Efficacy of diffuse source phosphorus reduction efforts: six decades of water quality management in Germany’s Ruhr River

Abstract Diffuse sources and resulting surface water quality impairments are major challenges for achieving advanced environmental goals as formulated in the EU Water Framework Directive or in the Zero Pollution Ambition of the European Green Deal. However, the effectiveness of legal and practical interventions is to date insufficiently understood. In the presented study, we quantitatively estimate the effectiveness of source control, use-related, and edge-of-field legislation in reducing phosphorus (P) loadings to the Ruhr River in Germany and the resulting multi-decadal trajectory of river water quality. Analyzing the legal setting and its implementation over a 66-year period, from 1957 to 2023, we found that P mineral fertilizer sales began to decrease in the 1980s even before the adoption of use-related legislation in 1989. The absolute amount of organic fertilizer used remained unchanged; thus, its relative importance increased during this period. Since the 1990s, an increasing share of agricultural land in protected areas contributed to strengthening the evolving codes of good agricultural practices (CGAP), and voluntary measures assisted a stronger compliance with legislative measures. The induced optimization of fertilizer application resulted in low P budgets in the catchment area at the end of the 1990s. However, edge-of-field measures are still relevant regarding the release of excess P stored in soils and the prevention of runoff and erosion caused by increasing extreme precipitation induced by climate change. Our methodological insights and findings on pollution trajectories can provide guidance for managing current and future pollution across catchments with comparable characteristics.