Role of Dissolved Organic Matter (DOM) in Copper Mobilisation in Soils

1. Role of Dissolved Organic Matter (DOM) in Copper Mobilisation in Soils Fien Amery, Fien Degryse, Wim Degeling, Thomas Noë and Erik Smolders 1

2. Overview Overview • Introduction • Cu Mobilising Potential (CuMP): a new method to quantify Cu affinity of DOM • Experiment 1: Variation of CuMP and DOM quality among soils, incubation and extraction methods • Experiment 2: Variation of CuMP and DOM quality with soil depth • Experiment 3: Incubation simulation of the field experiment • Conclusions • New methods and experiments: preliminary results and problems 2

3. Introduction • Introduction Dissolved Organic Matter (DOM) = organic matter in solution that passes 0.45 µm membrane In soils, DOM is a complex mixture of components with varying molecular weight, functional groups,… Sugars, small aliphatic and aromatic acids Humic acid Fulvic acid 3

4. Cu2+ Cu-DOM liquid solid Introduction DOM mobilises hydrophobic contaminants and trace metals in soils By complexing Cu, DOM brings Cu in solution → higher Cu mobility in soils Cu mobility is directly proportional to DOC concentration (= quantity), provided that the DOM quality is constant 4

5. Introduction Field experiment:

6. Introduction How variable is DOM quality and quantity: • before/after drying-wetting cycles • at varying duration of air-dry soil storage • at various incubation times • with different extraction procedures • with different soils • at different soil depths • after addition of soil amendments (manure, plant materials, waste water) 6

7. CuMP 2. Copper Mobilising Potential (CuMP) Limited research on the importance of DOM quality for Cu mobilisation Most studies compare Cu complexation capacities of DOM: not relevant for (uncontaminated) soils Need for a new method to measure Cu affinity of DOM at relevant conditions 7

8. Cu-DOM solution Cu2+ + DOM Cu Ca resin CuMP Use of a resin to buffer the free Cu2+ activity to a constant, relevant value: 10-11.3 M (checked with EDTA) Solution with DOM in equilibrium with Cu/Ca resin Constant pH 7.0 and [Ca]eq= 1.5 mM Equilibrium after 6 days end-over-end shaking CuMP = ≈ All conditions ((Cu2+), pH and [Ca]) kept constant and normalisation for DOC-concentration → CuMP only a measure for the Cu affinity of DOM 8

9. Exp 1: M&M 3. Experiment 1: Variation of CuMP among soils, incubation and extraction procedures 13 uncontaminated soils • varying soil characteristics • varying duration of air-dry storage (10 years air-dry - freshly from the field) • varying length of moist incubation: short (4 days) and long (> 1 month) • with and without 2 drying-wetting cycles before incubation • different soil solution extraction procedures: • Pore water (pw) by centrifugation • Water extraction (solid/liquid ≈ 1/5) • 0.01 M CaCl2 extraction (solid/liquid ≈ 1/5) Analysis on soil solution: • DOC-concentration • CuMP • Specific UV-absorbance SUVA = SUVA is a measure for the aromaticity of DOM • DOM acidity measured by potentiometric titration between pH 3 and 11 9

10. Hypothesis: • DOM flush after drying-rewetting of the soil • DOM released after drying-rewetting is derived from decaying biomass and has a low reactivity (not humified); this DOM is readily biodegradable

11. Results Exp 1: results Effect of incubation time and extraction procedure, soil 7: • Short incubation time: • Long incubation time 11

12. Exp 1: results Effect of extraction procedure and incubation time on CuMP of soil 1 to 9 CuMP of soil 1 to 9 with different extraction procedures and incubation times CuMP: water extract > CaCl2 extract > pore water long incubation > short incubation fresh from field > dry storage prior to incubation 12

13. Exp 1: results Effect of drying-wetting cycles and incubation period: • Ter Munck > ≥ < BS 1 ≈ ≈ 13

14. Exp 1: results Positive significant correlation between SUVA and CuMP SUVA SUVA: same trends as CuMP 14

15. Exp 1: results Total DOM acidity No clear relationships between total DOM acidity and CuMP Q7-11 (phenolic acidity) increased during incubation Note: soil samples kept 10 years air-dry: Extremely large DOC, low quality DOC (mg/L) CuMP (mmol Cu/kg C) soil 1 1702 4.2 soil 2 1740 3.9 15

16. CuMP ↓ SUVA ↓ Exp 1: discussion Discussion Long dry storage and/or drying-wetting cycles cause a flush of low quality DOM: low CuMP and low aromaticity Hypothesis: DOM from microbial cellysis (non-humified components, low Cu affinity) As CuMP, SUVA and Q7-11 (phenolic acidity) increases during incubation: low quality DOM is preferentially degraded by micro-organisms dry storage dry-wet cycles incubation CuMP ↑ SUVA ↑ Q7-11 ↑ 16

17. Exp 1: discussion Effect of extraction procedure soil with high quality DOM soil with low quality DOM solid organic matter solid organic matter extraction OM in pore water OM in pore water centrifugation 17

18. Exp 1: conclusions Conclusions experiment 1 • Quality (CuMP) varies up to 10-fold with soil treatment and extraction procedure • Cu affinity of DOM is related with aromaticity of DOM • DOM from air-dried soils differs in quality from soils not recently subjected to drying • Extracted DOM is different from true pore water DOM

19. Exp 2: M&M 4. Experiment 2: Variation of CuMP with soil depth Materials and methods • 3 soil columns from Ter Munck divided in 3 dephts: • 0-20 cm • 20-40 cm • 40-60 cm • Wetted until same water content • Moist incubation at 20°C during 4 days • Pore water extraction by centrifugation • Analysis on pore water: • DOC-concentration • CuMP • SUVA 19

