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Letter to the Editor - Nitrogen Critical Loads: Critical Reflections on Past Experiments, Ecological Endpoints and Uncertainties

Letter to the Editor
An International Journal
January-March 2022:12
© The Author(s) 2022
Article reuse guidelines:
DOI: 10.1177/15593258221075512
Nitrogen Critical Loads: Critical Reections
on Past Experiments, Ecological Endpoints
and Uncertainties
Jaap C. Hanekamp
and William M. Briggs
critical loads, dose-response, nitrogen, uncertainty
In our paper Nitrogen Critical Loads: Critical Reections on
Past Experiments, Ecological Endpoints and Uncertainties,
we tackle an important aspect of a fundamental ecological
topic that hitherto has little been reected upon within the
environmental discourse. As a working denition, nitrogen
critical loads (NCL) are believed to be the limits above which
there is a risk of signicant reduction in habitat quality, due to
the eutrophication, acidication and pressures on biodiversity
(species richness) from atmospheric nitrogen deposition.
Nitrogen critical loads were and are regarded as locus com-
munis on which labyrinthine edices of science and policy are
One such scientic edice of prime importance in the
Netherlands is the model AERIUS. It models both emissions
and deposition levels of nitrogen compounds (e.g. nitrogen
oxides and ammonia) for nature areas, affected by new or
expanding economic activities such as agriculture, trafcon
newly planned roads and industry. As ecological points of
reference, AERIUS contains nitrogen critical loads of depo-
sition for each type of habitat.
Ultimately, NCL are nothing other than ecological dose-
response outcomes created from observational and experi-
mental studies that have been published in the past 3 or so
decades. Although some of these studies are thus hardly re-
cent, they dene current nitrogenscience and policies in the
Netherlands and beyond. Therefore, my colleague and I de-
cided to assess the methodological quality of some para-
digmatical ecological studies on NCL. The NCL we
scrutinised are related to a certain class of habitats.
Overall, we show that NCL are not well dened, and are
subject to hitherto unrecognised forms of uncertainty, which
critically impact the precision and actionability of NCL. For
one, the ofcial denitions of what a critical load is are to
some extent clear with respect to political goals, but not clear
with respect to repeatable or consistent parameters. Also, we
show that overall there is a lack of real-world study design
in the assessed papers. Moreover, expert judgement, which
plays a substantial role in the NCL discourse, remains
unscrutinised; that is, the certainty in these judgements is
too high, and their bases are ambiguous. There needs to be
a way to verify the accuracy of these judgements, espe-
cially if and when costly decisions will be made relying on
The latter is particularly important, as the new Dutch gov-
ernment is willing to spend large sums of money on the reduction
of nitrogen deposition as to ostensibly ameliorate ecological
conditions of nature areas comprising in total 172 400 ha, which is
1724 km
In the coalition agreement between the four political
parties (15
of December 2021), 25 billion euros is specically
reserved on the topic of nitrogen deposition until 2035. Again, the
purported necessity for these large public funds is closely related to
the NCL and the manner in which these are understood with
respect to ecological impact of nitrogen deposition.
Sadly, this is not perceived by most, if at all. For that reason, we
encourage the scientic community active within this discourse to
take notice of our study as it also proposes study designs as to
improve best-estimates of NCL, which need substantial im-
provement, scientically and otherwise, as we show in our
contribution. More importantly, it is a call to self-reectively
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Science Department, University College Roosevelt, Middelburg, Netherlands
Department of Environmental Health Sciences, University of Massachusetts,
Amherst, U.S.A
Independent Researcher
Received 4 January 2022; accepted 4 January 2022
Corresponding Author:
Jaap C. Hanekamp, Chemistry, Roosevelt Academy, Lange Noordstraat 1,
Middelburg NL-4331 CB, Netherlands.
expand our knowledgebase on the ecological consequences, good
and bad, of atmospheric depositions of nitrogen.
Declaration of Conicting Interests
The author(s) declared no potential conicts of interest with respect to
the research, authorship, and/or publication of this article.
The author(s) received no nancial support for the research, au-
thorship, and/or publication of this article.
Jaap C. Hanekamp
1. Bij12. Landelijke monitoringsrapportage Natura 2000 en
Stikstof 2019. Utrecht: BIJ12; 2020.
Natura-2000-en-Stikstof-2019.pdf. Accessed January 3,
2Dose-Response: An International Journal
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