1. Organic Matter Characterization in Forest Soils
and Drainage Waters at Hubbard Brook
Chris E. Johnson
Dept. of Civil & Environmental Engineering
Syracuse University
David Ussiri
K’o H. Dai
David Kiemle

2. Outline Background: Carbon cycling at Hubbard Brook
Why Study Organic Matter?
Approaches to OM Characterization
Organic Matter Characterization at HBEF:
OM characterization – solid/dissolved
Clearcutting effects
Adsorption of OM in mineral soils

3. Global Change Context Carbon Sequestration
Understanding SOM decomposition
Adsorption/Desorption processes
Land-use Impacts on C cycling
Clearcutting effects
“Old fields” → Forest Conversion

5. Johnson et al. 1991, 1994

6. Johnson et al. 1994

7. Soil Solution DOC Concentrations
(Tension-Free Lysimetry)
Oa Horizon: 18 mg DOC L-1
Bh Horizon: 11 mg DOC L-1
Bs Horizon: 5 mg DOC L-1
Stream Water: 2 mg DOC L-1
Johnson et al. 2000

8. Natural Organic Matter
Biomass (Plant, Microbial, Animal)
Soil Organic Matter (SOM)
Dissolved Organic Matter (DOM)

9. Studying Organic Matter
Amount – Important, but coarse.
Elemental Composition – C,H,N,O,S,P
“Ecological Stoichiometry”
Ratios are often more interesting than actual concentrations (C:N, H:C, O:C)
Isotopic Composition
14C (“dating”)
13C (source of C)
Structure – Abundance of various C forms

10. Isolation, Concentration, “Fractionation”
ISOLATE OM from bulk soils.
CONCENTRATE OM in solutions.
FRACTIONATE OM according to desired characteristics.
We can study the products of any of these procedures

11. Fractionation Procedures
pH-Solubility Fractionation
“Humic Acid,” “Fulvic Acid,” “Humin”
Solubility Against Non-ionic Resins
“Hydrophilic,” “Hydrophobic” solutes

12. SOIL: extract with 0.5M NaOH
Solution
Fulvic acid
Displace with
0.1 NaOH
Purify and
and freeze-dry
Solution: Fulvic
acid fraction
Pass through acid washed XAD-8 resin
Precipitate:
Humic
Acid
Acidify and
centrifuge
Not Extracted:
Humin

13. Resin-Based Fractionation

14. Nuclear Magnetic Resonance Spectroscopy
Vibrations of chemical bonds differ due to elements, length, strength, other properties.
Magnetic
Resonance is induced/amplified by placing the sample in a strong magnetic field.
Nuclear
Certain nuclei have favorable spin properties, making them good candidates for NMR analysis:
1H 13C 15N 31P

15. Peak Assignments in 13C CPMAS NMR
Ketone
C=O
COOH/
Amide
O-Alkyl-C
(Carbohydrates)
Aromatic C
Alkyl-C

16. NMR in Decomposition Studies
Douglas Fir Wood
Preston et al. (1990)

17. Solid-State 13C CPMAS NMR: Whole-Soils

18. Issues and Limitations
Solid-state 13C-NMR is time consuming (>24-hr for some samples).
Paramagnetic interferences (Fe – in Spodosols!).
CP/MAS 13C-NMR is semi-quantitative:
Generally OK for comparisons.
Other NMR approaches exist (with their own limitations).
Low DOC concentrations: we used ~150 L of water to isolate stream DOC.
Difficult to collect high-volume soil solutions.

19. Clear-cutting – W5 at Hubbard Brook Winter/Spring 1983-84

20. Whole-tree harvest – boles and crowns

21. Most of the wood was chipped

22. Watershed 5 – Summer 1984

23. Change in O horizon mass after WTH

24. Clearcutting Effects – Whole-soil 13C CPMAS NMR
Uncut Oa Bh Bs
Clearcut Oa Bh Bs

25. Clearcutting Effects – Whole-Soil 13C CPMAS NMR

26. Soil Solution DOC
Uncut Oa Bh Bs
Clearcut Oa Bh Bs

27. Soil Solution DOC

28. How to index degree of decomposition?

29. Streamwater DOC – Spring Snowmelt

30. Contrasting Soil and Solution Composition: % Aromatic C

31. Some Observations There are distinct advantages to whole-soil NMR…
No extractions necessary → no “modification” issues
Non-destructive
…but looking for change is difficult.
Small change in large pool is ecologically meaningful, but hard to detect.
Perhaps better to focus on dynamic fractions of OM (and live with extraction issues).

