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  • Evaluation of years required for the progress of humification in soil using AMS radiocarbon dating technique(Summaries of Researches Using AMS)

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Soil organic matter is one of major C pools in biosphere, which play a key role not only in geochemical C cycle related directly to global warming, but also in soil fertility essential for sustainable food production. To understand the dynamics of soil organic matter, the evaluation of its stability associated with composition and chemical structure is important. The stability of humic acids, a major fraction of soil organic matter, has been considered to increase with increasing the degree of humification, which is expressed by two variables of A_<600>/C (absorbance at 600 nm per mg C of humic acids in 0.1 M NaOH) and log(A_<400>/A_<600>)(logarithmic value of the ratio of absorbances at 400 and 600 nm). However, whether the increase in the degree of humification occurs in a soil as time elapses and how many years it requires, if occurs, have never been clarified. In the present study, humic acids obtained from two buried soil layers, Tsubame 8th layer (Ts-8, Myoko, Niigata Prefecture) and Yubunebara IIA horizon (Yu-2, Gotenba, Shizuoka Prefecture), were fractionated into eight subfractions with different degrees of humification, respectively, and then their ^<14>C age were determined. For this purpose, precipitation method using 0.01 M-acetone solutions was applied. In the fractional precipitation, humic acid molecules with higher degrees of humification precipitated at lower concentrations of acetone. The ^<14>C age was also older for the subfractions precipitated at lower concentrations of acetone in both the soils with an exception. Consequently, significant correlations between the ^<14>C age and A_<600>/C or log(A_<400>/A_<600>) were observed in each soil, indicating that the degree of humification of humic acids had increased over the passage of time. The ^<14>C age of the Ts-8 humic acids ranged between 1321-1745 yBP excluding an abnormally old fraction. Since the degree of humification of a Ts-8 subfraction was highest level among the reported values for soil humic acids, it is considered that less than 500 years are enough to reach the final stage of humification. The ^<14>C ages of the Yu-2 samples, 524 to 914 yBP, differed largely from those of the Ts-8 samples. However, regardless of absolute ^<14>C age, the length of years required for the increase in the degree of humification was similar between them and could be estimated using an equation.



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