Phosphorus (P) and nitrogen (N) are the major limiting nutrients for plant growth in most ecosystems. However, it is unclear whether P fertilization exhibits higher effects on plant growth than N fertilization and whether the effects vary with season, since plant demands for N vs. P vary differently during different seasons. We seasonally measured N and P concentrations of fine roots, leaves, and soil as well as growth in Chinese fir (Cunninghamia lanceolata) plantations fertilized with P (5 g m⁻² a⁻¹) and/or N (5 g m⁻² a⁻¹ and 10 g m⁻² a⁻¹) over five years in subtropical China. Needle N and P concentrations were significantly influenced by P fertilization, whereas root N and P concentrations were significantly influenced by P fertilization and the interaction between P fertilization and season. Phosphorus fertilization generally decreased root N concentration in the warm season and increased P concentrations of needle and root in the cold season. Moreover, root P concentration with P fertilization was significantly higher during the cold season than during the warm season, regardless of N fertilization. Contrary to the cold season, in the case of high N fertilization, basal area increment was significantly lower with than without P fertilization during the warm season. Nitrogen fertilization in the warm season had a positive effect on basal area increment, contrary to the cold season. Contrary to N fertilization, P fertilization had a negative effect on basal area increment in the warm season, whereas it had a positive effect in the cold season. Phosphorus fertilization contributed more than N fertilization to basal area increment during both seasons. Moreover, root P concentration was the most crucial factor determining basal area increment during both seasons. The opposite effects of N and P fertilizations on plant growth depend on the season in the Chinese fir plantation, which suggested that the timing of P and N fertilizations should be considered for the Chinese fir plantations in subtropical areas.