, the study's authors said phosphorus pollution from agriculture needs better prevention and mitigation, which will be a challenge, especially as wetter storms drive increased rainfall and river flows. They said that this will likely require a combination of new technologies and changes to farming practices.