Tropical soils are critical terrestrial carbon reservoirs with abundant biodiversity that respond rapidly to environmental change. Globally, the expanding conversion of natural tropical forest to plantations in recent decades, to meet economic demands, has markedly influenced the cycling of soil organic carbon (SOC) pools; however, the mechanisms underlying the changes in SOC dynamics are poorly understood. In this study, we examined the SOC dynamics and soil microbial communities at five adjacent tropical forest sites characterized by different logging and plantation practices over the past few decades on Hainan Island, China, by applying natural abundance radiocarbon ( ¹⁴C) and phospholipid fatty acid (PLFA) analysis. At a >35-year rubber plantation site and a >50-year eucalyptus plantation site, an abnormal up-profile decrease in radiocarbon abundance was observed in the upper 30 cm soil layer. This could be indicative of continued soil organic matter decomposition long after forest–plantation conversion, and was consistent with the SOC inventories in the upper 30 cm soil layer at the two sites being significantly lower than those of NF. Both the SOC apparent radiocarbon ages and SOC inventory at a eucalyptus plantation site in which tillage was stopped 20 years ago were similar to those of NF, indicating that a recovery process had occurred. The soil microbial biomass was generally lower at the plantation sites than at the NF site. Both the radiocarbon abundance and SOC inventories in the upper 30 cm soil layer showed positive correlations with the soil microbial biomass, suggesting that microbes may have played a key role in the fate of SOC. This study provided evidence that forest–plantation conversion may facilitate the dissimilation of SOC, and also demonstrated that radiocarbon can serve as a powerful tool for assessing the potential changes of soil carbon dynamics resulting from forest-to-plantation conversion.