A hilltop completely denuded of A-horizon soil in a field near Guthrie Center, IA. Photo credit: Evan Thaler

University of Massachusetts Amherst – Erosion has depleted carbon-rich topsoil across a third of the United States Midwestern Corn Belt, or 100 million acres, according to a new study from the University of Massachusetts Amherst that indicates the U.S. Department of Agriculture has significantly underestimated the true magnitude of farmland erosion.

In a paper published in the Proceedings of the National Academy of Sciences, researchers led by UMass Amherst graduate student Evan Thaler, along with professors Isaac Larsen and Qian Yu in the department of geosciences, developed a method using satellite imagery to map areas in agricultural fields in the Corn Belt of the Midwestern U.S. that have no remaining A-horizon soil. The A-horizon is the upper portion of the soil that is rich in organic matter, which is critical for plant growth because of its water and nutrient retention properties. The researchers then used high-resolution elevation data to extrapolate the satellite measurements across the Corn Belt and the true magnitude of erosion.

Productive agricultural soils are vital for producing food for a growing global population and for sustaining rural economies. However, degradation of soil quality by erosion reduces crop yields. The study estimates that erosion of the A-horizon has reduced corn and soybean yields by about six per cent, leading to nearly US$3 billion in annual economic losses for farmers across the Midwest.

The A-horizon has primarily been lost on hilltops and ridgelines, which indicates that tillage erosion – downslope movement of soil by repeated plowing – is a major driver of soil loss in the Midwest, according to the research. Notably, tillage erosion is not included in national assessments of soil loss and the research highlights the urgent need to include tillage erosion in the soil erosion models that are used in the U.S. and to incentivize adoption of no-till farming methods.

The research suggests erosion has removed nearly 1.5 petagrams of carbon from hill slopes. Restoration of organic carbon to the degraded soils by switching from intensive conventional agricultural practices to soil-regenerative practices, has potential to sequester carbon dioxide from the atmosphere while restoring soil productivity.

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