Pressure, midpoint and endpoint metrics of N
pollution: current and proposed metrics
Ed Rowe, Jones L, Dise NB, Evans CD, Mills G, Hall J,
Stevens CJ, Mitchell RJ, Field C, Caporn SJ, Helliwell
RC, Britton AJ, Sutton M, Payne RJ, Vieno M, Dore
AJ & Emmett BA
Outline and acknowledgements
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•
•
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Impacts of atmospheric nitrogen pollution
Pressure metrics
Midpoint metrics
Endpoint metrics
Acknowledgments
• Defra Project AQ0823: Research into measures to evaluate the benefits to
semi-natural habitats of reductions in nitrogen deposition (REBEND)
• CEH Project NEC05574
Biological Conservation(2016)
DOI: 10.1016/j.biocon.2016.11.022
Impacts of N pollution
Deposition rate may change
Chemical response may be delayed
Biological response may be
further delayed
Adapted from Posch, 2004, Manual on
methodologies and criteria for modelling
and mapping…
Damage delay time
Recovery delay time
Evidence for biogeochemical change
Moss tissue N
2.5
2
1.5
%N
Surveys have shown effects on e.g.
• Plant tissue %N
• mineralisable N
• Soil C/N (in some habitats)
1
0.5
Effects of experimentally increased N
deposition rate have been observed on e.g.
• Soil total N
• Soil C/N (sometimes)
• Plant tissue %N
• N leaching
0
0
10
20
30
40
50
60
70
N deposition (kg
Total N deposition
kg N ha-1 yr-1
ha-1
Dicra scopa
yr-1)
Racom lanug
Thuid tamar
Mineralisable N
Rowe et al 2016 Biol Cons
10.1016/j.biocon.2016.11.022
Rowe et al 2012 Sci Tot Env 434: 62-70
Total N deposition kg N ha-1 yr-1
Evidence for biological change
Effect of N deposition rate on prevalence
of Bromopsis erecta in lowland calcareous
grassland
Analysis of large UK floristic datasets
Some changes observed at 5-10 kg N
ha-1 yr-1 i.e. below Critical Load
Probability of presence
Declines with N deposition in
prevalence and/or cover of 91 species
Total N deposition kg N ha-1 yr-1
Stevens et al. 2011, JNCC Report 447
Emmett et al 2011, JNCC Report 449
UK N emissions & deposition declining
NOx
NH3
UK emissions of NOx
UK emissions of NH3
Total N deposition
(% of maximum)
100
80
60
RoTAP report, 2012
Temporal deposition sequence from GANE
project (Fowler et al 2004 WASP:Focus 4: 9-23)
40
20
0
1950
1970
1990
2010
2030
Aims of the REBEND study
• Summarise current knowledge of cumulative impacts and
recovery
• Assess how habitat response to N deposition can be evaluated
using current tools and evidence
• Recommend alternative metric(s) to evaluate the benefits of
reductions in nitrogen deposition
Pressure, midpoint & endpoint metrics
It is useful to distinguish metrics that:
a) represent the degree of pressure on the ecosystem;
b) can be seen as markers or midpoints that represent progress towards a desired
endpoint, e.g. chemical conditions that make it likely that this endpoint will be
achieved in future;
c) illustrate achievement of a desired endpoint, e.g. a metric that can be directly
related to favourable conservation status.
The terms do not necessarily relate to the timescale of change, and ‘midpoint’ should
not be taken to indicate progress half-way towards a goal.
Pressure metrics
Decreased deposition, CLnutN still exceeded
• Many UK habitats receive
considerably more than their CLnutN
• Marginal reductions result in little
change in area exceeded
• Are the benefits of marginal
reductions under-represented?
• How persistent are the effects of N?
Jane Hall & Massimo Vieno
(REBEND report)
Ratio N dep / CLnutN
Is cumulative deposition informative?
