Abstract: The acceleration of nitrogen (N) deposition may aggravate phosphorus (P) limitation of subtropical tree species. We explored the response of 1-year-old Pinus massoniana seedlings to different P addition levels \[(without P fertilization, CK), low P (50 kg P·hm^-2·a^-1, LP) and high P (100 kg P·hm^-2·a^-1, HP)\]. The effects of different P addition levels on growth, biomass accumulation, non-structural carbohydrate (NSC), and root morphological indexes of P. massoniana seedlings were analyzed. The results showed that: (1) P addition promoted biomass accumulation in various organs of P. massoniana seedlings. HP treatment led to significant increases of 531%, 210%, 355%, 187%, and 321% in leaf, stem, coarse root, fine root biomass, and total biomass, respectively. Moreover, the proportion of leaf biomass increased in response to changes in soil P concentrations. (2) P addition significantly increased the NSC content in various organs of P. massoniana seedlings, and the NSC proportion in leaves gradually increased while that in stems gradually decreased with increasing in P addition. Root proportion showed a trend of first decreasing then increasing with increasing in P addition, but was lower than that in CK. (3) P addition significantly increased length, surface area, and volume of both coarse and fine roots, while root diameter was insensitive to P addition. Under conditions of P addition, fine roots adopted a resource utilization strategy. Our results indicated that P. massoniana seedlings responded to exogenous P addition by increasing the biomass and NSC content of each organ and increasing the proportion of leaves in biomass and NSC content. There was a trade-off between aboveground and underground parts in NSC distribution. P addition promoted the growth of seedling roots, while the coarse roots had certain internal stability in response to external changes. Therefore, P addition stimulated the growth of P. massoniana seedlings.

Key words: Pinus massoniana, phosphorus addition, biomass, non-structural carbohydrate, root morphological index