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

To examine the relationships between soil age and the structural stability and natural 13 C abundance (delta C-13) 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 delta C-13 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 C-13 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 A600/C value. The composition of the C functional groups did not vary appreciably among layers older than 1.0,000 YBP. The delta C-13 value of bulk soil organic matter (SOM) fluctuated in both profiles, suggesting that the proportions of C-3 and C-4 plants grown when each layer was the surface layer differed among the layers. Although, in general, variation in the delta C-13 value of each humus fraction corresponded to that of bulk SOM, there were significant differences in the delta C-13 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 delta C-13 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 delta C-13 values are discussed.