著者

和田 光史

出版者

日本ペドロジー学会

雑誌

ペドロジスト (ISSN:00314064)

巻号頁・発行日

vol.11, no.1, pp.46-58, 1967-06-30

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The accumulation of organic matter in volcanic ash soils has been discussed on the basis of a simple mathematical model; dX/dt=A-rX (1), where X is the organic matter in the soil, A the annual addition of plant residue, r the fraction of X decomposed of the soil organic matter per year and t time. By integrating the equation (1) on the assumption of the constancy of A and r, and substituting X by C, the following equation is derived; C=C_m-C_me^<-rt> (2), where C is the carbon % of the soil (corresponding to 10 cm depth of the original ash) and C_m the maximum carbon % of the soil at an equilibrium; dX/dt=0=A-rX. The values of A (=2.1-4.4 ton/ha), r (=0.005-0.0105) and C_m (=20%) are at first estimated from available data on the soils at the supposed equilibrium, and the curves representing the most rapid and slowest accumulation of the organic matter expecting in the volcanic ash soils are depicted in Fig. 1. No suitable datum has been availble to test the feasibility of these accumulation curves except for those in the very early period, which show a reasonable agreement. Then, the value of r has been checked by comparing the average age of the organic matter calculated on the basis of the above accumulation model with that estimated from the measurement of its C-14 specific radioactivity. The average age of the organic matter t^^- at some time t is t^^-=(_0∫^tt(C_m-C_me^<-r>) e^<-rt>・dt)/(_0∫^t(Cm_-C_me^<-r>) e^<-rt>・dt) (3), where (C_m-C_me^<-r>) is the annual increment of the soil organic carbon. By integration, the t^^--t relationship is calculated from the following eaqution ; t^^-=1/r((e^<-rt>(-rt-1)+1)/(-e^<-rt>+1)) (4) The t^^--t curve with r=0.005 (Fig. 2) that corresponds to the slowest accmulation in Fig. 1 shows the maximum average age of the organic matter to be 200 years. The available data of the C-14 age of surface volcanic ash soils, however, indicate much greater average age, and hence much smaller r values such as 0.000125 to 0.0005. This disagreement between the values of r calculated and observed has been solved formally by introducing a factor f, the fraction of A entered into the reservoir of the soil organic matter, into the equation (1). Then, the most probable C-t relationships expecting in the accumlation of the organic matter in the volcanic ash soils are calculated using the equation (2) and delineated in Fig. 3 with hatched lines, where the initial rapid accumulation has been also taken into consideration. The C-t (Fig. 3) and the corresponding t^^--t (Fig. 2) curves thus derived can be used for the dating of the deposition of a buried volcanic ash. As an example, analyses of the data related to the deposition of particular ashes " Imogo" in Kyushu Island have been attempted. The probable time limits for the initiation and termination of the organic matter accumulation (as shown in Fig. 4 as white and black rectangles, respectively) are calculated from the C-14 age of the soil organic matter (vertical arrows) which accumulated in the volcanic ash layers deposited below and above "Imogo" ashes. Comparisons of these time limits have led to conlcude that there are at least two different "Imogo" ashes; the one deposited at 5000〜5250±300 yr. B.P. (lateral arrows) and another deposited not earlier than about 4000 yr. B.P.