Plant competition can impose species-specific effects on the dynamics of soil carbon (C) and nitrogen (N) through rhizosphere processes and litter input. Therefore, it is crucial to quantify these effects in various terrestrial ecosystems for a better understanding of the mechanisms. Here, we collected subsoils containing low N from a subtropical forest and planted eight dominant tree species (two deciduous and six evergreens) in these soils in a greenhouse to explore the effects of interspecific plant competition on plant growth, soil C and N contents, and soil C and N mineralization rates after the plants had grown for 12 months. Soil labile organic C and N contents were represented by soil extractable organic C (EOC) and extractable organic N (EON) contents. We assessed the magnitude of the interspecific plant competition via the relative interaction intensity (RII) index, which was calculated from the biomass of seedlings in the mixed treatments compared with the single treatments. Our results showed that interspecific plant competition had species-specific effects on plant biomass and soil total C and N contents as well as soil C mineralization rates, whereas it tended to decrease soil N mineralization rates. However, interspecific plant competition significantly decreased plant C and N contents, and significantly increased soil EOC and EON contents with increasing RII. After classifying the communities into two functional types (i.e., deciduous–evergreen versus evergreen–evergreen), similar relationships were observed. These findings address the importance of quantifying interspecific plant competition on soil labile organic C and N contents, which is helpful for understanding soil C and N cycling in forest ecosystems.