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 extractable fraction (EF) and a nonextractable fraction (humin). The SOM of the EF was separated into six fractions: humic acid (HA), fulvic acid (FA), low-molecular-weight acids (LMA), low-molecular-weight neutrals (LMN), hydrophobic neutrals (HON) and hydrophilic neutrals (HIN). Liquid-state and solid-state ¹³C nuclear magnetic resonance (NMR) were applied to characterize the EF and the humin as well as the whole soils, respectively. The distribution of C species was calculated on the basis of relative integrated peak area. The liquid-state ¹³C NMR spectra of the extractable organic fractions demonstrated that O-alkyl C was concentrated in the low-molecular-weight fractions such as LMN and LMA, while the aromatic–unsaturated C was predominantly in the HA and FA. The solid-state ¹³C NMR of the whole soils showed that the Site 2 soil (Oe and O/A horizons) contained more O-alkyl C and less 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 soil, supporting the theory that organic matter in the Cf horizon could originate in the Oa horizon and was translocated by cryoturbation. Spectra of the humin, which make up 53 to 76% of total C, exhibited trends similar to those 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 seemed the same. Although cross-polarization, magic angle spinning (CPMAS) ¹³C NMR spectra intensities are nonquantitative, the spectral differences between humin and HAs and FAs indicated significant differences in their composition. The humin contained much higher alkyl C and lower aromatic–unsaturated as well as carboxyl–carbonyl C contents than the HA and FA. The results suggested that the humin fraction was different chemically from the HA and FA. It consisted of a large proportion of paraffinic carbons that may derive from algal or microbial sources.