1. Nitrogen status of Flemish forests is improving
Arne Verstraeten, Johan Neirynck, Nathalie Cools, Peter Roskams, Steven Sleutel, Stefaan De Neve
5th ICP Forests Scientific Conference
10–12 May 2016 – Luxembourg

2. 1. Background on our study
Atmospheric depositions of SO42‒ overall decreased by 6% per year between and 2010 in European Level II plots (Waldner et al., 2014 ATM ENV)
In a number of Level II plots this has initiated local recovery from acidification, e. g. in the Czech republic (Oulehle et al., 2011 GCB), the UK (Vanguelova et al., 2010 ENV POL) and Flanders (Verstraeten et al., 2012 ATM ENV)
Atmospheric DIN depositions decreased at a slower pace (on average 2% per year) and only in certain areas (particularly in western central Europe) (Waldner et al., 2014 ATM ENV)
The N critical loads and N critical limits are still exceeded in a large part of European Level II plots (Waldner et al., 2015 AFS)
N saturation is thus a matter of concern and is therefore included in the Pan- European Indicators for Sustainable Forest Management (SFM), more specifically in the indicator ‘Soil condition’ under the ‘Criterion 2: Maintenance of Forest Ecosystem Health and Vitality’ (
To what extent European forests could actually recover from N saturation needs to be further investigated

3. 2. Objective and setup of our study
The objective of our study was to evaluate recent evolutions in the N status of five ICP Forests Level II plots in Flanders, northern Belgium under recovery from acidification and high but decreasing DIN depositions
We used long-term monitoring data on the elemental concentrations in - deposition - soil solution - foliage
We evaluated trends (SMK or MK) for 3 indicators of N saturation: - the DON:TDN ratio (2005‒2014) (modified from ppt Williams et al., 2004) - the DOC:NO3− ratio (2002‒2014) (Taylor and Townsend, 2010 NATURE) - the foliar N:P ratio (1999‒2013) (Mellert and Göttlein, 2012 Eur J For Res)
We compared with critical limits for N saturation

4. 3. Hypothesis
We hypothesized that the foliar N:P ratio decreased and that the soil solution DON:TDN ratio and DOC:NO3− ratio increased, indicating an improvement in forest N status

5. 4. Results

6. 4.1 Trends in the DON:TDN ratio
DPSIR
The DON:TDN ratio generally increased (2005−2014)
Less marked recovery in the GON site. Due to higher clay content in the mineral soil? More clay -> higher CEC -> higher N retention capacity and N availability -> more NO3‒ (Callesen et al., 1999)
Verstraeten et al., in prep.

7. Contribution of N compounds (%)
N status
2: <33% DON
1: 33−67% DON
0: 67% DON
ppt Williams et al. (2004)
Accordingly, the proportion of DON in TDN increased, reflecting recovery from N saturation (improving N status)
Verstraeten et al., in prep.

8. 4.2 Trends in the DOC:NO3‒ ratio
DPSIR
The DOC:NO3‒ ratio in soil solution increased in four plots (2005−2014)
No recovery in the GON site. Due to higher clay content mineral soil? (Callesen et al., 1999)
Verstraeten et al., in prep.

9. DOC:NO3‒ ratio and N status
DPSIR
The DOC:NO3− ratio passed the critical inflection point for heterotrophic bacteria in soils (5.22) in 4 plots (improving N status) (Taylor and Townsend, 2010 NATURE)
Verstraeten et al., in prep.

10. What could explain the better N status?
1. Recovery from acidification
SO42‒ depositions decreased by 4.2‒7.5% per year between 1994 and 2010 in the five Level II plots
This resulted in chemical recovery in the soil solution: - decreasing Al3+ concentrations - increase of pH by 0.5 units during the past 10 years - thus better solubility of DOM and increasing DOC and DON concentrations (Verstraeten et al., 2012, 2014 ATM ENV) (Verstraeten et al., 2016 SCI TOT ENV)

11. What could explain the better N status?
2. Decreasing DIN depositions
Throughfall DIN depositions decreased by 2.4‒5.0% per year between and 2010 in the five Level II plots (mainly NH4+) (Verstraeten et al., 2012 ATM ENV)

12. Soil solution DON:TDN ratio vs. DIN deposition
We observed a predominantly negative correlation between the DON:TDN ratio and throughfall deposition of DIN
This indicates that decreasing DIN deposition contributed to a better N status
Verstraeten et al., in prep.

13. Soil solution DOC:NO3‒ ratio vs. DIN deposition
We observed a predominantly negative correlation between the DOC:NO3‒ ratio and throughfall deposition of DIN
Again this indicates that decreasing DIN deposition contributed to a better N status
Verstraeten et al., in prep.

14. 4.3 Trends in the foliar N:P ratio
DPSIR ns ns ns
Corsican pine
Scots pine
European beech ns ns
Common oak
European beech
The foliar N:P ratio remained unchanged between 1999 and 2013 (stable N status)
The foliar N:P ratio was near or above the upper critical limit, except in the HOE site (normal range) (Mellert and Göttlein, 2012)
Verstraeten et al., in prep.

15. Why foliage doesn’t follow soil solution?
Clues from the N saturation hypothesis (Aber et al., 1989, 1998, Galloway et al., ): NO3‒ leaching is negligible in the early stages of N saturation (0‒2), but increases exponentially in the late stages of N saturation (2‒3) Foliar N concentration is expected to increase ±linearly from N saturation stage 0 to 2 and to slightly decrease from stage 2 to Therefore, during initial recovery from N saturation foliar N content is expected to lag behind on soil solution NO3‒ concentration
Figure. Changes in several ecosystem functions with increasing Nr availability or degree of N saturation (modified from Aber et al. 1998). Abbreviations: C, carbon; N, nitrogen; N2O, nitrous oxide; NPP, net primary productivity. Galloway et al. (2003)

16. 5. Conclusions
Trends in soil solution chemistry (DON:TDN ratio and DOC:NO3‒ ratio) confirmed our hypothesis that the N status of forests in Flanders is improving under recovery from acidification and high but decreasing DIN deposition
However, the foliar N:P ratio seems to lag behind
Whether these results make part of a more general trend should be further checked on a wider spatial scale, using long-term data from international monitoring networks like ICP Forests
The DON:TDN ratio and DOC:NO3‒ ratio in soil solution allow to assess forest N status with a high frequency. Accordingly they could provide interesting information with regard to SFM-indicators. More work is needed to fine- tune the critical levels e.g. in relation to soil type

17. Acknowledgements I would like to thank:
The colleagues from our technical staff and laboratory for the collection and analysis of samples and advice on statistics
My PhD promoters from Ghent University: prof. dr. ir. Stefaan De Neve prof. dr. ir. Steven Sleutel

18. Thank you for your attention!

19. N status Foliar N concentration DPSIR
Corsican pine
Scots pine
European beech
Common oak
European beech
Foliar N concentrations were near or above the upper critical limit (luxury consumption) (Mellert and Göttlein, 2012)
The foliar N concentration decreased only in the BRA site between 1999 and 2013
Verstraeten et al., in prep.

20. N status Foliar P concentration DPSIR
Corsican pine
Scots pine
European beech
Common oak
European beech
Foliar P concentrations were in the lower normal range (Mellert and Göttlein, 2012)
The foliar P concentration decreased only in the BRA site between 1999 and 2013
Verstraeten et al., in prep.