development of a new method for peat acidity critical loads in the uk chris evans, jane hall, ed...
TRANSCRIPT
Development of a new method for peat acidity critical loads in the UK
Chris Evans, Jane Hall, Ed Rowe
Why revisit peat? Country Peat area (km2)Russia (European) 213,000 Finland 85,000 Sweden 66,000 Norway 28,000 Belarus 23,500 United Kingdom 17,500 Germany 13,000 Poland 12,500 Ireland 11,500 Estonia 10,000 Iceland 8,000 Ukraine 8,000 Latvia 6,600 Lithuania 3,520 Netherlands 2,350 France 1,500 Denmark 1,400 Romania 1,000 Europe total 515,000
Total area of peat in Europe 25 x the size of Wales
• Peats are inherently sensitive to atmospheric deposition
• Biodiversity and ecosystem functions (including C sequestration) are sensitive to acidity
Peat acidification in 1989 (Scotland)
-180
-160
-140
-120
-100
-80
-60
-40
-20
0
14:0
0
15:0
0
16:0
0
17:0
0
18:0
0
19:0
0
20:0
0
21:0
0
22:0
0
23:0
0
00:0
0
01:0
0
02:0
0
03:0
0
04:0
0
Alk
alin
ity
(eq
l-1)
31st January - 1st February 2014
3.5
3.6
3.7
3.8
3.9
414
:00
15:0
0
16:0
0
17:0
0
18:0
0
19:0
0
20:0
0
21:0
0
22:0
0
23:0
0
00:0
0
01:0
0
02:0
0
03:0
0
04:0
0
pH
31st January - 1st February 2014
-250
-200
-150
-100
-50
0
Alk
alin
ity
(eq
l1 )
Ditch Alkalinity
-200
-150
-100
-50
0
50
100
150
AN
C (
eql1 )
Ditch ANC
Mean ANC -11 eq l-1
Mean Alkalinity -82 eq l-1
Peat acidification in 2014 (Wales)Chemistry of water draining a
blanket bog, 2012-2013Acid episode in a peat stream,
January 2014
Problems with peats (bogs, not fens)
• Little or no base cation weathering• Little or no aluminium• Anaerobic• Acidity is strongly influenced by organic acids• So BC/Al ratios, SMB, VSD, MAGIC etc don’t really
work...
UK acidity critical loads based on a 0.2 pH change
UK peat critical loads: Method 1
• Critical load set as the amount of acid deposition required to cause peat porewater pH to fall 0.2 units below ‘pristine’ levels
• Described the 0.2 pH unit shift used to set critical loads by Smith et al. (1992) as “rather arbitrary”
• Proposed the use of ‘effective rainfall pH’, noting that this is essentially the same as peat porewater pH
UK peat critical loads: Method 2
UK peat critical loads: Method 2
• “Calluna vulgaris continued to flourish at acid deposition levels well above the existing critical load”.
• “Acid deposition adversely affected the growth of some moss species, notably Hypnum, Ceratodon purpureus, Sphagnum and Campylopus”.
• “A critical effective rain pH value of 3.6 is suggested”.
Why the focus on Calluna?
Parys Mountain Copper Mine, Anglesey, North Wales
Critical pH set to 4.4 – “reflects the buffering effects of organic acids upon peat drainage water pH”
UK peat critical loads: Method 2 continued
Problems with these methods1. Peat porewater pH is overwhelmingly determined by organic acidity
2. Many peatlands naturally have a pH < 4.4
3. Organic acid concentrations are (as demonstrated by Julian) difficult to model
4. The approach ignores three of the most important buffers against acidification in peatlands, namely:
i. Sulphate reduction and storage in accumulating peat
ii. Nitrogen retention and storage in accumulating peat
iii. Leaching of S and N in (non-acidifying) organic forms
Sulphur, nitrogen
DOS, DON
ANC = xBCn+ + NH4+ - xSO4
2- - NO3-
ANC = Ca2+ + Mg2+ + Na+ + K+ + NH4+ - SO4
2- - Cl- - NO3-
ANC = OH- - H+ + Organic acids – Aln+
An alternative approach?
