The Extent of Soil Loss Across the US Corn Belt
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The extent of soil loss across the US Corn Belt Evan A. Thalera,1, Isaac J. Larsena, and Qian Yua aDepartment of Geosciences, University of Massachusetts, Amherst, MA 01003 Edited by David Tilman, University of Minnesota System, St. Paul, MN, and approved December 29, 2020 (received for review December 20, 2019) Soil erosion in agricultural landscapes reduces crop yields, leads to remains a major gap in soil erosion research (19). Large-scale loss of ecosystem services, and influences the global carbon cycle. assessments of soil erosion are often based on model predictions Despite decades of soil erosion research, the magnitude of histor- (20–22) or qualitative information from soil surveys regarding ical soil loss remains poorly quantified across large agricultural the degree of soil degradation (23). In the United States, for ex- regions because preagricultural soil data are rare, and it is challeng- ample, nationwide soil loss trends (24) are simulated using water ing to extrapolate local-scale erosion observations across time and and wind erosion models that have been calibrated with erosion space. Here we focus on the Corn Belt of the midwestern United measurements made on small plots over a period of decades (21, States and use a remote-sensing method to map areas in agricul- 25). It has been debated whether upscaling such predictions to tural fields that have no remaining organic carbon-rich A-horizon. regional or national scales results in an accurate assessment of the We use satellite and LiDAR data to develop a relationship between current magnitude of soil loss in the United States (26, 27). A-horizon loss and topographic curvature and then use topographic Whereas such models are useful for assessing relative rates of 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 erosion for soil conservation planning, the soil loss predictions do lost A-horizon soil and that prior estimates of soil degradation from not provide information regarding the cumulative soil loss that has soil survey-based methods have significantly underestimated occurred since the initiation of cultivation, and hence the overall A-horizon soil loss. Convex hilltops throughout the region are often magnitude of agricultural soil degradation. completely denuded of A-horizon soil. The association between soil To assess the degree of cumulative soil degradation, soil sur- loss and convex topography indicates that tillage-induced erosion is veys conducted by the US Department of Agriculture have an important driver of soil loss, yet tillage erosion is not simulated in assigned erosion classes to soils based on the percentage of the models used to assess nationwide soil loss trends in the United original A horizon that has been eroded (28). Because the A States. We estimate that A-horizon loss decreases crop yields by horizon has the largest fraction of soil organic carbon within the ENVIRONMENTAL SCIENCES 6 ± 2%, causing $2.8 ± $0.9 billion in annual economic losses. Re- soil profile, it is a key component of water and nutrient retention gionally, we estimate 1.4 ± 0.5 Pg of carbon have been removed and soil productivity (29). Soils where 100% of the A-horizon from hillslopes by erosion of the A-horizon, much of which likely thickness has been removed are designated as Class 4 eroded remains buried in depositional areas within the fields. soils, and other classes represent lesser reductions in A-horizon thickness (<25%, 25 to 75%, and >75% for Classes 1, 2, and 3, soil erosion | remote sensing | soil organic carbon | agricultural productivity respectively). A major disadvantage of the use of erosion classes is that properly assigning classes based on the percentage of roductive agricultural soils are vital for producing food for a A-horizon loss requires accurate determination of the original Pgrowing global population (1–3). However, degradation of A-horizon thickness on all topographic positions (30). Hence, soil quality by erosion reduces crop yields, which can result in although soil erosion classes indicate soil degradation is wide- food insecurity, conflict (3), and the decline of civilizations (4). spread (23) we do not have a robust, quantitative understanding Degradation of soils leads not only to economic losses for farmers of how much soil has already been lost. but also a loss in ecosystem services (5), which alters the ability of soils to regulate hydrologic and biogeochemical cycles. Wide- Significance spread use of synthetic fertilizers to enhance the function of de- graded soils increases food production costs (6) and impairs water Conventional agricultural practices erode carbon-rich soils that resources (7), which negatively impacts human health (8) and are the foundation of agriculture. However, the magnitude of aquatic ecosystems (9). A-horizon soil loss across agricultural regions is poorly con- Globally, the reservoir of carbon stored in soils is three