Towards circular nutrient economies: An integrated manureshed framework for agricultural and municipal resource management

The manureshed concept provides a framework for understanding nutrient flows between agricultural operations. This study advances the manureshed framework by integrating agricultural and municipal wastewater treatment plant (WWTP) effluent sources at the watershed scale across the contiguous United States. Using 2016 Nutrient Use Geographic Information System and municipal WWTP effluent datasets, we quantified how WWTP integration transforms source–sink dynamics across U.S. watersheds. WWTP integration significantly altered classifications, with nitrogen source areas increasing by 167.9% and phosphorus by 24.4%. Agricultural nitrogen sources demonstrated strong regional concentration in the southeastern United States and eastern Texas, while the Pacific Northwest showed extensive phosphorus sources. Spatial transition probability networks revealed exceptional redistribution potential, with source areas showing over 70% adjacency to nutrient-deficit areas, indicating strong opportunities for short-distance transfers. This study provides the first comprehensive framework in theoretical manureshed analysis integrating agricultural and municipal WWTP effluent sources, creating new opportunities for circular nutrient economies.