32. Where does stream DOC originate?
Soil solution and stream water:
Soil solution collected from six tension-free lysimeter sites in W5 and bulked by horizon.
Stream water collected above W5 weir.
DOC characterized using procedure of Leenheer (1981) – hydrophilic/hydrophobic acids, bases, neutrals.
Soil solution and stream water samples concentrated by RO, freeze-dried, and analyzed by solid-state 13C CPMAS NMR.
Fulvic acid was extracted from soils sampled in summer 1998 on W5.

33. Soil Solution DOC Concentrations
(Tension-Free Lysimetry)
Oa Horizon: 18 mg DOC L-1
Bh Horizon: 11 mg DOC L-1
Bs Horizon: 5 mg DOC L-1
Stream Water: 2 mg DOC L-1
Johnson et al. 2000

34. DOC fractions in soil solution and stream water
Source pH
DOC
Hydrophobic
Hydrophilic
acid
base
neutral
mg L-1 % of Oa
4.19
10.5 51
4.0
4.5 40
0.1 Bh
4.45
10.6 37
0.2 63 Bs
5.10
9.6 23
3.9 68
4.4
Stream
5.33
0.9 36
6.4
1.7 29 27

35. Bulk Stream DOC Bulk soil solution DOC Chemical shift (ppm) -40 40 8040 80
120
160
200
240
Bulk Stream DOC
Bulk soil solution DOC

36. Soil fulvic acid
Soil solution

38. Why Isn’t Stream DOC like Soil Solution DOC?
Selective immobilization in deep mineral soil (below Bs lysimeters).
Importance of riparian-zone soils.
In-stream consumption of DOC.

39. Adsorption of OM by mineral soils
“Initial Mass Isotherm” Approach (Nodvin et al. 1986):
R = Net DOC sorbed (+) or released (-) per mass of soil
Ci = Initial concentration of DOC in solution per mass of soil
m = Regression slope ≈ partition coefficient (m ≤ 1)
b = Regression intercept ≈ desorption parameter (b ≥ 0)

40. Adsorption isotherms for hydrophilic and hydrophobic OM fractions extracted from Bh horizon soils (Similar results for OM extracted from O horizons) (R2 = )

41. Isotherm Parameters Hydrophobic Hydrophilic Horizon pH m b Bh 3 0.69
58.3
0.29
60.1 4
0.74
55.9
0.67
55.7 5
0.49
65.7
0.32
57.2
Bs1
0.79
9.2
0.44
7.7
0.88
8.3
0.68
9.7
0.34
8.6
Bs2
0.75
6.3
0.62
6.2
0.93
5.3
0.76
7.3
0.55
7.8 C
0.78
3.6
0.54
4.1
0.90
3.4
0.87
0.72
3.3
4.4

42. 13C CPMAS NMR Spectra of OM Used in Adsorption Experiments
fCOOH = 0.052
fCOOH = 0.078
fCOOH = 0.073
fCOOH = 0.107

43. The Cp:Fed ratio may be a good indicator of sorption potential

44. Adsorption “envelopes”

45. Conclusions
Organic matter in soils and soil solutions is depleted in O-alkyl-C and enriched in alkyl-C downward in the soil profile.
DOC in soil solutions decreases in hydrophobicity downward in the soil profile.
Consistent with (2), adsorption experiments indicated preferential adsorption of hydrophobic DOC over hydrophilic DOC.
DOC sorption in mineral soils at Hubbard Brook exhibits an “envelope” form, consistent with the electrostatic attraction between COO- groups in OM and positively charged Fe-oxyhydroxides in the soil.
Stream DOC differs from both soil solutions and soil fulvic acid.
Fifteen years after clearcutting, soil solutions are more aromatic than in a nearby control watershed.

46. Conclusions - NMR
NMR spectroscopy can yield insight into process-level questions related to organic matter:
Land-Use Effects
DOC Mobilization/Immobilization
While NMR is useful for OM characterization, changes in whole-soils can be hard to detect.
NMR is particularly useful in studying more dynamic OM such as DOM, water-soluble OM, and fractionated SOM.