The study, led by Sophia Hildebrandt and Dietrich Borchardt with Elisabeth H. Krueger and Aki S. Ruhl, focuses on the Ruhr catchment in North Rhine-Westphalia, a 4,478 square kilometer basin that mixes forest, which covers 51 percent of the land, with agriculture on 30 percent and built-up areas on 17 percent. The river has been managed by catchment-level authorities since the early twentieth century, and quality-assured monitoring of total phosphorus concentrations is available going back to 1965, making the basin an unusually well-documented case.

The researchers categorized the legal provisions into three types. Source control legislation targets the origin of phosphorus in fertilizers, use-related legislation specifies application requirements through codes of good agricultural practice and additional restrictions in protected areas, and edge-of-field legislation aims to prevent nutrient inputs into surface waters by creating buffer strips between fertilized land and water bodies. They identified 21 laws affecting phosphorus, tracing national acts back to their first versions through the German Federal Law Gazette, alongside EU and state-level legislation.

One of the most striking findings concerns timing. Mineral phosphorus fertilizer application in West Germany peaked at 33 kilograms per hectare in 1980 and then began to decline—before the use-related water protection legislation came into force in 1989. The authors attribute this to a combination of factors beyond legislation, including economic conditions, rising fertilizer prices, reduced availability, soil phosphorus saturation, and growing environmental awareness. After 1990, structural changes following German reunification also contributed to the drop in mineral fertilizer sales.

While mineral fertilizer use fell significantly, the absolute amount of organic fertilizer used remained unchanged, so its relative importance grew. In the Ruhr catchment today, phosphorus from organic fertilizers exceeds that from mineral fertilizers by a factor of five, compared with a national average factor of 1.6. Organic inputs make up 84 percent of all phosphorus inputs in the catchment, higher than Germany’s 61 percent, Europe’s 50 percent, and the global average of 29 percent. Demand for organic fertilizers rose in response to increasing energy and raw material prices in 2021, almost reaching supply in 2022.

Protected areas proved to be a key mechanism for tightening fertilizer rules. Drinking water protection zones, which forbid fertilizer application in their core zones and restrict it in outer zones, grew from a first ordinance in the catchment in 1974 to cover around 555 square kilometers, or 12 percent of the catchment, by 2022. Nature reserves covered 363 square kilometers, and eighteen eutrophic areas plus around 8 square kilometers of nitrate vulnerable zones added further restrictions. By 2020, only 71 percent of agricultural land was subject solely to the general codes of good agricultural practice, down from 95 percent in 1990.

Voluntary cooperation also played a supporting role. The Ruhr Cooperative, established in 1992 by the Chamber of Agriculture Westphalia-Lippe and the Working Group of the Water Companies along the Ruhr, provides free consultations to member farmers who commit to the cooperative’s fertilizing and plant protection standards, some of which exceed legal requirements. Membership grew from 569 farms in 2002 to 921 in 2019, declining to 875 in 2022 as smaller farms were sold to larger ones, while the agricultural area covered remained stable at about 460 square kilometers. The cooperative has no monitoring or penalizing power, and an estimated 20 to 25 percent of members are passive participants who appear mainly for financial support.

The combined effects of legal improvements, expanding protected areas, and the cooperative’s activity brought catchment phosphorus budgets down to around half of the legal limit of 4.3 kilograms per hectare, with a phosphorus use efficiency of 93 percent in 2016. In 2020, heat and drought reduced pasture outputs, raising the budget to 4.9 kilograms per hectare and lowering use efficiency to 79 percent. The authors note that despite these near-balanced budgets, German arable soils still contain excess phosphorus from long-term surpluses, with 76 percent of arable land nationally at risk of losses of dissolved phosphorus, and the same legacy accumulation is likely in the Ruhr catchment.

Edge-of-field measures showed a more modest contribution. Buffer strips covered 77 square kilometers, or 5.6 percent of the agricultural area, in 2020. Assuming full implementation of all legally required strips and an average phosphorus retention of 55 percent drawn from the literature, the researchers calculated an overall efficacy of 34 percent for riparian buffer strips, corresponding to a phosphorus reduction of about 5 percent at the river outlet. Use-related measures, by contrast, reached an estimated 85 percent efficacy in 2020. The authors stress that buffer strips remain relevant for constraining historical phosphorus stored in soils and for preventing runoff and erosion during the more extreme precipitation events expected under climate change.

The study also documents the changing balance of pollution sources. In 1990, point sources contributed 670 tonnes per year of total phosphorus to the Ruhr, while diffuse sources contributed 176 tonnes. Today the two are nearly on a par, at 190 tonnes per year from point sources and 160 tonnes from diffuse sources, the latter including groundwater, atmospheric deposition, and runoff from other land uses. Average total phosphorus concentrations have remained below the water quality objectives of 0.3 and 0.1 milligrams per liter since the 1990s. The authors caution that their efficacy estimates may be overestimated because fertilizer sales were already falling before the key legislation took effect, and that their budget approach cannot capture spatial variability or transport and lag times within the catchment. They suggest their methodology could be scaled to other river basins where diffuse phosphorus and other pollutants remain a major problem.

Subject of Research: The effectiveness of diffuse-source phosphorus reduction legislation in Germany's Ruhr River catchment from 1957 to 2023

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

Article References: Hildebrandt, S., Krueger, E. H., Ruhl, A. S., & Borchardt, D. (2026). Efficacy of diffuse source phosphorus reduction efforts: six decades of water quality management in Germany’s Ruhr River. Regional Environmental Change, 26(4), Article 223. https://doi.org/10.1007/s10113-026-02717-5

Image Credits: AI Generated

DOI: 10.1007/s10113-026-02717-5

Keywords: phosphorus, Ruhr River, diffuse pollution, agricultural legislation, fertilizer, riparian buffer strips, water quality, eutrophication, drinking water protection zones, Germany, river basin management, nutrient budgets