Targeted grassland restoration increases potential for reduced nutrient export in the Upper Mississippi River Basin

Abstract Agriculture remains a leading cause of poor water quality in the Upper Mississippi River Basin (UMRB), despite ongoing nutrient reduction strategies and incentivized conservation programs. Converting agricultural land to grasslands offers a potential pathway toward achieving nutrient reduction goals for this region. We evaluated whether conversion of agriculture to grassland four decades ago could have lowered contemporary nutrient loads at the basin outlet. Using an integrated modeling framework, we assessed how the extent and spatial configuration (random vs targeted) of land conversion affected nitrogen and phosphorus loading and identified the levels of conversion needed to meet U.S. Environmental Protection Agency goals for nutrient loading. Model results indicate a 45% nitrogen load reduction could have been achieved in 20 years after implementation in 1981 with a targeted 53% conversion of agricultural land, whereas a 45% phosphorus load reduction would require highly extensive conversion of agricultural land and 40+ years, or additional nutrient-reduction practices alongside transformative land-cover change. These simulations demonstrate that earlier and more extensive grassland restoration may substantially reduce nutrient loads, with the targeted approach outperforming random conversion. While a targeted approach offers greater water quality gains and potential for greater return on investment for funded programs, a random approach may be more feasible across large watersheds due to lower social-ecological coordination demands. Nonetheless, achieving UMRB water quality goals by mid-century will require immediate, transformative changes in land use and nutrient management, and support from targeted government policies.