Nitrogen Memoryscapes: Typologies of Nitrogen Inputs and Riverine Loads Across the Continental United States

Abstract

Nitrogen pollution has severely impacted inland and coastal waters, contributing to eutrophication, harmful algal blooms, and drinking water contamination. Reducing nitrogen inputs is widely expected to improve water quality, but watershed responses to changing nitrogen surplus are often nonlinear and delayed, complicating expectations of recovery. Here, we analyze long-term trends in nitrogen surplus and riverine nitrogen loads across 490 US watersheds from 1990 to 2017 to examine how changes in inputs translate into changes in load. By jointly classifying watersheds based on the direction of surplus and load trends, we identify four response regimes that capture the diversity of nitrogen dynamics across US landscapes. Using a multivariate classification framework, we show that watershed responses are shaped by interactions among nitrogen source type, hydrologic connectivity, climate constraints, and legacy nitrogen storage. Livestock density is associated with increasing nitrogen surplus, while declining atmospheric deposition is associated with decreasing surplus and loads. Groundwater-dominated watersheds exhibit increasing nitrogen loads despite declining surplus, reflecting the long memory of subsurface nitrogen storage. Connectivity modulates these responses: tile-drained watersheds exhibit tight coupling between surplus and load, whereas arid, low-connectivity systems show declining loads despite increasing surplus. These results show that spatial patterns in riverine loads cannot be used to infer recovery trajectories following reductions in nitrogen inputs, and effective nitrogen management requires accounting for watershed-specific response regimes.