News Nitrogen
In practice, nitrogen emissions deviate significantly from the model
The model that RIVM uses to calculate nitrogen emissions from individual farms and their depositions on vulnerable nature does not match measurements made by a research team from the University of Amsterdam (UvA). Traceable emissions from individual companies mainly precipitate within a radius of 100 meters. In general terms, the RIVM overestimates the measured deposition by 3,6 times.
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This is one of the most striking conclusions in the report 'Nitrogen deposition around dairy farms: spatial and temporal patterns', of which an Executive summary was published today (Tuesday, September 12). "We cannot claim a good 'match', nor any other form of validation, because the observed deposition measurements differ from those in the OPS model (the RIVM calculation model, ed.)," the researchers write.
Actual measurements taken
The study is important for several reasons. It is the first study in which actual measurements were made and nitrogen isotopes were also traced to the source. The RIVM mainly uses modeled methods and has omitted measurements.
The measurements taken by the UvA researchers led by Albert Tietema indicate that nitrogen deposition outside a radius of 500 meters around a farm can hardly be traced back to that farm. The largest amount of 'proprietary' nitrogen emissions occurs within a radius of 100 meters. At a distance of 25 meters, 75% of the ammonia concentration can still be traced back to it, at a distance of 500 meters only 25%.
PAS detectors helped out
The average 'background deposition' of traceable nitrogen within 500 meters is 5,6 kilos per hectare per year. This is well below the Critical Deposition Value (KDW) for most vulnerable nature, which means, for example, that all PAS detectors were easily rescued. The permit issuance is easier if a KDW within a radius of 25 kilometers around the applicant is not exceeded. While the OPS greatly overestimates the dry deposition, the model, on the other hand, significantly underestimates the wet deposition. The UvA research team does not have very precise figures about this, because the wet deposition is derived from the difference between the total deposition and the dry deposition.
The results quoted stem from a two-year independent study by the University of Amsterdam, which was financed by the Mesdag Fund. The research was conducted on two dairy farms in the north of the country. Of the total nitrogen emissions from farms, 91% pass directly to higher air layers, as is generally the case with other emissions from industry and traffic. Then it comes down again. The remaining 9% falls within 500 meters of a farm, but mainly very close.
Measurements and models should go hand in hand
The Amsterdam research group of the Institute for Biodiversity and Ecosystem Dynamics would like to conduct further research, but sees the measurement results as an important addition to the model-based work of the RIVM. "The need for measurements and modeling to go hand in hand is widely endorsed." Another conclusion from the study is that grassland emits a net amount of 12 kilos of ammonia per hectare, three quarters of which in the spring and summer, when fertilization and harvesting takes place.
Klaas van der Horst
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