1. Development of a new method for peat acidity critical loads in the UK Chris Evans, Jane Hall, Ed Rowe

2. Why revisit peat? CountryPeat area (km 2 ) 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

3. Peat acidification in 1989 (Scotland)

4. Mean ANC -11  eq l -1 Mean Alkalinity -82  eq l -1 Peat acidification in 2014 (Wales) Chemistry of water draining a blanket bog, 2012-2013 Acid episode in a peat stream, January 2014

5. 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...

6. 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

7. 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

8. “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”.

9. Why the focus on Calluna? Parys Mountain Copper Mine, Anglesey, North Wales

10. 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

11. Problems with these methods 1.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

12. ANC = xBC n+ + NH 4 + - xSO 4 2- - NO 3 - ANC = Ca 2+ + Mg 2+ + Na + + K + + NH 4 + - SO 4 2- - Cl - - NO 3 - ANC = OH - - H + + Organic acids – Al n+ 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:

13. Critical thresholds for UK ecosystems HabitatThreshold ForestsCa/Al = 1 FreshwaterANC = 0 or 20 HeathlandSkokloster weathering rate (ANC = 0) GrasslandSkokloster weathering rate (ANC = 0) MontaneSkokloster weathering rate (ANC = 0) Peat (old)pH = 4.4 Peat (new)ANC = 0

14. Prediction of mineral N and SO 4 leaching Mineral N = N deposition – DON - N peat SO 4 = S deposition – DOS - S peat NO 3 = Mineral N x f Nitrification Basic N and S flux equations:

15. Empirical basis for modelling N responses Sphagnum (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)

16. 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 %NPeat C accumulation rate Peat N accumulation rateMineral N leaching

17. Critical load function? CLF for an example site assuming 100% nitrification of mineral N

18. Option 3: Omit N from the acidity critical load? Acidity impacts of nitrogen deposition ANC = xBC n+ + NH 4 + - xSO 4 2- - NO 3 - 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 NH 4 +, all sitesSo: More mineral N -> higher ANC and pH

19. Conceptual C and S responses to S deposition Peat % SPeat C accumulation rate Predicted versus observed porewater SO 4 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

20. Drivers – xSO4 vs xBC deposition xSO 4 depositionxBC deposition

21. Old methodNew method Peat CLmaxS, old and new

22. Old methodNew method Peat CL exceedance by S, old and new

23. Why less exceedance with new method? Observed ANC vs observed pH

24. With peat S accumulation Without peat S accumulation Peat CLmaxS, with & without peat S sink

25. 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%)

26. 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 NH 4 + 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?

27. Thanks for listening Comments/questions? Max? Julian...?

28. 2012 Status Report – 1 hit Dutch contribution to 2012 CCE Status Report: “Based on the evaluation it seems that the current seepage maps are, at least partly, the cause of (unrealistic) low critical load values in peat soils.” “CLmaxS was five times higher with high seepage fluxes than at low seepage.” “In conclusion, it seems that the CLmaxS in wet peat soils are highly uncertain.” 2011 Status Report – 4 hits Nutrient N (Finland, Lithuania, Switzerland), VSD+ site application (NL) 2010 Status Report – nothing much.

29. 2020 baseline 2020 restoration

30. Cl deposition