The critical load is a key concept in air pollution, representing the maximum flux of pollutant that can be withstood by an ecosystem without causing damage immediately or in the long term. Nutrient nitrogen (N) damages ecosystems by changing the competitive balance among species. Excess N favours tall-growing, light-demanding species, which can shade out smaller species. For each sensitive habitat, an empirical critical load for nutrient N (CLempN) has been defined on the basis of experimental evidence. The values used are currently being assessed in the light of experimental and survey data that have emerged since the last review in 2011. Any changes in CLempN values would affect the statistics used to assess risks to UK ecosystems, and might affect decisions about land use and development.

1. Revision of critical
loads for nutrient
nitrogen
Ed Rowe
National Focal Centre for Modelling & Mapping
of Critical Loads & Critical Levels,
UK Centre for Ecology & Hydrology,
Bangor.

2. Outline
Rowe et al. (2021) Trends Report.
https://uk-air.defra.gov.uk/library/reports?report_id=1020
Average Accumulated Exceedance of Critical
Load for nutrient-N, annual mean 2017-19
1. Impacts of nitrogen pollution on ecosystems
2. Critical Loads and what they are used for
3. Setting values for the empirical CL for nutrient N
4. Revision of CLempN values - progress

3. Impacts of nitrogen on ecosystems
Maskell LC et al. (2010) Global
Change Biology 16, 671–679
Air pollution harms ecosystems through:
- direct toxicity (ammonia, ozone)
- accumulation (e.g. heavy metals)
- acidification (N, S)
- eutrophication (N)
IAQM Routes to Clean Air – 12th October 2021 – Ed Rowe ecro@ceh.ac.uk
Total N deposition kg ha-1 yr-1
Plant
species
richness
Effects of N deposition rate on plant
species-richness in UK habitats
(data from UKCEH Countryside Survey)

4. Eutrophication
Nitrogen favours the growth of tall, light-
competitive species. Short species are lost.
2.4
2.6
2.8
3.0
3.2
3.4
3.6
3.8
4.0
Threatened Rare Uncommon Decreasing Increasing
Conservation status
Canopy
Height
Class
c
bc
ab
a
d
Hodgson et al. (2014) Functional Ecology 28: 1284-1291
Hautier et al. (2009) Science 324 (5927) 636-638.
IAQM Routes to Clean Air – 12th October 2021 – Ed Rowe ecro@ceh.ac.uk

5. Critical Loads
Critical Load: “a quantitative estimate of an exposure to one or more pollutants
below which significant harmful effects on specified sensitive elements of the
environment do not occur according to present knowledge” Nilsson & Grennfelt (1988)
Critical Load values take into account ecosystem resilience
Values are set to prevent harm in the long term, by scientific consensus,
e.g. in the review of empirical Critical Loads for nutrient-N
N deposition
0 CLempN
Critical Load for nutrient-nitrogen
2D critical-load function for acidity
unfavourable
condition
favourable
condition
IAQM Routes to Clean Air – 12th October 2021 – Ed Rowe ecro@ceh.ac.uk

6. Exceedance of CL for Nutrient N – UK trends
Rowe et al. (2021) Trends Report.
https://uk-air.defra.gov.uk/library/reports?report_id=1020

7. National Focal Centre for Modelling & Mapping
UN-ECE Convention on Long-Range Transboundary Air Pollution (CLRTAP)
 International Cooperative Programme on Modelling and Mapping (ICP-M&M) of
Critical Loads and Critical Levels
 National Focal Centre (NFC)
ICP-Modelling and Mapping – aims:
(a) assess damage to forests, crops, natural vegetation, soils, surface and groundwaters, and materials by
determining critical levels and loads for the response of these systems, with particular attention to the direct
effects of air concentrations of sulphur dioxide (SO2), nitrogen oxides (NOx) and ozone (O3), and the indirect
effects of (long-term) deposition of sulphur and nitrogen compounds;
(b) map geographical areas to determine the scope and extent of pollutant depositions and concentrations
which exceed critical loads and levels;
(c) establish appropriate methods as a basis for assessing potential damage.
National Focal Centre (UKCEH Bangor)
• Generates UK and DA-scale statistics for air pollution pressures on ecosystems
• Coordinates UK involvement in ICP-M&M, e.g. coordinating review of CLempN
IAQM Routes to Clean Air – 12th October 2021 – Ed Rowe ecro@ceh.ac.uk

