Modelling saves a lot of time

For many years, Germany failed to apply the EU Nitrate Directive and was threatened with severe fines. These are now off the table thanks to the amended German fertilizer rules. A nationwide monitoring program is assessing the effects of these rules, and Jülich simulation models are part of this.

Each year, starting in February, the big tractors with their long tank trailers are back on the road. It is the visible and smellable start of the manure season. In Germany, however, fields are often fertilized more than is actually necessary. This is not without consequences: excess nitrate seeps into the soil and can pollute groundwater (see box). The EU limit of 50 milligrams of nitrate per litre is exceeded in several regions of Germany.

For years, the EU accused Germany of not doing enough against nitrate pollution. A decision by the European Court of Justice in 2018 indicated that the country could face multi-billion euro fines because of that. After Germany revised its fertilizer regulations several times, the EU finally terminated the infringement procedure in June 2023.

A key element of the new rules is a long-term monitoring, control and analysis system. This impact monitoring will record nitrogen emissions from agriculture and nitrate concentrations in groundwater. In this framework, Germany-wide modelling will be used to estimate how measures, such as less fertilization in agriculture, affect the nitrate contamination of groundwater and surface waters in Germany.

The model for analyzing nitrate fluxes is currently being developed by scientists in the RELAS project. AGRUM-DE, a model network for the analysis of nutrient fluxes at the level of federal states initiated in 2005, is used as the basis for this. “In the RELAS project, we are further developing AGRUM-DE into a nationwide standard for impact monitoring,” says Prof. Frank Wendland from the Jülich Institute of Bio- and Geosciences (IBG-3). His working group has contributed two models for analyzing nitrate fluxes to the network: “With mGROWA, we model the runoff components and the input pathways for nitrate, and with DENUZ-WEKU the nitrate turnover and the transport in soil and groundwater. In this way, we can simulate how much nitrate finds its way into surface waters and groundwater,” Wendland explains.

*RELAS stands for the mapping of regional agricultural nitrogen flows (REgionale LAndwirtschaftliche Stickstoffflüsse) as a basis for developing and optimizing agricultural policy measures to achieve climate protection targets. The project is funded by the Federal Office for Agriculture and Food (BLE).

Regional Differences

Moreover, the simulations with the Jülich models can spatially resolve the expected nitrate pollution quite precisely. According to Wendland, “That’s important because nitrate concentrations within an individual river catchment or groundwater system sometimes varies considerably.”

To this end, the researchers feed their models not only with data from the more than 10,000 groundwater monitoring stations in Germany, but also with a great number of input parameters such as climate data, land use and soil properties. The results of another AGRUM-DE model are also included: RAUMIS, developed by researchers at the Thünen Institute in Braunschweig. Based on various parameters such as fertilizer use, nutrient removal by plants and regional site characteristics, this model calculates how much excess nitrogen is produced by farming. “In the AGRUM-DE model network, we simulate how much nitrogen is in the system, so to speak. From that, we can draw conclusions about the nitrate concentrations as well as the amount of nitrate that reaches groundwater via leachate. With the help of the simulations, we can then also assess whether a planned measure will reduce the nitrate pollution of groundwater as desired,” Wendland explains.

The FZJ models, however, provide even more information: they determine how, via different input pathways, nitrate finds its way into surface waters. The related FZJ model results flow into another AGRUM-DE sub-model, MONERIS, developed by researchers at the Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB). MONERIS analyzes how certain nutrients, including nitrate, are distributed in rivers and eventually reach the sea.

Targeted Measures

“In this way, the AGRUM-DE-model network is able to identify the cause of high nitrate loads in surface waters including coastal areas,” says Wendland. This can be the excessive use of fertilizers, but also wastewater from sewage treatment plants or artificial drainage systems. “Based on this knowledge, we can identify starting points for additional targeted measures – for example, how much nitrate-based fertilizer farmers can use in a certain area without exceeding the EU limit there,” says the Jülich researcher.

“With the AGRUM-DE simulation tool, it will be possible to consistently predict how the amended fertilizer ordinance will affect the nitrate pollution of groundwater and surface waters – years before the effects are measurable. This will make it possible to readjust inadequate measures much earlier. The first nationwide modelling should be available by the end of 2025. In the end, the concept of impact monitoring helped to convince the EU Commission to drop the legal action against Germany. However, the discontinuation of the legal action is only a milestone, there is still a long way to go before the protection targets for water resources are achieved,” says Wendland.