Alkalinity-based definition of ANC (used for peat critical loads):
Charge-balance definition of ANC (MAGIC, VSD, etc...):
Omitting marine ions, this simplifies to:
Critical thresholds for UK ecosystems
Habitat Threshold
Forests Ca/Al = 1
Freshwater ANC = 0 or 20
Heathland Skokloster weathering rate (ANC = 0)
Grassland Skokloster weathering rate (ANC = 0)
Montane Skokloster weathering rate (ANC = 0)
Peat (old) pH = 4.4
Peat (new) ANC = 0
Prediction of mineral N and SO4 leaching
Mineral N = N deposition – DON - Npeat
SO4 = S deposition – DOS - Speat
NO3 = Mineral N x fNitrification
Basic N and S flux equations:
Empirical basis for modelling N responsesSphagnum (main peat-forming species)
becomes N enriched at higher N deposition
Lamers et al. (2000) in Noordwijkerhout report
Peat accumulation rates increase with low-to-moderate N deposition
Turunen et al. (2004)
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
1.60
1.80
0 1 2 3 4
%N
of p
eat f
orm
ation
N deposition (g S/m2/yr)
N1
N2
0
5
10
15
20
25
30
35
0 1 2 3 4
C ac
cum
ulati
on (g
C/m
2 /yr
)
N deposition (g S/m2/yr)
N1
N2 N3
N4
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
0 1 2 3 4
N ac
cum
ulati
on (g
C/m
2 /yr
)
N deposition (g S/m2/yr)
0.0
1.0
2.0
3.0
4.0
0 1 2 3 4
MIn
eral
N l
each
ing (
g N/m
2 /yr
)
N deposition (g N/m2/yr)
Conceptual C and N responses to N deposition
Increased peat N content Increased peat
C accumulation
Failure of peat C accumulation following species change
Peat %N Peat C accumulation rate
Peat N accumulation rate Mineral N leaching
Critical load function?
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.00 0.10 0.20 0.30
Tota
l S d
epos
ition
(keq
/ha/
yr)
N deposition (keq/ha/yr)
CLF for an example site assuming 100% nitrification of mineral N
Option 3: Omit N from the acidity critical load?
Acidity impacts of nitrogen deposition
05
101520253035404550
eq
l-1
Ammonium
Nitrate
0
10
20
30
40
50
60
70
80
90
100
0 10 20 30 40 50 60
AN
C du
e to
min
eral
N in
por
ewat
er (
eq l-1
)
Mineral N in porewater (eq l-1)
ANC = xBCn+ + NH4+ - xSO4
2- - NO3-
Option 1: Use default assumption of 100% nitrification? Unrealistic
Simple, consistent with data showing N is not acidifying, nutrient N handled separately
Option 2: Apply observed nitrification rates? Critical load function crashes!
X X
Mineral N mostly present as NH4+, all sites So: More mineral N -> higher ANC and pH
0
0.2
0.4
0.6
0.8
1
1.2
0 1 2 3
%S o
f pea
t for
mati
on
S deposition (g S/m2/yr)
S1
S2
0
5
10
15
20
25
0 1 2 3 4 5
C ac
cum
ulati
on (g
C/m
2/yr
)
S deposition (g S/m2/yr)
S3
S4
y = 1.5629x - 5.8901R² = 0.8687
0
20
40
60
80
100
120
140
0 20 40 60 80 100 120
Obs
erve
d SO
4(
eq l-1
)
Predicted SO4 (eq l-1)
Conceptual C and S responses to S depositionPeat % S Peat C accumulation rate
Predicted versus observed porewater SO4 concentrations
• %S content of peat follows (total) S deposition, up to a maximum of 1%
• DOS loss is assumed to follow DOC loss, with C/S ratio based on %S of peat formation
Drivers – xSO4 vs xBC deposition
xSO4 deposition xBC deposition
Old method New method
Peat CLmaxS, old and new
Old method New method
Peat CL exceedance by S, old and new
Why less exceedance with new method?
-100
0
100
200
300
400
500
3.5 4.0 4.5 5.0 5.5 6.0
ANC
(eq
l-1)
pH
Observed ANC vs observed pH
With peat S accumulation
Without peat S accumulation
Peat CLmaxS, with & without peat S sink
With peat S accumulation
Without peat S accumulation
CL exceedance, with & without peat S sink
Exceedance: 3725 ha (0.007%)
Exceedance: 77907 ha (14.3%)
Issues for further development
• Sulphur uptake into peat – what really controls %S?• Variable peat accumulation rates• Effects of drainage and management• Interactions with N deposition – if CLnutN is exceeded, will decreased
peat accumulation increase susceptibility to acidification, or will excess NH4
+ raise ANC and pH?
• Effects on downstream ecosystems – alkaline peats, acid waters?• Legacy effects of S and N accumulation – sensitivity to climate change
and management?• Internal xBC supply?• Can we relate ANC to evidence of ecological damage?
Thanks for listening
Comments/questions? Max? Julian...?