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Afforestation of former croplands has been proposed as a promising mitigation of the rising atmospheric CO2 emissions m view of the commitment to the Kyoto Protocol.In this study,we collected samples of forest floor,fine root and mineral soil in 23 paired plots of poplar plantations(Populus × xiaozhuanica W.Y.Hsu & Liang)with different stand basal areas(SBA,ranging from 0.2 to 32.6 m2 ha.1)and reference maize croplands using a paired-site design.Our results found that:(1)afforestation resulted in linear increases of Δc(refers to the difference in SOC content between a poplar plantation and the paired cropland)in the forest floor and 0-10 cm mineral soil with SBA(R2=0.67,p<0.001 and R2=0.34,p=0.003,respectively),but there were no clear relationships between SOC stocks in the soil deeper(>10 cm)and SBA;(2)fine root C stock increased by afforestation across all the soil layers(p<0.05),and root Ac had a quadratic curve(the first two mineral soil layers)or linear(the other mineral soil layers)relationship with SBA;(3)soil Δc stocks in the light fraction linearly increased with SBA(R2=0.40,p=0.02),but not for heavy fraction;(4)SOC derived from crop decreased significantly in the light and heavy fractions in the surface mineral layer,but tree-derived C in bulk soil and two fractions increased gradually with the increases of SBA.Our study implied that cropland afforestation could sequester new C in surface mineral soil accompanying the increases inputs of litterfall and root,but SOC stability didn't enhance in the semi-arid regions.