In a discovery that has repercussions for everything from domestic agricultural policy to global food security and the plans to mitigate climate change, researchers at the University of Massachusetts recently announced that the rate of soil erosion in the Midwestern US is 10 to 1,000 times greater than pre-agricultural erosion rates. These newly discovered pre-agricultural rates, which reflect the rate at which soils form, are orders of magnitude lower than the upper allowable limit of erosion set by the U.S. Department of Agriculture (USDA).
Soil erosion is the process by which soil is removed from the ground by wind, water, or other natural forces. This can happen gradually over time, or it can occur suddenly as a result of extreme weather events such as heavy rainfall or strong winds. Soil erosion can have numerous negative impacts, including reduced agricultural productivity, increased sedimentation in rivers and streams, and increased flooding. In addition, soil erosion can lead to the loss of valuable nutrients and organic matter, which can reduce the overall fertility of the soil.
The rate of soil erosion can vary depending on a number of factors, including the type of soil, the slope of the land, the amount of rainfall, and the type of vegetation present. In general, soils that are high in organic matter and well-vegetated are more resistant to erosion than soils that are low in organic matter and bare. In addition, gentle slopes are less prone to erosion than steep slopes. However, even on gentle slopes, soil erosion can occur if there is heavy rainfall or other factors that increase the flow of water across the land. The rate of soil erosion can also be affected by human activities, such as deforestation, overgrazing, and poor land management practices. Overall, the rate of soil erosion can vary greatly, and it is important to monitor and manage it to prevent negative impacts.
The study, which appears in the journal Geology, makes use of a rare element, beryllium-10, or 10Be, that occurs when stars in the Milky Way explode and send high-energy particles, called cosmic rays, rocketing toward Earth. When this galactic shrapnel slams into the Earth’s crust, it splits oxygen in the soil apart, leaving tiny trace amounts of 10Be, which can be used to precisely determine average 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,” says Isaac Larsen, professor of geosciences at UMass Amherst and the paper’s senior author. “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.”
This sample was then sent to a lab which counted the individual 10Be atoms, from which Larsen and his colleagues calculated a precise rate of erosion, stretching from the present day all the way 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,” says Caroline Quarrier, the paper’s lead author and 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 numbers are not encouraging. “Our median pre-agricultural erosion rate across all the sites we sampled is 0.04 mm per year,” says Larsen. Any modern-day erosion rate higher than that number means that soil is disappearing faster than it is accumulating.
Unfortunately, the USDA’s current limit for erosion is 1 mm per year — twenty-five times greater than the average rate Larsen’s team found. And some sites are experiencing far greater erosion, disappearing at 1,000 times the natural rate. This means that the USDA’s current guidelines will inevitably lead to rapid loss of topsoil. Therefore, the USDA is working to address soil erosion through a number of programs and initiatives, including conservation programs that help farmers implement practices that reduce erosion on their land and save the topsoil.
Not only is the topsoil crucial for U.S. agriculture — the annual cost of diminished agricultural productivity and environmental degradation due to erosion is estimated to be tens of billion dollars per year — as well as world-wide food security, but most climate-mitigation plans rely heavily on storing carbon in the soil. In addition, soil erosion can also lead to increased water pollution, as the eroded soil can wash into rivers and streams, carrying pollutants with it. This can have a negative impact on the quality of the water in these bodies, making it less suitable for drinking, irrigation, and other uses.
Yet, there’s no reason to despair. “There are agricultural practices, such as no-till farming, that we know how to do and we know greatly reduce erosion,” says Quarrier. “The key is to reduce our current erosion rates to natural levels,” adds Larsen.
Ways to Reduce Soil Erosion
There are several ways to reduce current erosion rates. Some of the most effective strategies include:
Implementing conservation tillage practices
Conservation tillage is a type of farming practice that involves minimal disturbance of the soil, such as no-till or reduced-tillage systems. This can help reduce erosion by keeping the soil in place and allowing vegetation to anchor the soil more effectively.
Planting cover crops
Cover crops are plants that are grown in between regular crop rotations to help protect the soil from erosion. These crops can help reduce erosion by covering the ground and holding the soil in place, as well as by adding organic matter to the soil to improve its structure and ability to retain water.
Using erosion-control structures
Erosion-control structures, such as terraces and sediment basins, can help reduce erosion by slowing the flow of water and trapping sediment before it can wash away.
Implementing riparian buffer zones
Riparian buffer zones are areas of vegetation along the banks of rivers and streams that help filter pollutants and reduce the erosion of soil.
In some cases, erosion can be reduced by enforcing regulations that limit activities that can contribute to erosion, such as logging and construction in sensitive areas.
Implementing these and other strategies can help reduce current erosion rates and protect our soil resources for the future.
This research was supported by the National Science Foundation.