Structural domains in peat as revealed by physical fractionation, sequential chemolysis and 15N and 13C CPMAS NMR spectroscopy

The distribution patterns of different C and N forms in peat seen using solid state 13C and 15N NMR spectroscopy were studied after applying a series of physical and chemical treatments. The material analysed includes four series of peat subfractions obtained by: (i) wet sieving, (ii) extraction with solvents specific for removing lipid or humic substances, (iii) mild degradation treatments yielding non-degraded residues (BF3–MeOH transesterification, HI degradation, HCl hydrolysis) and (iv) thermal treatment. In particular the sample series (i) would inform on the influence of particle size in the quantitative speciation of C and N forms in the whole peat material, whereas series (ii) and (iii) should give further insight on the C and N distribution patterns upon the influence of classical reactions considered to selectively remove structural moieties of the peat organic matter, such are carbohydrate-derived, ester-linked alkyl and hydrolyzable peptidic structures. The NMR spectra suggested: (i) a lack of heterocyclic N forms in the coarsest peat granulometric fractions, and their highest concentration in fractions less than 100 μm in size; (ii) the preferential accumulation of heterocyclic N forms in the humic acid fraction isolated with a chelating extractant (Na4P2O7); (iii) the survival of amide N forms after acid hydrolysis indicating that only a portion of the amide N forms were present in hydrolyzable form; (iv) the fact that heating of the peat was the treatment which most significantly increased the heterocyclic N moiety.