Water quality impacts of recycling nutrients using organic fertilizers in circular agricultural scenarios

CONTEXT: Most of the nutrients in food supply chains are lost as waste and pollution at great environmental and economic cost. A more circular agricultural system can be developed through recycling of organic waste streams such as manure and food waste into organic fertilizers. OBJECTIVE: This study assessed the agricultural system's circularity potential in the Susquehanna River Basin (SRB), the largest tributary to the Chesapeake Bay, by recycling organic waste streams such as manure and food waste into fertilizers. METHODS: Using the Soil Water Assessment Tool (SWAT), we developed and deployed three different scenarios beyond the baseline of current practices. First, we created the Manure Transport scenario to study improved manure transport beyond the sub-watershed where it is produced. Second, we developed the High Nitrogen Recovery Manure scenario to evaluate an increase of available nitrogen (N) in manure assuming developing technologies could reduce N losses from storage and transportation, leaving more N to apply to cropland. The final Compost scenario used food-waste as a source for compost fertilizer applied to areas without manure available. We evaluated the level of circularity from each scenario through the spatial distribution and the proportion of cropland using organic fertilizers sourced from manure and compost generated in the SRB. Additionally, we considered water quality at the outlet of the SRB to determine the impact a circular agricultural system could have on the water quality goals set forth for Chesapeake Bay. RESULTS AND CONCLUSIONS: The Manure Transport scenario showed circularity and water quality results similar to the baseline conditions. The High Nitrogen Recovery Manure scenario demonstrated the highest circularity, with 46% of cropland using organic fertilizers, resulting in the smallest phosphorus (P) loading per watershed area (3.26 kg/ha/yr) at the SRB outlet. However, it also had the highest N loading (9.97 kg/ha/yr) at the outlet. The Compost scenario doubled the area treated with organic fertilizers, with 16% of cropland utilizing compost application, and led to increased N (8.48 kg/ha/yr) and P (4.17 kg/ha/yr) loading at the SRB outlet. SIGNIFICANCE: The study highlighted that cropland area in this region is sufficient to expand the use of manure and food waste compost as fertilizers, thereby enhancing circularity and reducing commercial fertilizer inputs. However, it emphasized the importance of integrating field management strategies to address agricultural runoff concerns and achieve both waste reduction and improved water quality.