Disaggregating Landscape‐Scale Nitrogen Attenuation Along Hydrological Flow Paths

Evaluating how nitrogen (N) sources are attenuated throughout the landscape is critical to further our understanding of catchment‐scale N budgets. We developed a catchment‐scale N budget for a mixed land use karst springshed using in situ measurements (nitrate leaching fluxes and attenuation) and long‐term records (surface N inputs and spring exports) to estimate 20‐year average landscape‐scale N loading, attenuation, and export. We introduce a conceptual model framework to compute N export that can be applied consistently for point or nonpoint sources. The model is based on the product of only four components for each N source: population density or proportion of land cover, P; specific load, L; anthropogenic attenuation, A; and natural attenuation, N. The product of these components is computed for each N source and then integrated at the basin scale. The concise PLANmodel framework predicted attenuation of 90% ± 3% of N inputs, in close agreement with the estimate based on measured spring mass discharge (87% ± 3%). Further, when this attenuation is disaggregated along the hydrological flow path, we estimate that 64% of inputs are lost in the surface soil, 20% in the vadose zone, and 6% in the aquifer. Livestock and human wastes were estimated to be the dominant contributors to spring N export, which was independently supported by isotopic data. The PLANmodel is a simple, transferable framework that supports systematically computing N export based on proportioning of load and attenuation. Identifying the main sources of N ultimately contributing to discharged N loads is a critical step toward source‐related water‐quality management. Our concise model framework predicted attenuation of 90% of N inputsWe found that 64% of surface N inputs were attenuated in the surface soilManure and septic‐treated wastewater were the largest contributors to spring N export