Cumulative deposition (across sites) –
response of soil pH, little response of %N
“Accumulated dose” = total N deposition
since 1945
Log change indices
Phoenix et al 2012 Global Change Biology
18, 1197–1215. (Review of experiments)
Accumulated N dose, kg N ha-1
Calculating cumulative deposition
Integration period
Integrating from a fixed date e.g. 1970
• obscures current change
• never allows decrease in cumulative N
Integrating for a preceding window (e.g. previous 30 years) would be more responsive
Current deposition
1 year ago
10 years ago
100 years ago
Decreasing
effect
Suggested periods:
• soil-based ecosystems: 30 years (CE30)
• epiphytic & epilithic ecosystems: 3 years (CE3)
Less buffering  shorter delays
Strongly-buffered ecosystems
e.g. with soil
Less-buffered ecosystems
e.g. epiphytic, epilithic
Integration threshold
Integrating above zero
• means spatial pattern is identical to that of current deposition
• implies that even natural N deposition is/was harmful
Integration threshold could be
• Steady-state Mass Balance Critical Load
= uptake (in managed habitats) + immobilisation + acceptable leaching + denitrification
• Empirical Critical Load CLempN
= N deposition shown to not damage habitats in long-term
There are issues with:
immobilisation (for how long?)
long-term (evidence-base used to define CLempN)
Geographical implications of using CE30
N deposition
2004-6
Cumulative N
deposition,
1970-2005
Cumulative N deposition
above CLnutN (for heath) in
preceding 30 years
Midpoint indicators
Midpoint indicators represent progress towards a desired endpoint
Soil pH
Signal dominated by recovery from sulphur pollution
Vegetation productivity
Rarely measured
Soil total N
Stock usually large, so affected little by changes in flux
Soil total N/C ratio
Increases with productivity; decreases with immobilisation
Soil mineral N or NO3
Variable, and zero over a large range, but still informative
Soil mineralisable N
Expensive for little extra information (cf. mineral N).
No standard method.
Tissue N/P
Can decrease with N deposition if phosphatase production
is stimulated
Promising midpoint metrics
(at lower N dep rates)
(at higher N dep rates)
Moss %N
normalised for species 
Moss Enrichment Index
N leaching
Midpoint metrics from models
deposition
‘passive’ N
‘slow’ N
Increases in soil N pools (extra over constant
low deposition scenario) with different
turnover rates to a hypothetical increase in
N deposition from 2 to 20 kg ha−1 yr−1 for the
period 1970-2000, as predicted by the
MADOC model for a peatland system.
available N
‘Pressure’
(deposition)
‘Risk’
(‘slow’ N pool)
‘Damage’
(plant-available N)
Endpoint metrics
• Illustrate achievement of a desired endpoint (NO3 in drinking water, biodiversity, …)
• e.g. species richness
• Hettelingh et al. 2013 (CCE workshop presentation) used a species-richness: N deposition
relationship to map European calcifuge grassland responses under different scenarios
• Assumes instantaneous recovery response
• Species-richness may not be an appropriate endpoint metric for all habitats
Stevens et al. 2010
Env Poll
richness
Species
Species richness
Relationship between N-deposition and species richness
35
y = 24,39e-0,0244x
R2 = 0,4023
30
25
20
Species richness
15
Expon. (Species
richness)
10
5
0
0,00
10,00
20,00
30,00
40,00
Total N-deposition (kg N/ha/yr)
50,00
Total N deposition, kg N ha-1 yr-1
Biodiversity – what is the ‘desired’ endpoint?
• Increased species richness due to invasion by eutrophiles is not a desired outcome
• Distinctive / specialist species may better represent beta-diversity
Heaths
Species
richness
Ranking according
to metric
Scarcity
Positive indicator
species
Ranking according
to specialists
Rowe et al. (2016) PLOS-ONE. doi:10.1371/journal.pone.0161085
Subshrub
cover
Conclusions: what are the best indicators?
Pressure
• Recent N deposition above CLempN
Midpoint
• 0-25 kg N ha-1 yr-1: moss enrichment index
• 25+ kg N ha-1 yr-1: N leaching rate
Endpoint
• nitrate concentration in drinking water
• greenhouse gas balance
• presence of (or habitat-suitability for) species distinctive for the habitat