Systems approach to nitrogen modeling in the Chesapeake Bay: Advancing production chain analysis under future changes.

Agricultural runoff accounts for >40% of Chesapeake Bay's nonpoint source nitrogen (N) pollution, representing a complex problem requiring systems-level management approaches. Traditional nitrogen management strategies, focused primarily on field-level best management practices, have proven insufficient to achieve watershed restoration goals, underscoring the need for systems-level management approaches that account for interactions across the entire food production chain. In this study, we investigate the effects of simulated future agricultural changes and management scenarios on agricultural N loss in the Chesapeake Bay Watershed using a systems approach production chain analysis that tracks nitrogen flows through seven distinct stages of food production, processing, and consumption chains. We developed scenarios including future agricultural intensification, efficiency improvement management strategies, and combined scenarios to evaluate system-wide responses. Our results show that a combination of interconnected factors is the most influential in controlling total nitrogen loss, including expected factors like the production amounts of crops and animals and fertilizer application rates, as well as several less widely discussed factors, including live animal weight gained and feed conversion ratios. Although live animal weight gain and feed conversion ratio were previously identified as sensitive variables in earlier applications of the nitrogen flow model, our study advances this work by quantifying their relative importance to total nitrogen loss. In addition, the present analysis extends these insights to future climate-impacted scenarios by incorporating climate-driven changes in crop yields and projected growth in animal production. This combined approach clarifies not only which factors matter most but also how their influence evolves under changing environmental and production conditions, thereby identifying the most consequential leverage points for future nitrogen management. These findings demonstrate that systems-level perspectives across interconnected food production chains can provide viable information for identifying pathways to meet watershed quality objectives while accommodating projected agricultural intensification under changing climate conditions.