OOS 44-6 - Shifts in the composition of soil organic matter during decomposition as revealed by 13-C nuclear magnetic resonance and fourier transform infra-red spectroscopy

Thursday, August 6, 2009: 3:20 PM
Galisteo, Albuquerque Convention Center
Marc Kramer, Earth and Planetary Sciences, University of California, Santa Cruz, CA, Theodore K. Raab, Stanford University, Stanford, CA and Kathryn L. Amatangelo, Ecology and Evolutionary Biology, Brown University, Providence, RI
Background/Question/Methods

We assessed the chemical composition of soil organic matter under various stages of decomposition using two analytic methods; FT-infrared spectroscopy and solid-state 13C-nuclear magnetic resonance spectroscopy. Samples of (1) live foliage/young leaf litter from a fast-decomposing tree (Cheirodendron trigynum) and a slow-decomposing fern (Dicranopteris linearis);  (2) leaf litter differing in mass-loss % from greenhouse and field decomposition studies;  (3) root stands below each plant species, and (4) O- and A-soil horizons beneath each plant, intact and as a <53µm size fraction were run on each instrument.   Ball-milled plant materials, litter and soils were run on a Fourier Transform infrared spectrometer (Thermo-Nicolet Nexus 470; Madison WI) in transmission mode, using KBr-mulls at a sample/KBr loading of 1% (w/w). :  The 13C-1H cross-polarization (CP)-MAS NMR data were recorded with a Bruker AVANCE500 spectrometer equipped with an 11.74T magnet. Solid powder samples were loaded in a 4 mm Zirconia rotor with Kel-F end cap. The rotor was spun at 15 kHz to eliminate spinning sidebands. A CP pulse sequence with: (i) ramped-amplitude mixing power, and (ii) a two-pulse-phase-modulated (TPPM) decoupling technique used. The contact time was 1 ms, the recycle delay was 1 s, the spectrum width was 500 KHz, and 100-300 Hz line broadening applied.
Results/Conclusions

Both FT-IR and NMR spectra revealed that Cheirodendron litter contained a greater proportion of lipids, while Dicranopteris contained more aromatic comounds. As litter aged, this effect became more pronounced in Cheirodendron – and the proximate source of the litter (derived either from greenhouse- or  field- experiment), mattered little.  Further, the broadening of the main lipid-derived NMR bands (in the 20-45 ppm region) as material worked its way into the finer soil fractions, supports the notion that chemical diversity of the microbially-derived lipids increases over time beyond the starting materials provided by Cheirodendron. Both 13C-NMR derived polysaccharide/protein ratios and those obtained by FT-IR for Cheirodendron  and Dicranopteris litter time-series decreased during the decomposition experiment. Litter lipids, expressed as fraction of total signal intensity, were directly compared between 13C-NMR and FT-IR and the regression was highly significant. In summary, NMR and IR spectra agreed in many chemical patterns evident in the litter decomposition samples. The comparison between NMR and FT-IR became more difficult in mineral soil horizons, even for total lipids.

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