Mixed-species forests have potentially more benefits than monocultures particularly in terms of greater carbon sequestration. A 16-year-old Chinese fir (Cunninghamia lanceolata) monoculture in Zhejiang, China, was converted to a Chinese fir—broadleaved plantation by thinning from below (TFB) and crop tree release (CTR) methods coupled with planting of Chinese sweet gum (Liquidambar formosana) seedlings. Carbon pools in trees, snags, shrubs and herbs, seedlings, litterfall, forest floor, and mineral soil were measured for 5 years. The total tree biomass carbon increase in un-thinned control and CTR stands was approximately 15% higher than that in TFB. The average individual tree biomass C stock increased by 20.5% and 9.2% in CTR and TFB, respectively. Carbon flux through litterfall decreased after thinning but recovered, thereafter, to a level similar in the un-thinned control. Compared to the control, lower tree mortality and higher growth of seedlings in both converted stands resulted in no change in ecosystem C stocks. Carbon stocks in trees and seedlings increased more in CTR than in TFB stands, implying that CTR should be favored when converting pure plantations into mixed-species stands.