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Afforestation is an important strategy to increase soil organic carbon (SOC) stocks and stabilize soils against degradation and erosion. While the area under afforestation has increased globally during the last 20 years, the dynamics of SOC accumulation due to afforestation are still poorly known, as it cannot be directly compared to that in cropland. The goals of this study were: i) to investigate the dynamics of SOC accumulation after afforestation on the Loess Plateau; ii) to determine SOC contents and distribution in density fractions and aggregates most strongly affected by afforestation; and iii) to analyze the relationships between old and new SOC in relation with afforestation periods. A chronosequence of forest plots aged 1, 5, 12 (with Chinese pine, Pinus tabulaeformis Carr.) and 30 years (with white poplar, Popidus alba L.) were selected within the large scale "Grain for Green" project. A maize field nearby was chosen as a control, which represents the land use prior to afforestation. At each location, soils were sampled at depths of 0-10 and 10-30 cm, respectively and aggregates were fractionated into < 250, 250-2000 and > 2000 mu m sizes, which were subsequently separated into light (rho < 1.85 g cm(-3)) and heavy (rho > 1.85 g cm(-3)) density fractions. SOC content was generally higher in afforested soils than in cropland and increased with stand age. The mean SOC accumulation rate was 0.11 g C kg(-1) year(-1) for the 0-10 cm layer of the 30-year-old forests, whereas the maximum rate was found for 5-year-old forest at 0.24 g C kg(-1) year(-1). For each period of afforestation, the maximum SOC content was recorded in the macroaggregates. The C turnover of the light fraction (calculated based on delta C-13 of SOC) peaked in the large macroaggregates and ranged from 21 to 23 years). Whereas SOC turnover in the heavy fraction, peaked in the microaggregates at a relatively longer period of 46 to 70 years. We conclude that the initial SOC accumulation under afforestation occurs mainly in the macroaggregates, with a faster turnover compared with microaggregates. During the first 30 years of afforestation, the SOC accumulation and stabilization is ongoing mainly in the upper 10 cm, while the C sequestration in 10 to 30 cm depth needs much longer time.