Comparison of humic acid fractions derived from thermally created plant residues and natural soils: Spectroscopic and elemental analyses

To clarify the formation condition of Type A humic acids (HAs) from plant residues and to contribute to reducing CO2 emission, three plant residues were incubated for 0, 25, 50, 100 and 160 days with fresh volcanic ash and water at 90 deg C to produce humic acid like substances (HALS). The HALS of all incubation samples showed a decrease in log (Asub(400)/Asub(600)) values and an increase in Asub(600)/C values with increasing incubation time. When HALS were classified, only rice straw HALS of 160 days (RS160) belonged to Type A, whereas broad leaf HALS of 160 days (BL160) and Japanese cedar sawdust HALS of 160 days (JCSD160) belonged to Type B. For elemental and nuclear magnetic resonance (NMR) analysis, the HALS samples were prepared according to the International Humic Substances Society (IHSS) method. Data from the elemental composition of all HALS showed high contents (45.98 to 56.55) of carbon (C), low (3.19 to 5.16) hydrogen (H), and low (0.33 to 2.36) nitrogen (N). Carbon content increased in RS and BL, and increased or fluctuated to some extent in the JCSD, whereas H and N contents decreased in all samples as incubation progressed. The H/C and O/C ratios showed a marked decrease, simultaneously the O/H ratio increased, with increased incubation days. These results suggested that oxidation occurred in the incubation system. Although some HALS samples (RS160 and BL160) appeared in the Type A region in both H/C versus O/C and H/C versus O/H diagrams. HALS plotted in the figures of log (Asub(400)/Asub(600)) versus Asub(600)/C, H/C versus O/C and H/C versus O/H diagrams were generally placed out of the area of natural soil HAs. In the case of sup(1)H NMR, the spectral shape of RS, BL and JCSD differed from that of natural soil HAs. Only the sup(1)H NMR spectrum of RS160 was similar to that of Type A soil HA. Changes in the composition of proton species in HALS showed that percentages of aromatic protons (Hsub(ar)) increased with increasing incubation. Spectra obtained using sup(13)C NMR revealed that RS and BL HALS were different from natural soil HAs, whereas RS160 was similar to Type A HA spectra. In the spectra of RS HALS, the signal intensities of carbohydrate C (approximately 74 and 105 p.p.m.) and methoxyl C (approximately 56 p.p.m.) decreased gradually as incubation time progressed, while carboxylic C (approximately 175 p.p.m.) increased. Simultaneously other distinct signals became broad and overlapped each other. As a result, the changes in RS HALS spectra suggested that progress in the modification of lignin and the oxidative degradation of polysaccharide structure, and the spectrum of RS160 became similar to the spectra of Type A HAs. Although the only signal resulting from phenolic C (approximately 150 p.p.m.) still appeared in RS160, the signal intensity decreased and became broader and weaker from RS0 to RS160 with incubation time. Therefore, it was concluded that RS HALS might form a completely similar spectra to the spectra of natural Type A HAs with longer incubation time.