times strained, hindering the ability to assess soil degradation. Using that in the atmosphere (10) and given the extent of agricultural a remote-sensing method for quantifying the absence of land use (11), understanding soil carbon dynamics in agricultural A-horizon soils and the relationship between soil loss and to- systems is critical to understanding the carbon cycle (12). Whether pography, we find that A-horizon soil has been eroded from soil erosion constitutes a net carbon sink or source depends on roughly one-third of the midwestern US Corn Belt, whereas both the depositional fate of the eroded carbon and the ability to prior estimates indicated none of the Corn Belt region has lost – replace carbon in degraded soils (13 15). If biological productivity A-horizon soils. The loss of A-horizon soil has removed 1.4 ± replaces eroded carbon, and decomposition of carbon stored in 0.5 Pg of carbon from hillslopes, reducing crop yields in the sedimentary deposits is halted or slowed, then soil erosion is a net study area by ∼6% and resulting in $2.8 ± $0.9 billion in annual sink of atmospheric carbon (14–17). However, if eroded carbon economic losses. rapidly decomposes and is not replaced in eroded soil horizons, then soil erosion constitutes a carbon source. Restoring carbon to Author contributions: E.A.T., I.J.L., and Q.Y. designed research; E.A.T. performed research; degraded soils therefore has potential to both reestablish soil E.A.T. analyzed data; and E.A.T., I.J.L., and Q.Y. wrote the paper. The authors declare no competing interest. function and sequester atmospheric CO2 (10). However, quanti- fying the impacts of soil degradation on agricultural productivity This article is a PNAS Direct Submission. and the carbon cycle first requires robust estimates of the mag- Published under the PNAS license. nitude of agriculturally induced soil loss (14, 16). 1To whom correspondence may be addressed. Email: [email protected]. Although thousands of soil erosion measurements have been This article contains supporting information online at https://www.pnas.org/lookup/suppl/ made globally (18), the lack of a robust and scalable method for doi:10.1073/pnas.1922375118/-/DCSupplemental. estimating the magnitude of erosion in agricultural landscapes Published February 15, 2021. PNAS 2021 Vol. 118 No. 8 e1922375118 https://doi.org/10.1073/pnas.1922375118 | 1of8 Downloaded by guest on September 27, 2021 Here we present results from a remote sensing method used to thick A-horizon soils (SI Appendix, Fig. S1 and Table S1). In the estimate the spatial extent of agriculturally induced loss of decades following European settlement, the prairie was plowed, A-horizon soil for a major global agricultural region, the Corn Belt and the landscape was rapidly and extensively converted to row- of the midwestern United States. Rather than simulate or measure crop agriculture. For example, in Iowa, Indiana, and Illinois, less short-term soil loss rates, we combine measurements of soil- than 0.1% of the original tallgrass prairie remains (32). The fertile surface reflectance in the visible spectrum (soil color) with high- soils and temperate climate make the Corn Belt one of the world’s resolution satellite imagery to directly measure the proportion of most agriculturally productive regions. The United States is the the agriculturally cultivated landscape that has completely lost its world’s largest producer of corn and soybeans (34), and 75% of original A horizon. Combining our spectral analysis with rela- the corn and 60% of the soybeans produced in the United States tionships between A-horizon soil loss and topography derived are grown in the Corn Belt (35). from high-resolution LiDAR topographic data allows us to predict Despite the importance of the region’s agricultural produc- A-horizon soil loss in areas where images are not available. We tivity, model predictions indicate the Corn Belt currently has the find that historical soil erosion has completely removed A-horizon highest soil erosion rates in the United States (24). The historical soil from approximately one-third of the Corn Belt. The spatial magnitude of A-horizon soil loss from the initiation of agricul- patterns of soil loss suggest that key erosion mechanisms are not ture to the present is unknown, but prior work in Iowa and simulated in nationwide assessments of soil erosion trends in the Minnesota noted that in some areas the magnitude of soil ero- United States and that soil survey data greatly underestimate the sion has been great enough to completely remove dark, carbon- extent of A-horizon loss. rich A-horizon soil, exposing light-colored B-horizon soil that is poor in organic carbon (36, 37). The Corn Belt Region of the Midwestern United States Our study focuses on the midwestern United States, on a A-Horizon Soil Loss in Individual Fields ∼ 2 390,000-km region that encompasses much of the area collo- We combined high-resolution satellite imagery, a newly devel- quially known as the Corn Belt (Fig.