| Abstract: | Abstract To determine the rates of increase in C and N stocks in the soil and organic layers following afforestation in Andisols, we measured C and N densities in the organic and soil layers at depths of 0–5, 5–15 and 15–30?cm, together with a chronosequence analysis of 4-year-old, 14-year-old and 23-year-old Japanese cedar (Cryptomeria japonica) and 4-year-old, 12-year-old and 25-year-old Hinoki cypress (Chamaecyparis obtusa) plantations. The short-term changes in C and N were confirmed by repeated sampling 5?years after the first sampling. Tree growth, biomass accumulation and organic layers were much greater in Japanese cedar than in Hinoki cypress plantations. Soil C density (kg?m?3) increased and bulk density decreased with stand age in the surface layer (0–5?cm). The average soil C accumulation rate was 22.9?g?C?m?2?year?1 for Japanese cedar and 21.1?g?C?m?2?year?1 for Hinoki cypress. Repeated sampling showed that the rate of increase in C in the surface soil was relatively slow in young stands and that soil C density (kg?m?3) in the subsurface soil did not change over a 5-year period. Although N accumulated in the tree biomass and organic layers, the soil N density (kg?m?3) did not change after afforestation. Although the andic properties of the soil and differences in the planted species did not influence the rate of increase in soil C, soil C density was expected to increase to a concentration greater than 80?g?kg?1, possibly because of the large C accumulation capacity of Andisols. |
