Spectroscopic Investigations of Cultivation Effects on Organic Matter of Vertisols

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Abstract
Solid state carbon‐13 nuclear magnetic resonance (13C NMR) spectroscopy with cross polarization and magic angle spinning (CP/MAS) was used to study the effect of cultivation on the nature of organic matter in a Vertisol (mesic, mixed, clayey, Typic Chromusterts). Continuous cultivation for 20, 35, and 45 yr resulted in 50, 61, and 66% loss in organic matter, respectively, from the 0‐ to 0.1m layer. However, 13C NMR spectroscopy performed directly on samples from these cultivated soils (0–0.1 m depth) and on adjoining virgin soil under native, virgin brigalow (Acacia harpophylla) scrub showed only minor differences in the in situ organic matter. Resonance bands representing long‐chained alkyl groups were the dominant feature of all four spectra and represented >50% of the organic C. Fractions obtained from density separations (2.4 Mg m−3) made on the soil under native scrub and the soil that had been cultivated for 35 yr showed that the organic C content of the light fractions (−3) was greatly reduced after cultivation, while the heaviest fraction remained unchanged. Carbon‐13 NMR spectra determined on the fractions with a density −3 showed that the organic matter with the highest alkyl content occurred in the 2.0‐ to 2.4‐Mg m−3 fraction. Also, the relative amount of humic acid decreased after cultivation, while the relative amount of clay‐bound organic matter increased. Ultraviolet/visible and infrared spectroscopic analyses of the humic acid and humin fractions agreed well with the NMR data. It is concluded that the major mechanism responsible for the relative stability of the persistent soil organic matter is a physical association with the inorganic components of the soil, rather than an inherent chemical or biochemical inertness of the organic matter itself.
Funding Information
  • Queensland Wheat Industry Research Committee