Long-term dynamics of soil organic carbon and effect of temperature increase on the residence time of soil organic carbon

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1. Relationship between structural properties and ^<14>C age of soil organic carbonTo elucidate the long-term dynamics of soil organic carbon, relationships between ^<14>C age and degree of humification, composition of C functional groups as estimated by ^<13>C CPMAS NMR spectra, composition of polynuclear aromatic components released on oxidative degradation with ruthenium oxide oxidation (RTO), and δ^<13>C of humic acids in a soil layer were analyzed. Humic acids were obtained from the A2 or A3 horizon of two brown forest soils, Inabu (Toyota, Aichi) and Miyazaki (Miyazaki, Miyazaki), and fractionated by fractional precipitation with acetone-0.01 M NaOH into seven or eight fractions. In each soil, there were significant correlations between ^<14>C age and two variables indicating degree of humification. With increasing ^<14>C age and degree of humification, the proportion of humic acid C present as alkyl C and aromatic C decreased and increased, respectively. GC analysis of RTO produ … More cts showed the increases in the content and degree of condensation of polynuclear aromatic structures with increasing ^<14>C age. On the other hand, the rate of increase in the degree of humification with time differed between the two soils: 2800 years for whole humification process in Inabu while 190 years in Miyazaki. Nevertheless comparison with the previous results suggested that the Inabu result is a specific case and the difference in temperature conditions does not affect the rate of structural changes of humic acids. Removal of saccharides and peptides by acid hydrolysis changed ^<14>C age of humic acid fractions only by 30 years. Therefore, such components were incorporated in the humic acid structure during the initial stage of humification and have been survived.2. Effect ofincrease in temperature on the residence time of soil organic carbonTo understand the influence of global warning on the residence time of soil organic carbon at different stage of humification, the A1 horizon of Inabu brown forest soil was incubated at 15, 20, 25, 30, 34, 37, and 40℃. Soil was collected after 40, 120, 260, and 365 days, and total C and humic acid C content as well as the degree of humification of humic acid were determined. Change in the composition of humic acids with respect to the degree of humification was also examined using factional precipitation with acetone-0.01 M NaOH. Total C in soil decreased with a larger extent at temperatures of〓30℃ than at less than 30℃. However, humic acid contents after 365 days did not differ significantly among the seven treatments. Fractional precipitation showed no decrease in the fractions with smaller degree of humificatioan. Thus, the increase in temperature from 15 to 40℃ did not influence the residence time ofhumic substances within a year. Less