Advancing circular nutrient economy to achieve benefits beyond nutrient loss reduction in the Mississippi/Atchafalaya River basin

S ince the establishment of the US Hypoxia Task Force (HTF) in 1997, billions of dollars have been invested in Nutrient Reduction Strategy (NRS) implementation in the Mississippi and Atchafalaya River basins (MARB) to reduce the Gulf of Mexico hypoxic zone size to less than 5,000 km2 (1,930 mi2) by 2035 (USEPA 2022). However, after 25 years of continuous efforts, substantial improvement in water quality has yet to be achieved. The largest hypoxic zone measured was 22,730 km2 (8,776 mi2) in 2017, more than four times the targeted goal (NOAA 2022). Farmers’ adoption of best management practices proposed by state NRS and collaboration among diverse stakeholders are vital to achieving the HTF goals because the majority of nutrient pollution is from agricultural sources (USEPA 2022; Robertson and Saad 2021). Therefore, reorienting the strategy to implement NRS more effectively and motivate farmers’ involvement has been a top priority at the scientific and policy levels. A circular nutrient economy encompasses responsible nutrient management practices for the reduction of nutrient losses and increased recovery of nutrients from waste streams for reuse in agricultural production. The concept is based on the principles of the circular economy, which seeks to decouple economic growth from resource consumption and environmental degradation. Some countries (e.g., Netherlands and Singapore) have been pioneers in implementing circular nutrient economy practices to close nutrient loops, such as the Phosphate Platform and Singapore's NEWater program. In this viewpoint, we suggest that a circular nutrient economy in the MARB could accelerate NRS implementation and achieve benefits beyond nutrient loss reduction. Hongxu Zhou is a graduate research assistant in the Department of Agricultural and Biological Engineering, University of Illinois at UrbanaChampaign, Urbana, Illinois (ORCID: 0000-00021746-8182). Andrew J. Margenot is an associate professor in the Department of Crop Sciences, University of Illinois at Urbana-Champaign, Urbana, Illinois (ORCID: 0000-0003-0185-8650). Wei Zheng is a principal research scientist at the Illinois Sustainable Technology Center, University of Illinois at Urbana-Champaign, Urbana, Illinois (ORCID: 0000-0002-0307-0915). Chloe B. Wardropper is an assistant professor in the Department of Natural Resources and Environmental Sciences, University of Illinois at Urbana-Champaign, Urbana, Illinois (ORCID: 0000-0002-0652-2315). Roland D. Cusick is an assistant professor in the Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois (ORCID: 00000002-4037-2939). Rabin Bhattarai is an associate professor in the Department of Agricultural and Biological Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois (ORCID: 0000-0002-3433-299X). Received March 23, 2023. ADVANCING A CIRCULAR NUTRIENT ECONOMY COULD MOTIVATE NUTRIENT REDUCTION STRATEGY ADOPTION A significant emphasis in the current NRS of MARB states has been placed on preventing off-field nutrient loss to achieve environmental benefits. In contrast, less attention has been paid to the potential economic benefits to farmers. A 2019 comprehensive review on the adoption of conservation practices illuminated a range of significant social, economic, and operational factors influencing adoption (Prokopy et al. 2019). The study highlighted that the farmers whose primary operational motivation was financial were less likely to adopt conservation practices than those with noneconomic primary motivations. Apart from some cost-share programs supported by governments to implement conservation practices (USEPA 2022), many practices may not result in immediate and/or direct agriculture-related economic benefits, such as an increase in crop yield or a reduction in fertilizer cost (Robertson and Saad 2021), and may even decrease yields (e.g., cover crops) (Deines et al. 2022). Even with cost-share programs, many practices may still incur net or opportunity costs. Advancing a circular nutrient economy in the MARB would achieve multiple benefits in terms of the economy, ecology, efficiency, and long-term sustainability. The MARB rivers discharged 90,500 t (99,759 tn) of nitrate (NO3 –) and 15,600 t (17,196 tn) of phosphorus (P) into the Gulf of Mexico in May of 2021 alone (NOAA 2021). A recent study indicated that the midwestern United States has a large potential for nutrient recovery and reuse due to extensive centralized wastewater and maize (Zea mays L.) ethanol infrastructure with a high degree of co-location with agricultural nutrient consumption, highlighting the untapped potential for a circular nutrient economy in this globally significant grain-producing region (Ruffatto et al. 2022). Nutrient recycling as a strategy to meet the NRS goals can generate income for farmers and partially offset reliance on inputs, leading to greater profitability. Shifting to a systems perspective, recovering lost nutrients offers an excellent opportunity for more effective sharing and implementation of NRS to deliver direct cost recovery from nutrients’ fertilizer value and indirect cost recoveries by mitigating the nutrient clean-up or damage costs. Indeed, systems thinking has been associated with the adoption of cover crops by individual farmers (Church et al. 2020).