Impact of dissolved organic carbon (DOC) deposition on soil solution DOC Intern(ation)al data evaluation Arne Verstraeten ICP Forests combined Expert Panel meeting Zagreb, 27-31 March 2017 Joint session EP on soil and soil solution and EP on deposition

1. Background Atmospheric deposition and canopy leaching are a considerable input of DOC and DON to the forest floor, according to a study that compiled published data from case-studies in 42 temperate forest sites: variation in throughfall fluxes could explain 46% and 65% of the variation in DOC and DON fluxes from the forest floor, respectively (Michalzik et al. 2001) However, a study on 5 level II plots in Flanders found that deposition fluxes of DOC were strongly correlated with soil solution fluxes of DOC, but their seasonal peaks were not simultaneous, suggesting a limited impact of deposition DOC on soil solution DOC (Verstraeten et al. 2014 AE)

The input of canopy-derived DOC and DON is expected to increase because climate warming increases the length of the growing season and could trigger more frequent and intense attacks of phytophagous insects (Pitman et al. 2010 STE) More information about the role of DOC and DON deposition on soil solution is thus needed, with particular attention for seasonality effects

2. Objective Gain more insight on the relationship between concentrations (and fluxes) of DOC and DON in deposition and soil solution by studying the link between seasonal patterns and temporal peaks in open field precipitation, throughfall and different soil layers

2. Research questions How strong do DOC and DON concentrations in soil solution depend upon the concentrations in deposition? Are there differences between coniferous and (deciduous) broadleaved forests? Can we refine our knowledge of the seasonal patterns of DOC and DON and find links to events in the annual forest cycle (growing season, bud burst and unfolding of leaves, flowering and pollen distribution, shedding of leaves, …)?

3. Hypotheses The concentrations of DOC and DON in throughfall are related to the concentrations in open field precipitation The concentrations of DOC and DON in soil solution are influenced by the concentrations of DOC and DON in stand deposition (throughfall + stemflow) => peaks in soil solution coincide with or are preceded by peaks in stand deposition The concentrations of DOC and DON in soil solution can be linked to major seasonal events in the forest, and more particularly in spring (unfolding of leaves, pollen distribution) and autumn (shedding of leaves)

4. Methods - general analysis (all data) - coniferous, broadleaved deciduous, (broadleaved evergreen?) - selected number of individual tree species (Fagus sylvatica, Quercus robur, Pinus sylvestris, Picea abies, …) (?) Seasonal patterns (expected to be non-linear): general additive mixed models (gamm’s) Relationship between concentrations in deposition and soil solution: Linear Mixed Models (LMM’s)?

5. Data requirements Corrected aggregated deposition data (open field and throughfall) (1990-2011) but: no DOC included yet! Corrected aggregated soil solution data (1991-2013) Thanks to Peter, Elisabeth, Jim & co.! zero-tension and tension lysimeters were selected DOC - Available data (raw) => 145.444 analyses (O horizon: 23.372) Soil Layer Type # Countries # plots Total <30 obs. >100 obs. Organic 10 102 7 68 Hydromorphic 2 6 5 Mineral 24 222 14 144

DOC - O horizons The number of countries with DOC measurements is quite limited, which restricts the possibilities for translating the result to the European level

DOC - O horizons But there seem to be quite some plots with sufficient DOC data

DOC - O horizons

DON – available data (raw) => 27.982 analyses (O-horizon: 4.497) Soil Layer Type # Countries # plots Total <30 obs. >100 obs. Organic 9 84 21 8 Hydromorphic 1 Mineral 20 146 31 79

DON - O horizons Also the number of countries with DON measurements is quite limited, which restricts the possibilities for translating the result to the European level

DON - O horizons The number of plots with long data series on DON is limited, but many plots have at least 3 (but will be reduced to 2!) years of data

DON - O horizons

Main issues: - Depositions of DOC still need to be calculated - Deposition dataset is now limited to 2011 (2012-2013 not yet included) - Some data cleaning may be still needed: # small number of plots/countries with high year-to-year variation ~ analytical problems?) # some negative DON values occur

6. Cooperation Communication of progress via project group ‘Impact of DOC deposition on soil solution DOC’ on the ICP Forests website. http://icp-forests.net/group/impact-of-doc-deposition-on-soil-solution-doc Please join if you would like to contribute! (currently: 15 participants) Participants will be invited for coauthorship (paper, conference abstracts, …) Meeting via Skype (~ 1 time per month) to discuss methods, data issues, results,… Document sharing: google drive, sharepoint, …? Supervision: EP on deposition, EP on soil and soil solution

7. Timing Update of aggregated deposition dataset: 2017-… (Peter?) Data screening and analysis: 1/1/2018 – 30/9/2018 Present first results at 7th ICP Forests Scientific Conference (May 2018) Writing draft paper: 1/10/2018 – 01/03/2019 (to be discussed at next EP meeting in March 2019?) Submission: April-May 2019

DOC – Seasonal pattern (O horizon)

DOC – Seasonal pattern (O horizon)

DON – Seasonal pattern (O horizon)

Thank you for your attention! Any questions?