A recent study led by the University of Massachusetts Amherst has found that the rate of soil erosion in the Midwestern U.S. is 10 to 1,000 times greater than pre-agricultural erosion rates. These newly discovered pre-agricultural erosion rates –reflecting the rates at which soils form – are much lower than the upper allowable limit of erosion set by the U.S. Department of Agriculture (USDA). These findings could have major repercussions for everything from domestic agricultural policy to global food security and climate change mitigation strategies.
The scientists made use of a rare element beryllium-10 (10Be), which forms when stars in our galaxy explode and send high-energy particles – called cosmic rays – rocketing towards the Earth. When this galactic shrapnel smashes into our planet’s crust, it splits oxygen in the soil apart, leaving small trace amounts of 10Be, which can then be used to determine average soil erosion rates over the span of thousands to millions of years.
“We went to fourteen small patches of remnant native prairie that still exist in Iowa, Minnesota, South Dakota, Nebraska, and Kansas, and used a hand auger to collect deep soil cores, in material that dates back to the last Ice Age,” said study senior author Isaac Larsen, a professor of Geosciences at UMass Amherst. “We brought this soil back to our lab at UMass, sifted it to isolate individual sand grains, removed everything that wasn’t quartz, and then ran these few spoonfuls through a chemical purification process to separate out the 10Be – which was just enough to fit on the head of a pin.”
The researchers then sent this sample to a laboratory, where the individual 10Be atoms were counted in order to calculate a precise rate of erosion, stretching from the present day back to the last Ice Age (about 12,000 years ago). “For the first time, we know what the natural rates of erosion are in the Midwest,” said study lead author Caroline Quarrier, who completed this research 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 results were far from encouraging. “Our median pre-agricultural erosion rate across all the sites we sampled is 0.04 mm per year,” Larsen reported. Any modern-day erosion rate higher than that value means that soil is disappearing at a faster pace than it is accumulating.
Unfortunately, USDA’s current limit for erosion (one millimeter per year, so 25 times greater than the average rate the researchers found) will inevitably lead to rapid loss of topsoil, which plays a critical role not only for U.S. agriculture, but also for global food security, as well as climate change mitigation strategies that rely on soil as an important carbon sink.
However, according to the scientists, there is no reason to despair yet. “There are agricultural practices, such as no-till farming, that we know how to do and we know greatly reduce erosion,” Quarrier said. “The key is to reduce our current erosion rates to natural levels,” Larsen concluded.
The study is published in the journal Geology.
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