In Europe, significantly more nitrate from over-fertilized soils is apparently entering the groundwater and water bodies than previously thought. According to a new model calculation, nitrogen is washed out of the root zone of the soil in around three quarters of European usable areas during the winter months. This means that the proportion of areas at risk from nitrate leaching is almost twice as large as previously estimated.
From the field to the root zone: In agriculture, the nitrogen required for crops is often applied as fertilizer in such large quantities that the plants cannot completely absorb the nitrogen. As a result, the excess nutrients are washed out of the field with the seepage water and reach the root zone as nitrate. If it is not denitrified to nitrogen again by bacteria there, the nitrate eventually penetrates into the groundwater and into the surface waters. This nitrate leaching becomes a problem in many places in Europe, especially in winter, as the plants then do not grow and therefore absorb less nitrogen.
What does nitrate leaching depend on?
How much of the nitrogen released through fertilization reaches groundwater and surface waters as nitrate depends, among other things, on complex processes in the soil. Researchers working with Rohini Kumar from the Helmholtz Center for Environmental Research have now analyzed in more detail which of these decide the path taken by nitrogen. To do this, they primarily investigated the processes in the root zone, which extends from the surface of the soil to a depth of one meter. “The root zone is the most dynamic and active part of the subsoil, where precipitation, dry phases and evaporation act,” explains Kumar. It acts as both a hydroclimatic and a biogeochemical filter between the earth’s surface and the deeper soil layers.
How strong the leaching of nitrate from this zone is has so far only been statically described: Information on land use, soils and the topography of the landscape was combined with mean precipitation and groundwater levels. However, their temporal variability was not taken into account. “However, the amount of precipitation and temperatures change daily, affecting evaporation, the ground water and ultimately the time spent there and the water transport into deeper layers,” explains Kumar. “Therefore, mean values, such as those used to describe the static state, are less effective from today’s perspective.”
The scientists therefore chose a different approach for their study: They used a specially developed model to simulate the runoff dynamics of the root zone. In doing so, they combined observational data on the weather and environmental conditions with calculations of the daily change in water residence and nitrate in the root zone as well as denitrification. This enabled them to determine exactly to the day how long the nitrate dissolved in the water remains in the root zone in different regions of Europe before it migrates into the depths – and also how this leaching has changed over the last 65 years.
75 percent of the usable space vulnerable
The analyzes showed that the usable areas in Europe are significantly more susceptible to nitrate leaching in groundwater, rivers and the like than previously assumed. According to the scientists, almost 75 percent of European usable land is at risk for a period of at least four months per year – especially in winter. Compared to the static calculations previously assumed, this area share is more than 30 percent larger. Areas in the east and north-east of Germany, on the Iberian Peninsula and in some Eastern European countries are particularly at risk. “Since the temporal-spatial dynamics of water transport has not yet been taken into account in the risk assessment of nitrate-endangered areas in Europe, the spatial extent of the nitrate input is underestimated,” explains Kumar’s colleague Andreas Musolff.
The new findings are now intended to help adjust fertilization in agriculture accordingly: “With the more precise information, farmers could tailor their fertilization regime more precisely to ensure that there is as little nitrate as possible in the soil in the particularly critical months,” says Musolff. In future studies, the transport and denitrification in the subsoil and in the water network would also have to be considered.
Source: Helmholtz Center for Environmental Research, Article: Nature Communications, doi: 10.1038 / s41467-020-19955-8