Mixed plantations have more soil carbon sequestration benefits than pure plantations in China

•Conducting mixed plantations promotes more SOC sequestrations than pure plantations.•Arbor-shrub mixtures have the highest soil carbon sequestration rate (RS).•Soil carbon stock was decreased in the early stage since afforestation.•Afforestation on the cropland had a higher RS than on non-agricultural land.•Afforestation in C-deficit regions had the higher RS than C-rich regions.