Droughts and heavy rain drive nitrate into groundwater
Clean drinking water is not something to be taken for granted. Anyone who drinks from the tap relies on the water not having been contaminated with pollutants as it travels through the ground. But nitrate threatens this safety. It enters the soil primarily through fertilizers.
Climate change also influences how much of the substance ultimately ends up in water bodies and groundwater. This is shown by a new study published in the journal Science. Researchers from the Helmholtz Centre for Environmental Research (UFZ) in Leipzig were involved.
Nitrate is nitrogen in a specific chemical form. Plants need it to grow. That is why it is applied to fields as fertilizer. However, what plants and soil do not absorb is washed away with the water – into streams, rivers, and ultimately into the groundwater from which our drinking water is sourced. Excessively high levels can pollute water bodies and complicate drinking water treatment.
The speed at which water flows determines the risk
Until now, humans were primarily considered the main culprits – too much fertilizer, too little consideration. “However, we were able to show that flow velocity also plays a decisive role in nitrate leaching,” says Dr. Songjun Wu of the Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB), who led the study.
The principle is simple: When water flows quickly, plants and microorganisms have little time to break down the nitrate. It enters rivers and groundwater unimpeded. When water flows slowly, natural processes have more time. The substance is broken down before it causes damage.
For their study, the research team analyzed more than 3,800 river basins in Europe and used a specialized model to reconstruct the pathways of water and nitrogen through the landscape.
Extremes Make It Worse
This is exactly where climate change comes into play. Heavy rainfall accelerates water flow and washes nitrate out of the soil. Droughts slow it down dramatically. Plants and microorganisms can barely function, causing the substance to accumulate. When rain finally does come, everything is washed away all at once.
To describe these relationships, the researchers introduce the concept of so-called moisture thresholds. As long as the water balance remains within these limits, nitrate levels stay relatively stable. If the limits are exceeded due to extreme wetness or extreme dryness, the risk of nitrate leaching increases significantly.
Project leader Prof. Doerthe Tetzlaff of the IGB and Humboldt University of Berlin explains that the moisture limits could “help define a safe margin of action that is resilient to hydrological changes.”
The researchers have calculated two future scenarios for Europe. With low greenhouse gas emissions, nitrate pollution would decrease in more than 70 percent of Europe by 2100. With high emissions, however, the opposite is likely, particularly in Eastern and Southern Europe: persistent drought, higher nitrate levels, and poorer water quality.
The study shows that nitrate pollution can only be understood within the “complex interplay of transport and retention in the landscape,” summarizes UFZ hydrogeologist Dr. Andreas Musolff. The projections highlight where Europe is particularly at risk.
To reduce nitrate pollution in soils and water bodies, more targeted fertilization and improved wastewater treatment are needed. At the same time, climate change also plays a role, as droughts and heavy rainfall can increase the leaching of nitrate.
Original publication:
Songjun Wu, Chris Soulsby, Yi Zheng, Andreas Musolff, Doerthe Tetzlaff: Divergent Evolution of Nitrogen Cycling Along Gradients of Landscape Water Velocities; Science, 2026.