20. Exp 2: hypothesis Hypothesis: • DOM in deeper layers more humified and of higher quality because easily degradable, low quality DOM is degraded in upper soil layers OR • DOM in deeper layers less humified and of lower quality because higher quality DOM (with higher aromaticity and hydrophobicity) is preferentially adsorbed by soil particles

21. Exp 2: results Results 21

22. Exp 2: discussion Discussion • Layer 0-20 cm not significantly different from 20-40 cm → field was ploughed 1 month prior to soil sampling • Good positive correlation between SUVA and CuMP (R² = 0.80) • CuMP and SUVA was lowest in 40-60 cm → preferential adsorption of hydrophobic and aromatic DOM on soil particles in higher soil layers (low quality DOM migrates easier through the soil) New depth experiment in February-March: same trends? Additional parameters will be measured (total C, total Cu) 22

23. Exp 3: M&M 5. Exp 3: Variation of DOM quality and quantity with soil amendments Materials and methods • Fresh top soil from the field experiment • 6 treatments: • Blank • Pig manure (60 mL/kg soil) • Waste water (770 mL/kg soil, in 5 drying-wetting cycles) • Waste water without DOM (770 mL/kg soil, 5 d-w cycles) • Straw (3 g/kg soil) • 14C-labeled maize roots (1 g/kg soil) • Pore water sampled after 3, 20 and 64 days incubation (20°C) • Analyses: DOC, SUVA, CuMP, 14C, respiration between day 38(34) and 64 23

24. Exp 3: hypothesis Hypothesis: • Day 3: higher DOC concentrations and lower DOM quality in soils with amendments (except for amendments with high quality DOM) • During incubation: DOC concentrations decrease and DOM quality increases due to humification • Maize-derived DOM in soil solutions diminishes in time

25. Exp 3: results Results anaerobic? high Ca-conc

26. Exp 3: results CuMP pig manure: 79 mmol/kg C • CuMP • CuMP vs SUVA

27. Exp 3: results 14C 14C: 3.3% of DOC Respiration from day 38(34) till 64 14C: 3.1% of respired C

28. Exp 3: discussion Discussion • Blank: DOC, SUVA and CuMP rather constant in time • Soil with waste water and waste water without DOM: Flush of low quality DOM due to drying-wetting cycles, this flush is easily degradable • Soil with pig manure: same trends as for the waste water But CuMP and SUVA of pig manure very high? • Soil with straw and maize: DOC remains high SUVA comparative to blank: extra DOM of plant material comparative aromaticity to basic DOM? BUT: CuMP initially high, decreases during incubation • Respiration ~ DOC concentration in soil solution

29. Conclusions 6. Conclusions • Large variation in Cu affinity of DOM → not only DOM quantity (= DOC-concentration) but also DOM quality is important in prediction of Cu mobilisation in soils DOM of soils kept dry in the lab ≠ DOM of soils freshly from the field Differences in DOM quality in depth, and with soil amendments 29

30. New questions 7. New questions • Other DOM characteristics? → Some tests on fluorescence measurements: no additional information • Can the variation in DOM quality be explained by the relative presence of different DOM fractions with more homogeneous quality? • Hydrophobic/hydrophilic fractions • Low / high molecular mass fractions → Tests at this moment

31. Fractionation DAX-fractionation • Fractionation based on hydrophobicity • Sample at pH 2 over DAX-resin (methylmethacrylate polymer): hydrophobic components adsorb, hydrophilics don’t Elution with 0.1 M NaOH: hydrophobic components desorb • Technique normally used for large volumes, over large DAX-columns • Goal: adapt the method for small soil solution volumes • Hypothesis: • DOM with higher CuMP and SUVA: relatively more hydrophobics • SUVA and CuMP rather homogeneous in one fraction Low quality DOM High quality DOM Hydrophilic Hydrophobic Hydrophobic Hydrophilic

32. Fractionation DAX in column • Some tests with one soil solution • SUVA (hydrophilic): +/- 7 L/(g.cm) SUVA (hydrophobic): 40 → 20 L/(g.cm) • Advantages: nice distribution, less dilution (concentration of the hydrophobic part), less resin bleeding • Disadvantages: less reproducible (certainly when done manually) → Purchase of chromatographic apparatus?

33. Fractionation DAX in batch • Some tests with one soil solution • Resin bleeding → apply blank correction • SUVA (hydrophilic): +/- 8.5 L/(g.cm) SUVA (hydrophobic): 30 L/(g.cm) • Other tests: good reproducibility, also when using diluted samples • Advantages: more reproducible • Disadvantages: no fluent distribution, more dilution, more resin bleeding

34. Fractions not homogeneous Hydrophilic Hydrophobic Fractionation DAX in batch • Tests with short- and long-term incubated Zegveld soil • SUVA (short) = 12.6 L/(g.cm); SUVA (long) = 22.0 • Hydrophilic/hydrophobic = • 3.1 +/- 0.1 for short-term incubated soil • 2.1 +/- 0.5 for long-term incubated soil • BUT:

35. Fractionation What now? • Purchase of chromatographic apparatus (also for gel permeation chromatography) for fractionation over columns • OR: serial batch fractionation (but high dilution and resin bleeding!) • Further fractionation in acids, neutrals, bases?

36. Fractionation Gel permeation chromatography • Fractionation based on molecular size • Which molecular size ranges? • Hypothesis: high quality DOM consists of relatively larger molecules

37. Future 8. More future research • New depth experiment • Field experiment: modelling Cu fluxes by means of DOM quality measurements • Soil column experiments: adding DOM with different quality • analyse Cu fluxes • measuring Cu speciation by Donnan dialyse • measuring Cu-DOM dissociation kinetics by the competitive ligand method 37

38. Thank you for your attention!