Seamless National-Scale Assessment of Legacy Nitrogen Pollution in German Rivers

Nitrogen pollution of surface water bodies remains one of the most persistent environmental challenges across European landscapes, largely resulting from widespread agricultural intensification since the early 20th century. In this study, we present a seamless national-scale framework for the comprehensive assessment of nitrogen dynamics in German river systems by combining long-term nitrogen input sources (e.g., mineral fertilizers, manure applications, and wastewater effluents) with a process-oriented modeling framework to estimate both current and legacy sources of nitrogen pollution. We built a harmonized dataset covering both diffuse and point nitrogen sources from the 1950s [1,2] to the present and tracked the terrestrial transfer of nitrogen through soils, groundwater, and river networks. Our analysis is based on the multiscale water quality model mQM [3], which explicitly accounts for nitrogen legacy storage and delayed release and transport processes across terrestrial compartments. Model parameterization follows a stepwise approach: catchment-scale parameters are first constrained using riverine N concentration data from more than 100 gauging stations, compiled within the QUADICA database [4,5]. This allows for a robust basin-scale model configuration covering diverse German landscapes: natural forested regions, intensive croplands, and livestock-based systems; and capturing a range of varying hydroclimatic conditions, subsurface soil and groundwater characteristics, and socioeconomic factors. 

Subsequently, to enable seamless model application across the entire German river network, we apply a transferable model parameterization that utilizes spatial proximity, physiographic similarity, and landscape characteristics using statistical methods and machine-learning techniques (e.g., regression relationships and Random Forests). Our near-century-long data and model-based analysis show pronounced spatial and temporal heterogeneity in N input sources and riverine N concentrations across German landscapes. Regions with excessive nitrogen surplus are associated with livestock-intensive systems in northern Germany as well as mineral fertilizer-dominated cropland regions in central Germany, while differences in catchment functioning and hydroclimatic conditions modulated how excess N signals propagate to riverine N concentration levels. Despite national-scale reductions in nitrogen surplus and substantial improvements in wastewater treatment, there are regions of Germany (e.g., in the Central Elbe River) that continue to exceed critical nitrogen thresholds (>2.5 mg/L). Our analysis provides a seamless framework for assessing nationwide nitrogen pollution and supports the development of intervention strategies for sustainable nitrogen management.

[1] https://doi.org/10.1038/s41597-022-01693-9
[2] https://doi.org/10.5194/essd-16-4673-2024
[3] https://doi.org/1029/2022GL100278
[4] https://doi.org/10.5194/essd-14-3715-2022
[5] https://doi.org/10.5194/essd-2025-450