1. 1 Trends in soil solution chemistry in temperate forests under on-going recovery from acidification and climate change in Flanders, Belgium Arne Verstraeten Workshop – Environmental change and forest ectomycorrhizal fungi 8–9 October 2015, Kew Botanic Gardens, London, UK

2. 2 Forest dieback in Central Europe in the 1970’s  clear link with air pollution and ‘acid rain’ UNECE Convention on Long-Range Transboundary Air Pollution (CLRTAP), 1979, Geneva  abate acidifying emissions International Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests (ICP Forests), 1985 Monitoring network  monitoring of forest condition: ±6000 Level I plots  clarify cause-effect relationships: ±500 Level II plots Background on European forest monitoring

3. 3 72 Level I plots crown condition 5 Level II core plots (are also LTER-sites) deposition, soil solution, litterfall, tree mineral nutrition, growth, meteorology, ground vegetation, crown condition, LAI, soil,… 6 Level II additional plots growth, ground vegetation, crown condition ICP Forests monitoring network in Flanders

4. 4 Level II core plots in Flanders

5. 5 Results

6. 6 Flanders is a high-deposition area, but: Acidifying depositions (NH 4 + and SO 4 2- ) showed decreasing trends (1994–2014) NO 3 - ± stable Depositions of base cations (Ca 2+, K +, Mg 2+ ) also decreased 1. Depositions (open field) A) Trends in atmospheric deposition

7. 7 Similar trends were observed below the canopy  Could be considered as inputs to the soil Verstraeten et al. (2012) 2. Depositions (throughfall + stemflow) Trends in atmospheric deposition

8. 8 The average rate of decrease is comparable to the trend observed in other forests in western and central Europe: SO 4 2- ± 6% per year N ± 2% per year (Waldner et al. 2015) Trends in atmospheric deposition

9. 9 The pH of precipitation and throughfall samples decreased until around 2004, and started to increase thereafter pH of precipitation and throughfall samples Open field Throughfall Trends in atmospheric deposition

10. 10 SO 4 2- concentrations in soil solution strongly decreased in the 5 plots NO 3 - concentrations decreased in 4 plots NO 3 - leaching halted suddenly in 2 plots; remains very high in 1 deciduous plot B) Trends in soil solution chemistry 1. Acidifying ions

11. 11 Ionic strength decreased in all plots Trends in soil solution chemistry

12. 12 At the beginning of the monitoring period pH was stable or declining at most depths and most plots In the mineral soil, pH is very low (3–4 at most plots), but significantly increasing since 2005 Trends in soil solution chemistry 2. pH

13. 13 Trends in soil solution chemistry

14. 14 In 3 plots the BC/Al ratio is below the critical limit for damage to fine roots (which could affect ectomycorrhiza?) In 4 plots the BC/Al ratio is further declining. A possible reason is the decreasing trend of BC depositions Trends in soil solution chemistry 3. BC/Al ratio

15. 15 The ANC is still below 0 at most depths, but shows a general increasing trend in the organic layer and in the mineral soil, indicating that soil acidification is slowing down. Trends in soil solution chemistry 4. Acid Neutralizing Capacity (ANC)

16. 16 DOC concentrations showed a tendency to increase in the mineral soil (2002–2014) Verstraeten et al. (2014) Trends in soil solution chemistry 5. Dissolved organic carbon (DOC)

17. 17 DON concentrations showed a tendency to increase in throughfall and in the mineral soil (2005–2014) Verstraeten et al., submitted Trends in soil solution chemistry 6. Dissolved organic nitrogen (DON)

18. 18 Given the stable inputs by precipitation and the significant correlation of DOC and DON with air temperature, climate warming might explain the increase of DON in throughfall water Trends in soil solution chemistry

19. 19 DOC:DON showed a tendency to decline (2005–2014), but variability is high DOC:DON increased after an exceptionally long spring drought in 2007, suggesting an effect of drying and rewetting DON increased more than DOC under acidification recovery and climate change Trends in soil solution chemistry 7. DOC:DON

20. 20 Conclusions Acidifying depositions decreased strongly in five ICP Forests Level II plots in Flanders This initiated chemical recovery of the soil solution: - increasing pH - decreasing SO 4 2– and NO 3 – concentrations - decreasing ionic strength - increasing DOC and DON concentrations (through increasing solubility) - increasing ANC But the BC/Al ratio is further decreasing

21. 21 Question Could these signs of chemical recovery also indicate a biotic recovery (fungi, bacteria, other groups of organisms)?

22. 22 Acknowledgements I would like to thank: These colleagues for their valued input: Johan Neirynck Bruno De Vos Peter Roskams Pieter Verschelde Gerald Louette Many other colleagues from our technical staff and laboratory for the collection and analysis of samples My phD promoters from Ghent University: prof. Dr. Ir. Stefaan De Neve prof. Dr. Ir. Steven Sleutel

23. 23 Thank you for your attention!