Grassland-to-cropland conversion increased soil, nutrient, and carbon losses in the US Midwest between 2008 and 2016
After decades of declining cropland area, the United States (US) experienced a reversal in land use/land cover change in recent years, with substantial areas in grassland cover converting to cropland in the US Midwest. Although previous studies estimated soil carbon (C) loss due to cropland expansion in the Midwest, other important environmental indicators, such as soil erosion and nitrogen (N) loss, remain largely unquantified. Based on the land use changes derived by Lark et al 2020, we simulated environmental impacts from the conversion of grassland to corn and soybeans using the EPIC (Environmental Policy Integrated Climate) model. Between 2008 and 2016, over 2 Mha of grassland were converted to crop production in the US Midwest, with much less active cropland concomitantly abandoned or retired from production. The net change in cropland-grassland conversion increased soil erosion by 8.0%, N loss by 3.6%, and soil organic C loss by 5.7% relative to existing cropland, despite an associated increase in cropland area of only 2.1%. The net conversion also offsets benefits derived from Conservation Reserve Program (CRP) lands, from negating approximately 5.4% of the greenhouse gas sequestration benefits to almost 20% of the N runoff prevention benefits nationally. Even though corn and soybeans are two common biofuel feedstock crops, the results are not solely attributable to biofuel driven land use change, since other factors such as global food demand and increased food prices are also key drivers. In addition, the above estimates represent the worst-case scenario of converting unmanaged grassland to tilled corn and soybeans. Notably, the substantial benefit of perennial cover, such as for cellulosic feedstocks, and reduced tillage practices suggest that environmental degradation from cropland expansion could be attenuated if managed using either of these practices. Further, additional conservation measures, such as CRP set-asides, are likely necessary to counterbalance the soil quality impacts shown here, particularly in areas with high rates of grassland conversion (e.g., the Dakotas, or southern Iowa).