Manureshed Nutrient Dynamics Across the Conterminous United States: Three Decades of Agricultural Flows and Integration With Municipal Wastewater Sources

ABSTRACT Integrated agricultural and municipal nutrient flows across the conterminous United States (CONUS) exhibit substantial interannual variability and spatial scale dependence that single‐year analyses cannot characterize. Addressing this gap has been limited by manual geospatial workflows that are time‐intensive and non‐reproducible. Here we present manureshed , an R package that enables multi‐temporal (1987–2016) and multi‐scale analysis across county, Hydrologic Unit Code (HUC) 8 watershed, and HUC2 basin levels, integrating nitrogen (N) and phosphorus (P) agricultural nutrient data from the Nutrient Use Geographic Information System (NuGIS; 1987–2016) with municipal wastewater treatment plant (WWTP) discharge data from the U.S. Environmental Protection Agency (EPA; 2007–2016). Automation of spatial harmonization, classification, and visualization workflows—distributed via the Comprehensive R Archive Network (CRAN) with on‐demand data access via the Open Science Framework (OSF)—makes reproducible continental‐scale analysis accessible as a routine operation. Over three decades, nitrogen source county counts exhibit marked interannual variability with no persistent directional trend, demonstrating why single‐year snapshots can misrepresent structural patterns. WWTP integration more than doubles the number of nitrogen source watersheds (from 28 to 72 at HUC8 scale for 2016), with new sources emerging predominantly in urbanized areas. Validation against published HUC8‐scale classifications confirms computational accuracy (error < 0.1%), and 30‐year multi‐state batch analysis completes in under 3 min compared to multiple days for equivalent manual workflows. An interactive Shiny dashboard, five vignettes, and comprehensive documentation support broad accessibility.