The conversion of croplands to plantations has been broadly implemented to mitigate climate change. This mitigation effect has been achieved mainly via the sequestration of CO2 from the atmosphere and the storage of carbon within tree biomass and the soil, but this effect is not fully understood. To study this mitigation effect, a poplar plantation, a metasequoia plantation, and a poplar and metasequoia agroforestry system converted from croplands were studied. As such, we investigated the soil nutrient contents, soil organic carbon (SOC) fractions and SOC stocks in the three types of plantations to a depth of 100 cm. High total nitrogen, ammonium-nitrogen, and nitrate-nitrogen contents along the soil profile were detected in the poplar-based plantations. After 10 years of development, the poplar plantation had the greatest SOC fraction contents. The SOC stocks of the poplar plantation and poplar and metasequoia agroforestry system reached 90.35 Mg ha−1 and 98.12 Mg ha−1, respectively. The 0–10, 10–20, and 20–40 cm soil layers accounted for a greater share of the SOC stock in the metasequoia plantation than in the other plantations, and the largest proportion of SOC stock in the poplar and metasequoia system was detected in the subsoil layers. Overall, the different afforestation practices resulted in different SOC accumulation patterns among soil depths, especially in the subsoil. The proportion of the SOC stock in the subsoil layers was greater in the agroforestry plantation than in the pure plantations.