SOIL CARBON CYCLING IN A JAPANESE CEDAR (CRYPTOMERIA-JAPONICA) PLANTATION

Soil carbon cycling was measured synthetically and quantitatively throughout a year in a Japanese ceder plantation, Mt. Amida, Hiroshima Prefecture, west Japan, and analyzed by a compartment model. The results observed and analyzed were compared with those obtained by the same methods in natural secondary (Japanese red pine) and climax (evergreen oak) forests developed in the same warm-temperate zone, west Japan. There was no clear seasonal trend in litterfall rate and accumulation of A0 layer, but soil respiration increased in summer and decreased in winter with the change in soil surface temperature. The relative decomposition rate of the A0 layer (0.187 year-1) observed in the cedar plantation was one-third of that in an evergreen oak forest and rather less than in a Japanese red pine forest. Transfer of humans from the A0 layer to mineral soil was faster than in the pine forest, and thus the loss rate of the A0 layer (0.262 year-1) was somewhat larger than that in the pine forest. However, the relative decomposition rate of humans in mineral soil (0.0063 year-1) was only one-third of those in the pine and evergreen oak forests. This suggests that soil carbon cycling was extremely slow in the cedar plantation. This is probably due to the great resistance of cedar litter to decomposition and to less broadleaf litterfall than in the pine forest, because the soil environmental conditions were no worse than in other two forests. Based on these results, a discussion is presented on the management of cedar plantations, how to maintain their productivity under conditions with slow material cycling and how to enhance this cycling.