Factors contributing to greater soil organic carbon accumulation after afforestation in a Japanese coniferous plantation as determined by stable and radioactive isotopes

Although afforestation is known to change the soil organic carbon (SOC) stock, studies have reported variations in the direction and the amount of SOC stock change after afforestation. We used stable carbon isotopes and an adjacent C3 natural forest to estimate the SOC stock that accumulated after establishment of a coniferous plantation (for 55 years) on a former C4 grassland. Based on studies using radioactive isotopes (cesium-137, 137Cs; unsupported lead-210, 210Pbexc), we suggest some factors that affect the alteration in SOC dynamics following afforestation. The results show that the SOC stock was greater in the plantation than in the natural forest. Little difference was observed in the estimated amount of cumulative C input (for 55 years) from aboveground vegetation between the stands, although the quality of organic matter supplied (C/N ratio) differed in both stands. In addition, the previous C4 grass-derived SOC accounted for about 36% of the difference in SOC stock between stands. The results suggest that the possible factors leading to greater SOC stock in the plantation stand were preservation of older SOC and greater net C accumulation rate due to the change of litter quality after afforestation. The results of the radioactive isotope studies also suggest that afforestation changes the dynamics of vertical C transportation. Before afforestation, C downward transportation was higher in the plantation than in the natural forest, but after afforestation transportation equilibrated to a level that was comparable between the two stands. We postulate that this change in the dynamics of the radioactive isotopes, which was closely related to SOC dynamics, was due to changing litter quality and the lack of disturbance with afforestation at this study site. The results suggest that the study site coniferous plantation has a high potential for SOC accumulation. The findings indicate that restriction of the initial SOC reduction by controlling soil disturbance with afforestation and the quality of organic matter supplied after afforestation are important for enhancing soil C sequestration after afforestation.