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.
