Assessing sustainable soil loss by quantifying natural erosion rates in agricultural landscapes of the Midwestern U.S.

Abstract:

Nutrient-rich soil is the foundation of sustainable and resilient food systems. The long-term viability of agriculture requires maintaining productive soils by reducing agricultural erosion to levels that match natural rates of erosion and soil formation. Quantifying soil formation and erosion is therefore necessary in order to determine sustainable rates of soil loss. The USDA assigns soil loss tolerance values to farmland, which delimits the maximum amount of soil erosion that can still sustain agricultural productivity indefinitely. However, there is a dearth of quantitative data regarding rates of natural erosion and soil formation in agricultural landscapes, making it difficult to assess whether established soil loss tolerance values lead to sustainable soil management. Here we quantify natural erosion rates in the Midwestern U.S., one of the world’s most productive and economically important agricultural regions. We use the cosmogenic nuclide ¹⁰Be to quantify pre-agricultural soil erosion rates averaged over millennial timescales. We measured ¹⁰Be concentrations in samples from 3- to 4-meter deep soil profiles collected from 14 native (i.e. undisturbed) prairies throughout the Midwest. Preliminary results from a subset of the sites indicate natural erosion rates range from of 0.02 to 0.04 mm yr^-1. The presence of thick, organic-rich prairie soils in the Midwest indicates that these low rates of natural erosion promote the formation of fertile topsoil. Soil loss tolerance values assigned to these locations are 1 mm yr^-1 – nearly two orders of magnitude higher than the natural erosion rates we measure – and agricultural erosion often exceeds tolerable levels. Hence, as currently defined, tolerable soil loss rates will lead to the depletion of Midwestern soils. Quantifying natural erosion rates with cosmogenic nuclides offers a means for developing scientifically-based values of soil loss tolerance and a baseline against which to compare anthropogenic erosion rates.