Characterization of soil organic matter fractions of tundra soils in arctic Alaska by carbon-13 nuclear magnetic resonance spectroscopy

Soil organic matter (SOM) was extracted with 0.1 M NaOH resulting in an ext ractable fraction (EF) and a nonextractable fraction (humin), The SOM of th e EF was separated into six fractions: humic acid (HA), fulvic acid (FA), l ow-molecular-weight acids (LB PA), low-molecular-weight neutrals (LMN), hyd rophobic neutrals (HON) and hydrophilic neutrals (HIN), Liquid-state and so lid-state C-13 nuclear magnetic resonance (NMR) were applied to characteriz e the EF and the humin as well as the whole soils, respectively. The distri bution of C species was calculated on the basis of relative integrated peak area. The liquid-state C-13 NMR spectra of the extractable organic fractio ns demonstrated that O-alkyl C was concentrated in the low-molecular-weight fractions such as LMN and LMA, while the aromatic-unsaturated C was predom inantly in the WA and FA, The solid-state C-13 NMR of the whole soils showe d that the Site 2 soil (Oe and O/A horizons) contained more O-alkyl C and l ess alkyl C content than the Site 1 (Oa1 and Oa2) and 3 (Cf horizon) soils; the Site 3 Cf horizon exhibited spectra similar to those of the Site 1 soi l, supporting the theory that organic matter in the Cf horizon could origin ate in the Os horizon and was translocated by cryoturbation. Spectra of the humin, which make up 53 to 76% of total C, exhibited trends similar to tho se of the whole soils. Humin appeared to possess greater alkyl C and less O -alkyl C content than whole soils; aromatic, carboxyl-carbonyl C contents s eemed the same. Although cross-polarization, magic angle spinning (CPMAS) C -13 NMR spectra intensities are non-quantitative, the spectral differences between humin and HAs and FAs indicated significant differences in their co mposition. The humin contained much higher alkyl C and lower aromatic-unsat urated as well as carboxyl-carbonyl C contents than the HA and FA, The resu lts suggested that the humin fraction was different chemically from the HA and FA, It consisted of a large proportion of paraffinic carbons that may d erive from algal or microbial sources.