There still exist uncertainties in the trend, magnitude and efficiency of carbon sequestration with regard to the changes in soil organic carbon (SOC) pools after afforestation. In this study, SOC turnover times of the meadow steppe and planted forests at Saihanba Forest Station of Hebei Province, China are estimated by means of the radiocarbon ( 14 C) method. Our results show that the SOC turnover times can be as long as from 70 to 250 years. After planting the Pinus sylvestri var. mongolica

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... the Leymus chinensis meadow steppe, the turnover times of organic carbon in both bulk samples and soil aggregate fractions of the topsoils are decreased with an increase of the stand age. Such a lowering of the turnover time would cause an increase in soil CO 2 flux, implying that afforestation of grassland may reduce the capacity of topsoil to sequestrate organic carbon. Combined stable isotope and 14 C analyses on soil aggregate fractions suggest that there are different responses to afforestation of grassland between young and old carbon pools in topsoils. In the young and middle-age planted forests, the proportion of CO 2 emission from the older soil carbon pool shows an increasing trend. But in the mature planted forest, its proportion tends to decline, indicating that the stand age may influence the soil carbon sequestration mechanism. The CO 2 emission from the topsoils estimated using the 14 C method is relatively low compared to those by other methods and may be caused by the partial isolation of the young carbon component from the soil aggregates. For more accurate estimation of CO 2 flux, future studies should therefore employ improved methodology for more effective separation of different soil carbon components before isotope analyses. 14 C, soil organic carbon turnover, aggregate fractions, Saihanba, afforestation of grassland, stand age Citation: Tan W B, Zhou L P, Liu K X. Soil aggregate fraction-based 14 C analysis and its application in the study of soil organic carbon turnover under forests of different ages.