8. Critical Loads for nutrient nitrogen
Bobbink & Hettelingh (Eds.) (2011) Review
and revision of empirical critical loads and
dose-response relationships: Proceedings of
an expert workshop, Noordwijkerhout, 23-25
June 2010.
2011
IAQM Routes to Clean Air – 12th October 2021 – Ed Rowe ecro@ceh.ac.uk

9. Summary of assigned Critical Loads
IAQM Routes to Clean Air – 12th October 2021 – Ed Rowe ecro@ceh.ac.uk
• Bobbink & Hettelingh (2011) proposed CL ranges and reliability
• Hall et al. (2011) proposed mapping values for the UK. Some are modified by rainfall.

10. Timeframe for CLempN Review
IAQM Routes to Clean Air – 12th October 2021 – Ed Rowe ecro@ceh.ac.uk

11. e.g. Grasslands
IAQM Routes to Clean Air – 12th October 2021 – Ed Rowe ecro@ceh.ac.uk

12. Evidence: experimental additions
IAQM Routes to Clean Air – 12th October 2021 – Ed Rowe ecro@ceh.ac.uk
• Clear evidence for effects of N alone
• But previous N deposition at a site may mean it has already been affected

13. IAQM Routes to Clean Air – 12th October 2021 – Ed Rowe ecro@ceh.ac.uk
Evidence: gradient studies
• Effects of N may be confounded with other environmental gradients
• Evidence from a wide range of sites
• Gradient studies are being included in the 2021 review, for the first time

14. IAQM Routes to Clean Air – 12th October 2021 – Ed Rowe ecro@ceh.ac.uk
• Species may decline or increase with N deposition
• Some wildflowers, bryophytes and lichens are eutrophilic (fertility-loving)
• Relatively small declines in overall species richness obscure local extinctions
Evidence: effects on individual species
Sanionia uncinata
Cerastium arvense
Cerasium semidecandrum
Viola canina
Trifolium arvense
Vicia lathyroides
Stevens et al.
(2011) JNCC
Report 447.

15. IAQM Routes to Clean Air – 12th October 2021 – Ed Rowe ecro@ceh.ac.uk
• Above 8 kg N ha yr-1, the probability of finding common restharrow is
significantly less than it is at 5 kg N ha yr-1
• Is this a harmful effect?
• Chapter authors are assessing the relevance and certainty of evidence
Significant harmful effect?
Ononis repens

16. IAQM Routes to Clean Air – 12th October 2021 – Ed Rowe ecro@ceh.ac.uk
• Point of most-rapid change on the N gradient
• Some species have changepoints below current CL
Evidence from changepoint analyses?
Payne et al. (2013)
PNAS 110: 984-987.

17. IAQM Routes to Clean Air – 12th October 2021 – Ed Rowe ecro@ceh.ac.uk
• Atmospheric nitrogen pollution is extremely damaging to biodiversity and ecosystems
• Very small loadings of N cause measureable change, especially in historically clean areas
 Pressure to decrease value used for Critical Load
• Some changes may be acceptable
• For policy users, changes in CL are inconvenient
 Pressure to maintain the current value
• Chapter authors are weighing these pressures, as well as the evidence
• Final version of report April 2022
• Then UK NFC will decide on mapping values within the new ranges
Review outcomes

18. Acknowledgements
Rowe et al. (2020) Trends Report.
https://uk-air.defra.gov.uk/library/reports?report_id=1001
The National Focal Centre for modelling and mapping critical loads and critical levels is
supported by the UK Department for the Environment, Food and Rural Affairs (Defra) under
Contract AQ0849.