To examine the relationships between soil age and the structural stability and natural 13 C abundance (δ 13 C) 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 δ 13 C 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 13 C 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 δ 13 C 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 δ 13 C value of each humus fraction corresponded to that of bulk SOM, there were significant differences in the δ 13 C 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 δ 13 C 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 δ 13 C values are discussed. Kew words:13 C NMR, degree of humification, fulvic acids, humic acids, stable carbon isotopic ratio.

We evaluated the effect of soil conservation by weeds on the degradation and generation of humic acids, fulvic acids, and water-soluble non-humic substances (WS-NHS) in a red-acid soil (Vertic Dystrudept) (Indonesia) from the changes in humus composition and stable carbon isotopic ratio (d 13 C). Three plots, a weeded plot (T-1; the common practice), a plot covered with Paspalum conjugatum Berg., a C 4 plant (T-2), and a plot in which native weeds were allowed to grow (T-3), were prepared. An incubation experiment determined the d 13 C values of the humus fractions generated from Paspalum in soil. Based on the increase in d 13 C value, the proportion of total C that originated from Paspalum C after 4 years under coffee was 16 AE 4% in the T-2 topsoil (0-10 cm). Humic and fulvic acids in the T-1 topsoil decreased to 46 and 84%, respectively, whilst both increased or remained constant in the T-2 and T-3 soils. The WS-NHS content varied little and was independent of land management. The preferential loss of the humic acids with a smaller degree of humification as assessed by their darkness in colour was shown in T-1. The decrease in the degree of humification suggested the accumulation of the weedderived humic acids in T-2 and T-3. In the T-2 topsoil, 36 AE 2%, 13 AE 3% and 15 AE 2% of C in the humic acids, fulvic acids and WS-NHS, respectively, were estimated to be Paspalum-derived after 4 years. The estimated initial C loss during the same period was 17 AE 3%, 14 AE 2% and 7 AE 2%, respectively, for those fractions, which suggests the fastest turnover rate for the humic acids and significant retardation of their degradation in soil colonized by weeds.