13C NMR and Elemental Composition of Hot Water Extractable Organic Matter in a Forest Soil'

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13C NMR and Elemental Composition of Hot Water
Extractable Organic Matter in a Forest
Soil. Chris Johnson, Syracuse University, Dept.
of Civil Environmental Engineering, Syracuse,
NY 13244 Zhihong Xu, Centre for Horticultural and
Forestry Research, Griffith University, Brisbane,
QLD 4111, Australia

• NMR Observations
• Spectra of HWEOM (Figure at Right) are dominated
  by O-alkyl C (44.9 71.4 of total signal
  intensity). This region includes carbohydrate C,
  methoxyl C, and N-alkyl C.
• HWEOM spectra have much lower signals for alkyl C
  than the corresponding whole soils (Figure
  Below). HWEOM also has smaller peaks in the aryl
  C and O-aryl C regions (aromatic C).
• Spectra for whole soils (Figure at Left) show
  decreasing signals for O-alkyl C with increasing
  depth in the O horizon, with corresponding
  increases in alkyl C. The fraction of signal
  intensity associated with alkyl C approximately
  doubled from the Oi horizon to the bottom of the
  Oa horizon, while the O-alkyl C fraction declined
  by about 35 (Bar Charts at Lower Left).
• HWEOM (Bar Charts - Lower Right) exhibited a
  depth pattern opposite to the whole soils, with
  O-alkyl C increasing and alkyl C decreasing with
  increasing depth.
• Humification results in the accumulation of
  recalcitrant organic matter compounds rich in
  alkyl C structures (e.g., waxes, resins,
  suberin). These data suggest that humification
  also results in decreased solubility of these
  alkyl-dominated structures.

• Elemental Composition
• HWEOM has higher O and N concentrations than the
  soil from which it was extracted. CN ratios of
  HWEOM are 30-40 lower than the soil (Table at
  Right).
• Plotted on a Van Krevelen diagram (Figure Below),
  HWEOM is distinct from the whole soils. HWEOM
  has higher OC and slightly lower HC ratios than
  the whole soils.
• Whole soils plot in a region of the Van Krevelen
  diagram corresponding to typical lignin
  composition. HWEOM samples generally plot away
  from the whole soils in the direction of
  carbohydrate. This suggests that hot water
  extracts a carbohydrate-rich fraction that is
  otherwise typical of soil organic matter.
• The model formula for chitin plots among the
  HWEOM samples. If chitin is taken to be a proxy
  for microbial biomass, then microbial biomass
  could be contributing to the HWEOM fraction in
  these soils.

Molar CN ratios in hot-water extractable organic
matter and associate soil
Sparling et al. (1998)

• Objectives
• To examine the composition of HWEOM and compare
  it to the soil from which it was extracted.
• To identify changes in HWEOM concentration and
  composition with depth in the forest floor.
• To compare the composition of HWEOM to other soil
  organic matter fractions and model compounds
  (carbohydrate, lignin, chitin).

• Site and Field Methods
• Hubbard Brook Experimental Forest, NH
• Predominantly hardwoods (beech, maple, birch)
• Some conifers (red spruce, balsam fir)
• Spodosols (Typic Haplorthods) gt Inceptisols
  (Typic Dystrochrepts)
• Acidic soils pHw 3.5 - 4.0 in O horizon
• Four sample sites
• 2 hardwood, 2 conifer
• Oi (litter), Oe (fermented layer) sampled whole
• Oa (humus) sampled in 1-cm depth increments to 5
  cm
• Samples frozen until analysis

• Conclusions and Ideas
• The elemental and 13C NMR composition of HWEOM
  suggests that it is a carbohydrate-rich fraction
  of soil organic matter.
• The composition of HWEOM is also consistent with
  the hypothesis that HWEOM contains microbial
  biomass and by-products of microbial activity.
• The composition of HWEOM varies significantly
  with depth within the forest floor, increasing in
  O-alkyl C and decreasing in alkyl C with depth,
  contrary to the pattern observed in the soils
  from which the HWEOM was extracted.
• Next Steps Use dipolar dephasing (DD) analyses
  to further examine HWEOM composition tannins,
  rigid vs. non-rigid structures.
• Next Steps Examine NMR quantitation using Bloch
  Decay (BD) analyses, spin counting experiments.
• Next Steps Examine effects of a calcium
  amendment on the amount and composition of HWEOM.

Yield from hot-water extractions of O horizon
soils from the Hubbard Brook Experimental Forest,
NH. Mean values are shown 1 std. error (N 4).
References Boyer, J.N., and P.M. Groffman. 1996.
Bioavailability of water extractable organic
carbon fractions in forest and agricultural soil
profiles. Soil Biol. Biochem. 28783-790. Paul,
E.A., and F.E. Clark. 1989. Soil microbiology and
biochemistry. 2nd Ed. Academic Press, San Diego,
CA. Sparling, G. et al. 1998. Hot-water soluble C
as a simple measure of labile soil organic
matter The relationship with microbial biomass
C. Soil Biol. Biochem. 301469-1472. Stevenson,
F.J. 1994. Humus chemistry Genesis, composition,
reactions. 2nd Ed. John Wiley Sons, New
York. Ussiri, D.A.N. and C.E. Johnson. 2002.
Characterization of organic matter in a northern
hardwood forest soil by 13C NMR spectroscopy and
chemical methods. Geoderma 111123-149.
Thanks! Funding National Science Foundation,
Australian Research Council, USDA NRI Competitive
Grants Program Field Help Megan Nosovich Sample
Prep Analysis Megan Nosovich, Mary Margaret
Koppers NMR Acquisitions David Kiemle