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

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

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

Johnson et al. 1991, 1994

Johnson et al. 1994

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

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

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

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

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

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

Resin-Based Fractionation

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

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

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

Solid-State 13C CPMAS NMR: Whole-Soils

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.

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

Whole-tree harvest – boles and crowns

Most of the wood was chipped

Watershed 5 – Summer 1984

Change in O horizon mass after WTH

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

Clearcutting Effects – Whole-Soil 13C CPMAS NMR

Soil Solution DOC Uncut Oa Bh Bs Clearcut Oa Bh Bs

Soil Solution DOC

How to index degree of decomposition?

Streamwater DOC – Spring Snowmelt

Contrasting Soil and Solution Composition: % Aromatic C

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).

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.

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

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

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

Soil fulvic acid Soil solution

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.

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)

Adsorption isotherms for hydrophilic and hydrophobic OM fractions extracted from Bh horizon soils (Similar results for OM extracted from O horizons) (R2 = 0.96-0.99)

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

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

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

Adsorption “envelopes”

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.

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.