Possible Mechanisms Leading to a Delay in Carbon Stock Recovery after Land Use Change

Changes in land use sometimes lead to soil C loss, and a long time may be required for the C stock to recover to initial levels. Thus, it is important to evaluate the mechanisms related to accumulation of newly input C following land use changes. In this study, we sought to determine the signature of newly input C in the soil profile after land use change. 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.