Virtual fractionation of charcoal from soil organic matter using solid state 13C NMR spectral editing

Abstract

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.