Evan A. Thaler,  Isaac J. Larsen, and Qian Yu

PNAS February 23, 2021 118 (8) e1922375118

Significance

Conventional agricultural practices erode carbon-rich soils that are the foundation of agriculture. However, the magnitude of A-horizon soil loss across agricultural regions is poorly constrained, hindering the ability to assess soil degradation. Using a remote-sensing method for quantifying the absence of A-horizon soils and the relationship between soil loss and topography, we find that A-horizon soil has been eroded from roughly one-third of the midwestern US Corn Belt, whereas prior estimates indicated none of the Corn Belt region has lost A-horizon soils. The loss of A-horizon soil has removed 1.4 ± 0.5 Pg of carbon from hillslopes, reducing crop yields in the study area by ∼6% and resulting in $2.8 ± $0.9 billion in annual economic losses.

Abstract

Soil erosion in agricultural landscapes reduces crop yields, leads to loss of ecosystem services, and influences the global carbon cycle. Despite decades of soil erosion research, the magnitude of historical soil loss remains poorly quantified across large agricultural regions because preagricultural soil data are rare, and it is challenging to extrapolate local-scale erosion observations across time and space. Here we focus on the Corn Belt of the midwestern United States and use a remote-sensing method to map areas in agricultural fields that have no remaining organic carbon-rich A-horizon. We use satellite and LiDAR data to develop a relationship between A-horizon loss and topographic curvature and then use topographic data to scale-up soil loss predictions across 3.9 × 10⁵ km² of the Corn Belt. Our results indicate that 35 ± 11% of the cultivated area has lost A-horizon soil and that prior estimates of soil degradation from soil survey-based methods have significantly underestimated A-horizon soil loss. Convex hilltops throughout the region are often completely denuded of A-horizon soil. The association between soil loss and convex topography indicates that tillage-induced erosion is an important driver of soil loss, yet tillage erosion is not simulated in models used to assess nationwide soil loss trends in the United States. We estimate that A-horizon loss decreases crop yields by 6 ± 2%, causing $2.8 ± $0.9 billion in annual economic losses. Regionally, we estimate 1.4 ± 0.5 Pg of carbon have been removed from hillslopes by erosion of the A-horizon, much of which likely remains buried in depositional areas within the fields.

See: https://www.pnas.org/content/118/8/e1922375118

Figure 2:

Calculation of A-horizon loss and topographic curvature. (A) True-color image of a field near Clear Lake, IA (open square in Fig. 1); black polygons surround light-colored areas predicted to have exposed B-horizon soil. Gray polygons indicate ±1 SD of the predicted area of B-horizon exposure. (B) Topographic curvature map where red pixels denote negative curvature (convex hillslopes) and blue pixels denote positive curvature (concave hillslopes). (C) Cumulative fraction of area with curvature values for the entire field (black line) and curvature values for the area with exposed B-horizon soil (gray line); 34 ± 7% of the area is exposed B horizon soil. Gray shading shows the ±1 SD prediction of B-horizon exposure. (D) Mean (±1 SD) of the fraction of pixels with a spectral signature of B-horizon soils versus topographic curvature, shaded by median topographic slope per curvature bin, which shows that exposure of B-horizon soil occurs predominantly on convex topography. (Inset) The probability density of topographic slope values for A-horizon pixels and for B-horizon pixels; gray shading shows the ±1 SD prediction of B-horizon exposure. Image credit: ©2013, DigitalGlobe; NextView License/Maxar, Inc.