Reducing national water degradation: Development and application of a manureshed-identification framework

Context: The manureshed concept minimizes nutrient imbalance in livestock-intensive agricultural systems by transporting surplus manure to agricultural fields with nutrient demands. The impacts of manureshed-based manure management across the contiguous United States (CONUS) and its potential to improve soil nutrient dynamics and water quality are not well known. Objective: This study developed a framework to evaluate the impacts of manureshed-based manure nutrient management at the CONUS scale. Methods: Across CONUS, county-scale manure imports and exports were balanced by delineating manuresheds according to historic agronomic nitrogen (N) and phosphorus (P) demands and the transportation potentials of the nearest manure types (wet vs. dry). The water quality impacts of manureshed-based nutrient management were assessed for the Oconee River Watershed, in the southeastern United States using the National Agroecosystems Model (NAM) developed from the Soil Water Assessment Tool (SWAT+). Results and conclusions: The analysis identified that nearly 78 % of the wet manure surplus counties (152 counties) required only two neighboring counties to balance excess wet manure P, and about 77 % of dry manure surplus counties (428 counties) required only three neighboring counties to balance excess dry manure P. The P-balanced manureshed-based manure management reduced organic phosphorus (P) by 29 % and organic nitrogen (N) by 5 % in the manure surplus case study watershed (Upper Oconee). When the excess manure was redistributed based on crop P demand, the total nutrient load at the watershed outlet decreased, with a 0.44 % reduction in total nitrogen (TN) and a 3.41 % reduction in total phosphorus (TP). However, the redistribution of excess manure led to a slight increase in nutrient loads in the Lower Oconee, the manure sink watershed. Significance: The study demonstrates the potential of manureshed-based manure management to reduce nutrient loads. The findings emphasize that with spatially explicit and adaptive management strategies, manureshed-based approaches can successfully balance nutrient flows across regions, leading to better manure nutrient use efficiencies and water quality improvement.