The solid state 13C NMR spectral editing technique
proton spin relaxation editing (PSRE), which generates subspectra of
components that have different proton relaxation rates and are spatially
separated by at least 30–100 nm, was applied to hydrofluoric acid
treated <53 m soil fractions from 8 Australian surface soils. Most of the
aromatic signal was partitioned into the rapidly relaxing subspectrum,
especially for the soils known to have high charcoal contents. However, the
presence of other rapidly relaxing soil organic matter (SOM) components
prevented a clean separation of charcoal from non-charcoal components. PSRE
analysis was repeated after the samples had been treated with high energy
ultraviolet photo-oxidation, which brings about the mineralisation of most SOM
other than char. Excellent separation of the charcoal fraction by PSRE was
achieved after photo-oxidation for 5 of the samples with the highest charcoal
content. The rapidly relaxing subspectra for these samples also suggested that
the charcoal present in soil contains significant carbonyl functionality,
possibly as a result of in situ weathering.
A new PSRE methodology is described, designed to best suit SOM samples. Data
from inversion-recovery experiments were fitted to a model consisting of 2
components with different T1H relaxation rate constants,
thus providing an objective best fit to the inversion-recovery data and
avoiding the subjective judgements required in other PSRE methodologies.
