At the request of the Ministry of Agriculture, Nature and Food Quality, the PBL assessed Professor Arthur Petersens expert opinion on a mathematical lower limit for nitrogen deposition. This review concludes that the proposed lower limit by Mr. Petersen is reasonably to well substantiated. However, the PBL makes several suggestions to strengthen the argumentation.
The argumentation of Professor Petersen is logically constructed and aligns well with the scientific literature. Moreover, his reasoning fits with the usual practice for applying safety margins and detection values in risk assessments, such as the approval and use of chemicals. The PBL makes several proposals to improve the argumentation. Additionally, the PBL presents three points that can clarify the discussion on a mathematical lower limit:
- To begin with, the PBL suggests referring to a detection limit instead of a mathematical lower limit, as recent discussions have shown that the term mathematical lower limit can cause confusion regarding the value of a precise calculation model like AERIUS. The accuracy of the AERIUS model is distinctly different from the interpretation of the mathematical lower limit as proposed by Professor Petersen. Petersen describes a limit below which no detection of nitrogen deposition can occur with the current state of science.
- Furthermore, the PBL advises, in line with the Habitats Directive, to make a clear distinction between a detection limit for nitrogen deposition for permitting and the obligation of a member state to improve nature.
- Finally, the PBL emphasizes the importance of the existing monitoring system that allows for the registration of cumulative activities below the detection limit.
Three Frameworks
The PBL assessed this expert opinion within three frameworks:
- the Birds and Habitats Directives,
- the jurisprudence on which Professor Petersen bases his opinion, and
- the current policy on nitrogen deposition, including the associated AERIUS calculation system.
These frameworks are important for assessing the effects of an additional nitrogen deposition of 1 mol per hectare per year on nitrogen-sensitive nature, estimating the expected cumulative impact of activities that will be developed under a detection limit, and determining who is responsible for preventing potential deterioration of nature quality due to this accumulation.