The rate of soil erosion in the midwestern United States is 10 to 1,000 times faster than pre-agricultural rates, according to a new study from University of Massachusetts researchers.
Home to some of the richest soil in the country, much of the Midwest has been converted from prairie to agricultural fields over the past 160 years. Experts have long warned that the shift has dramatically increased how quickly topsoil wears away, but it’s been difficult to quantify that increase because scientists didn’t know what natural rates of erosion were in the region.
The authors of the new report, published in the journal Geology Wednesday, set out to change that. To do so, they made use of a metallic element known as beryllium-10, which occurs when stars in the Milky Way explode and send high-energy particles, known as cosmic rays, down toward Earth. When the material hits the Earth’s crust, it splits oxygen in the soil apart, leaving tiny trace amounts of beryllium-10 behind.
The scientists sought out those trace bits of the element to determine what erosion rates were like thousands of years ago — a technique previous research has used to calculate historic erosion rates around the world. They headed out to 14 small patches of remaining untouched prairie across Iowa, Minnesota, South Dakota, Nebraska, and Kansas to collect soil samples that contained bits of the material, which date back some 12,000 years to the last Ice Age.
Then, they brought the samples back to their lab in Amherst and sifted out the tiny, ancient bits of beryllium-10. There was “just enough to fit on the head of a pin,” Isaac Larsen, professor of geosciences at UMass Amherst, who worked on the study, said in a statement.
The scientists sent those bits of beryllium-10 to a lab, where experts were able to count its individual atoms. From there, the UMass Amherst team was able to calculate how quickly the soil was eroding when the beryllium-10 fell.
“For the first time, we know what the natural rates of erosion are in the Midwest,” says Caroline Quarrier, the paper’s lead author, who worked on the study as part of her master’s thesis at UMass Amherst. “And because we now know the rate of erosion before Euro-American settlement, we can see exactly how much modern agriculture has accelerated the process.”
The report’s conclusions are staggering. They found that before farmers began tilling land in the Midwest, the median rate of erosion was 0.04 millimeter per year.
That natural erosion rate represents the soil’s equilibrium point, where soil accumulates as quickly as it wears away. Any increase in the pace above that number, the study says, suggests the soil is disappearing faster than it is building up.
Current measured rates of erosion in the Midwest are up to 1,000 times faster than that natural pace, suggesting that soil in those locations is eroding 1,000 times more quickly than it’s accumulating.
Another finding: The US Department of Agriculture says an erosion rate at or below 1 millimeter per year is sustainable, but that’s 25 times greater than the average historic rate the new study documents, suggesting that even a rate of erosion deemed unremarkable by current federal guidelines will inevitably lead to rapid loss of topsoil.
Rapid erosion, if left unchecked, could spell disaster for midwestern crop production. Previous studies suggest it’s already reducing yields. It’s also a problem for the climate, because soil stores planet-warming carbon dioxide, but eroded soil stores less carbon.
But it doesn’t have to be the Midwest’s fate. As the authors note, by implementing more sustainable farming practices, the region could reduce the rate at which topsoil slips away.
“The key is to reduce our current erosion rates to natural levels,” said Larsen. It’s an ambitious goal, but the stakes couldn’t be higher.