Carbon and nitrogen accumulation mehcamnisms in forested ecosystem and its estimation

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In this study, we used stable and radioactive C isotopes with soil fractionation methods in a C_3 coniferous plantation converted from C_4 grassland, in Japan. The difference in δ^<13>C values between the surface litter and the soil organic carbon (SOC) below the litter was 5‰ or greater; this large isotopic difference was attributed to rapid decomposition in the litter layer and preservation of C derived from the previous C_4 vegetation. Most SOC Δ^<14>C values were negative throughout the soil profile, suggesting that most of the SOC in the soil profile was recalcitrant and had been preserved for a long time. Only the surface sand values were slightly positive. These results suggest that most newly input C is consumed at the soil surface. The low ability of these soils to preserve newly input C is one factor in the slow recovery of soil C. From the results of soil organic matter with vegetation re-growth, there was no significant change in soil C, although the surface soil organic matter changed with forest management. It reached the plateau after 40 years of re-growth. The value was almost similar to the accumulated organic matter in the climax forest. It is considered that carbon and nitrogen accumulation in the forested ecosystem reaches the potential amount after 40 years of forest re-growth.