Few studies have focused on the effects of long-term forest plantations on the soil profile of carbon (C) and nitrogen (N) stocks. In this study, we selected 78-year-old tree plantations that included three coniferous tree species (i.e., slash pine, hoop pine and kauri pine) and a Eucalyptus species in subtropical Australia. We measured soil extractable organic C (EOC) and N (EON) contents and total C and N stocks under different tree species on the forest floor and along a soil profile to 100 cm depth. The results showed that Eucalyptus had significantly higher soil EOC contents (3.3 Mg ha−1) than the other tree species (EOC of 1.9–2.3 Mg ha−1) and had significantly higher EON (156 kg ha−1) contents than slash pine (107 kg ha−1). Eucalyptus had significantly higher soil C (58.9 Mg ha−1) and N (2.03 Mg ha−1) stocks than the other tree species (22.3–27.6 Mg C ha−1 and 0.71–1.23 Mg N ha−1) at 0–100 cm depth. There were no differences in soil C stocks at the 0–100 cm depth among the coniferous tree species. Forest floor C stocks had stronger effects on mineral soil total N stocks than fine root biomass, whereas fine root biomass exerted stronger effects on soil total C stocks at the 0–100 cm depth than forest floor C and N stocks. Our results addressed large differences in soil C and N stocks under different tree species, which can provide useful information for local forest management practices in this region.