Typologies of Nitrogen Surplus Trajectories using the new TREND-nitrogen dataset: Shifting Hotspots and Dominant Controls

Abstract:

Worldwide increases in the flows of reactive nitrogen (N) threaten drinking water quality, disrupt aquatic ecosystems, and escalate greenhouse gas emissions. At both global and local scales, an increase in anthropogenic N flows over the last century is driven by increases in population, shifting diets, and increased use of commercial N fertilizers.

Increasingly, efforts are being made to quantify the magnitude of changes in anthropogenic N cycling. A limitation of existing approaches to linking the N mass balance to water quality is the assumption that the current-year N balance is the primary driver of current-year N fluxes from the landscape to the environment. To better understand long-term N dynamics, and in particular, the role of N legacies in driving both dissolved and gaseous emissions of N from human-impacted landscapes, it is crucial to develop long-term datasets of N inputs and outputs.

We addressed these limitations by constructing the Trajectories Nutrient Dataset for Nitrogen (TREND-nitrogen) – a long-term (1930–2017) county–scale N mass budget for the contiguous U.S. We quantify individual components of the anthropogenic N balance and also calculate county-scale N surplus magnitudes. Here, N surplus is defined as the difference between N inputs (fertilizer N application, biological N fixation, atmospheric N deposition, manure N, human waste) and non-hydrologic outputs (crop N uptake). We use cluster analysis to develop a spatially explicit typology of N surplus trajectories. We find ten primary trajectory types, with trajectories being identified across diverse geographic regions, indicating a widespread functional homogenization of human-dominated landscapes. This newly developed typology of N surplus trajectories improves our understanding of long-term N dynamics. The underlying dataset provides a powerful tool for modeling the impacts of legacy N on past, present, and future water quality.