Nitrate pollution of soil and water bodies is a pressing global environmental problem caused by the use of fertilisers. As a new study in the journal Science shows, both the volume of water flowing through the landscape and its flow velocity play a decisive role in the risk of nitrate pollution.
The study by the Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB) in collaboration with the Helmholtz Centre for Environmental Research (UFZ) and led by Prof. Doerthe Tetzlaff of the IGB and Humboldt-Universität zu Berlin shows: An increase in nitrate is often climate-related and is influenced by extreme weather events such as heavy rainfall or periods of drought, which accelerate or slow down the water cycle. To explain this, the authors introduce the concept of ‘moisture limits’. Exceeding these limits – either below or above them – consequently increases nitrate pollution in soils and the leaching of nitrate into surface water or groundwater. Their projections suggest that nitrate risks across much of Europe will decrease by 2100 under a climate scenario with low emissions of anthropogenic greenhouse gases, but highlight the increasing risks across large parts of Eastern and Southern Europe under a high-emission scenario.
3,800 European river basins analysed in the model
Since pre-industrial times, human nitrogen inputs into the terrestrial biosphere have doubled, primarily due to the large quantities of synthetic and organic fertilisers. Excess nitrogen enters water bodies, posing a risk of eutrophication and thus endangering food production, drinking water supply and the sustainability of ecosystems. To better understand the nitrate cycle, the research team developed a process-based model to track water and nitrogen flows using stable water isotopes. The model was applied to over 3,800 European river basins, and the research team mapped the rates of the water cycle. In doing so, they assessed how the acceleration and deceleration under wetter and drier climatic conditions since the 1980s have affected nitrate pollution. The study also forecasts how this will develop by the end of the 21st century.
Why flow velocity plays a role in nitrate pollution
Flow velocity plays a crucial role in nitrate pollution, as it determines the balance between transport and metabolism. Higher flow velocities, such as those found on the north-western coast of Europe and in mountainous regions, reduce the time available for nitrate degradation. This increases the risk of nitrate being leached from the soil into groundwater and surface water. In contrast, slower flow velocities, such as those found in lowland regions, give plants and microorganisms more time for nitrate degradation. “Until now, nitrate pollution has mainly been attributed to human inputs such as fertilisers. However, we have been able to show that flow velocity also plays a decisive role in nitrate leaching,” says lead author Dr Songjun Wu from the IGB.
How can such negative developments be prevented?
To mitigate these risks, the study emphasises the importance of keeping hydrological changes within moisture limits. This can be achieved either by managing climate extremes or by expanding these limits through the reduction of human nitrate inputs, for example through crop rotation, optimised fertilisation and improved wastewater treatment.