It is well known that phosphorus (P) is one of the most important elements frequently limiting primary productivity in tropical and subtropical ecosystems. However, the effect of P availability on the decomposition rate of fine roots has yet to be confirmed by experimental manipulation. In order to examine fine root decomposition in response to P addition, we conducted litterbag experiments using the fine roots of Castanopsis carlesii and Cunninghamia lanceolata, which are two tree species typical of subtropical forests. The experimental site is located in Sanming Castanopsis Kawakamii nature reserve of Fujian. We examined the effects of four levels of P addition:high P (HP, 360 kg P hm^-2 a^-1), medium P (MP, 240 kg P hm^-2 a^-1), low P (LP, 120 kg P hm^-2 a^-1), and control (CT, 0 kg P hm^-2 a^-1). During the 2-year experimental period, it was found that C. carlesii fine roots decomposed faster than those of C. lanceolata, and that the decomposition rates were relatively rapid during the initial phase and slow during the later phase. P addition increased the fine root decomposition rates in both species, although it had a greater stimulatory effect on C. lanceolata fine roots. However, the decomposition rates did not accelerate with increasing P addition level. During the decomposition process, the enzyme and accumulated enzyme activities in C. carlesii fine roots were significantly higher than those in C. lanceolata. The pattern of cellulose-degrading enzyme activity showed an initial increase, but subsequently decreased after reaching a peak. In contrast, degradation of recalcitrant lignin gradually increased during the decomposition process. There was a significant correlation between hydrolase activity and the fine root decomposition rate (P < 0.05), and an exponential relationship between oxidase activity and the fine root decomposition rate. P addition decreased acid phosphatase activity, but significantly increased the activities of β-1,4-glucosidase, cellobiohydrolase, β-1,4-N-acetylglucosaminidase, phenol oxidase, and peroxidase during fine root decomposition. The results show that P is one of the main factors affecting subtropical tree fine root decomposition, and that changes in enzyme activities during the decomposition process can explain the observed decomposition rates. The present study reveals the mechanism of fine root decomposition in a subtropical forest with low soil phosphorus availability, which is conducive to both understanding nutrient cycling and decision making in forest management.