Demonstration of spatial heterogeneity in the organic matter of de-ashed humin samples by solid-state 13C CPMAS NMR

Abstract

A variant of solid-state 13C CPMAS NMR demonstrated the presence of carbon in spatially distinct domains within three de-ashed samples of humin from a forest soil profile under Douglas-fir near Victoria, British Columbia. Differences in 1H T1 values indicated incomplete spin diffusion in the solid state and separation of organic matter in domains with dimensions of tens of nanometres or more. These relaxation differences were used to separate subspectra of carbons associated with the slower- and faster-relaxing protons. The former were dominated by methylene chains, while the latter contained more aromatic and O-alkyl carbon. Most of the organic matter appears to remain associated with iron even after extensive de-ashing with HCl/HF, but the aromatic and carbohydrate structures appear to be more closely associated with iron than methylene groups in long chains. This NMR technique offers a novel approach to probing the structure of insoluble soil organic matter. Key words: Humin, CPMAS NMR, relaxation, iron, forest soil, Brunisol