1. Soil micro-organisms play important roles in ecosystems and respond quickly to environmental changes. We examined how understory removal and tree girdling influence the composition of soil microbial community and the litter decomposition in two subtropical plantations. 2. Phospholipid fatty acids (PLFAs) analysis was used to characterize soil microbial community. Redundancy analysis and principal response curves (PRC) were used to investigate the relationships between soil microbial community and environmental factors. 3. Understory removal significantly reduced the amount of fungal PLFAs, the ratio of fungal to bacterial PLFAs, and the litter decomposition but did not affect bacterial PLFAs and total PLFAs. In contrast, tree girdling did not affect the soil microbial characteristics. The changes in soil microbial community caused by understory removal were mainly attributed to the indirect effects such as increased soil temperature and soil NO₃⁻-N availability. In addition, PRC analysis showed that the relative abundance of most PLFAs increased in response to understory removal in the 2-year-old plantation but decreased in the 24-year-old plantation. 4. We propose that understory plants are important components in subtropical forest ecosystems, and play different roles in maintaining soil microbial community and driving litter decomposition processes in young vs. old plantations. The functions of understory plants should be considered in forest management and restoration. The negligible effect of tree girdling on the soil micro-organisms can be attributed to the resprouting trait and mycorrhizal interactions of Eucalyptus.