Effects of nitrogen addition on the seedling biomass and its allocation of three Pinus tabuliformis provenances

Nitrogen deposition is much heavier in the north of China, which exerts a great influence on forest ecological system dynamics. To improve our understanding about the effects of elevating N deposition on the growth and the biomass allocation pattern of Pinus tabuliformis seedlings, a nitrogen addition experiment with three provenances of P. tabuliformis seedlings was carried out on Lingkong Mountain in Shanxi Province, northern China. Three-year-old P. tabuliformis seedlings of three different provenances, Beijing (BJ), Shanxi (SX) and Neimenggu (NMG), were planted in a common garden, and five nitrogen treatments with three replications were designed, including: control (N0, 0kg/(hm²·a), in N, similarly hereinafter), low nitrogen level(N1, 15kg/(hm²·a)), middle nitrogen level (N2, 25kg/(hm²·a)), high nitrogen level (N3, 50kg/(hm²·a)) and the supersaturated nitrogen level (N4, 150kg/(hm²·a)). The base diameter, plant height, monthly growth amount, biomass and the biomass allocation pattern of P. tabuliformis seedlings under different nitrogen addition levels were measured. The results showed that: 1) Adding nitrogen had remarkable effect on plant height for BJ provenance seedlings (P < 0.05), but there was no significant impact on the average base diameter. The base diameters of SX and NMG provenance seedlings were influenced by nitrogen addition treatments, but these effects were not significant. 2) The height growth of BJ provenance seedlings increased under adding nitrogen treatments, while their base diameter growth decreased, but these effects were not significant; The month base diameter growth of NMG provenance seedling decreased significantly under the N4 treatments (P < 0.05). 3) The growth of BJ provenance was affected by nitrogen addition, with dry biomass, dry biomass of the whole plant, the aboveground part and stem increasing significantly under the N1 nitrogen treatment (P < 0.05). Leaf dry biomass of SX provenance seedlings decreased significantly under different nitrogen addition treatments, while there was no significant changes of the whole plant biomass of NMG provenance seedling. 4) Leaf mass ratio of SX provenance seedlings increased significantly under the N3 level nitrogen addition treatments (P < 0.05); The root mass ratio of NMG provenance seedlings reduced significantly with the incensement of concentration of nitrogen addition. while stem mass ratio showed inversed trend; 5) The analysis of sources of variation showed that the variation of base diameter was related closely with both nitrogen treatment and provenance, but those of month growth of base diameter, leaf mass ratio and root-leaf ratio were only associated with the provenance. In conclusion, continuous nitrogen added has affected the internal stability of Pinus tabuliformis populations.