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)

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Alk

alin

ity

(eq

l-1)

31st January - 1st February 2014

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pH

31st January - 1st February 2014

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alin

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(eq

l1 )

Ditch Alkalinity

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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)

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%N

of p

eat f

orm

ation

N deposition (g S/m2/yr)

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N2

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cum

ulati

on (g

C/m

2 /yr

)

N deposition (g S/m2/yr)

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N2 N3

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cum

ulati

on (g

C/m

2 /yr

)

N deposition (g S/m2/yr)

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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?

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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

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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

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%S o

f pea

t for

mati

on

S deposition (g S/m2/yr)

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C ac

cum

ulati

on (g

C/m

2/yr

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S deposition (g S/m2/yr)

S3

S4

y = 1.5629x - 5.8901R² = 0.8687

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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?

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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...?