【Objective】Any changes, even minor ones, in forest soil carbon pool would have profound effects on atmospheric CO2 concentration and global climate change. Particulate organic carbon is mainly of semi-decomposed plant residues and very high in biological activity and short in turnover cycle, so it can be used as an indicator of changes in soil organic carbon. Change in forest type or forest conversion is an important pattern of changes in land use, and affects storage and transformation of soil organic carbon through increasing or reducing biomass of the plants and litter on the ground. So, it is an important driving force of soil organic carbon (SOC) dynamics. This paper focused on conversion of natural broad-leaved forest to mixed coniferous-broad-leaved forest or Chinese fir plantations (Cunninghamia lanceolata [Lamb.] Hook) in subtropical China to explore effects of the conversion on content of SOC and its distribution in the forestland. Therefore, three tracts of forests were selected as subjects of the study, i.e. a 28-year-old natural broad-leaved forest, a Chinese fir plantation and an artificial mixed coniferous broad-leaved forest converted from natural broad-leaved forest in November 2016. 【Method】Soil samples were collected from different soil layers (0~20 cm, 20~40 cm, and 40~60 cm) in the forestlands for analysis of effects of the conversion on contents of soil coarse particulate organic carbon (CPOC), soil fine particulate organic carbon (FPOC), and mineral-associated organic carbon (MOC) and their ratio.【Result】Results show that once natural forest was converted into artificial ones, (1) content of soil organic carbon in various soil layers and stock of litter on the surface decreased significantly; (2) CPOC decreased in content by 60.0% and 54.6% in the mixed coniferous broad-leaved forest and Chinese fir plantation, respectively and in proportion, too in the 0~20 cm soil layer, while FPOC did reversely in both content and proportion; however, MOC declined in content, but rose in proportion, by 8.6% and 20.1%, respectively; and (3) The soil POC/MOC and MOC/SOC ratios in all the soil layers exhibited decreasing trends. In the 0~20 cm soil layer, soil CPOC was the most closely related to SOC, whereas in the 40~60 cm soil layer, MOC was.【Conclusion】Therefore, the conversion of subtropical natural broad-leaved forests into coniferous-broad-leaved mixed forests and Chinese fir plantations reduces soil total organic carbon (SOC) as the latters produce less litter and fine root biomass, thus leaving the soils more susceptible to soil erosion caused by rainfall, which in turn enhances reduction of total SOC, and POC/SOC and POC/MOC in all soil layers to a varying extent, However, it improves stability of SOC. Soil CPOC is a better indicator reflecting the effect of forest conversion on topsoil organic carbon, while MOC is, reflecting the impact of forest conversion on deep soil organic carbon.