Structural stability and natural 13C abundance of humic acids in buried volcanic ash soils

Abstract

To examine the relationships between soil age and the structural stability and natural 13C abundance (δ13C) of humic acids, buried humic layers in volcanic ash soil profiles near Mt Fuji, Japan, namely Yubunebara (present to 10,000 years before present (ybp); n = 5) and Ashitaka-Onoue (<1,000–30,000 ybp; n = 6), were analyzed. The contents and δ13C values of the fulvic acids (humic substances) and non-humic substances in the fulvic acid fractions were also determined. The degree of humification of the humic acids differed remarkably between the surface and buried layers, and the A 600/C value (absorbance at 600 nm mg−1 C mL−1 of humic acids in 0.1 mol L−1 NaOH) increased toward the older buried layers. The proportion of aromatic C in total C of the humic acids, estimated using cross polarization/magic angle spinning 13C nuclear magnetic resonance spectroscopy, also increased, whereas the proportions of alkyl and O-alkyl C decreased from the surface toward the older layers. These observations suggested higher stability in the aromatic than aliphatic components and the contribution of aromatic C to the increase in the A 600/C value. The composition of the C functional groups did not vary appreciably among layers older than 10,000 ybp. The δ13C value of bulk soil organic matter (SOM) fluctuated in both profiles, suggesting that the proportions of C3 and C4 plants grown when each layer was the surface layer differed among the layers. Although, in general, variation in the δ13C value of each humus fraction corresponded to that of bulk SOM, there were significant differences in the δ13C value between the humus fractions and bulk SOM, and the differences were more conspicuous in the younger layers and less conspicuous in the older layers. A negative correlation (with an exception) was also observed between the difference in the δ13C values of humic acids from that of bulk SOM and the relative content of aromatic C in the humic acids. Possible causes of the differences and changes in the δ13C values are discussed.