Abstract: [Objective] This study aimed to provide recommendations for N fertilizer management of Populus tomentosa under drip fertigation. [Method] A field experiment was conducted in 2010-2012 (stand age of 3-5 years) to investigate the effects of nitrogen rate (NR) and application frequency (NF) on biomass accumulation and N uptake of P. tomentosa (clone B301) in a fast-growing pulp plantation. The experiment included six N fertigation treatments, which were a factorial combinations of three NR (115(N₁₁₅), 230(N₂₃₀) and 345(N₃₄₅) kg·hm^-2a^-1) and two NF (two (F₂) or four (F₄) times per year). A control treatment (0 N with irrigation, CK) was also included in the experiment. Diameter at breast height, height, biomass, N uptake and soil N were measured periodically during the experiment. [Result] Under natural condition (ambient environment), the soil N supply capacity (includes apparent N mineralization (23-42 kg·hm^-2a^-1 in the P. tomentosa plantation of age 3-5) and soil inorganic N) could not provide P. tomentosa with adequate nutrient for optimal growth. The amount of apparent N mineralization was 23-42 kg·hm^-2a^-1 in the plantation of age 3-5. The trees in CK incorporated nearly 60 kg·hm^-2a^-1 from soil N sources, which could not maintain a high level of yield of P. tomentosa plantation. N fertigation showed a positive effect on tree growth, and especially trees under the N₁₁₅F₄ treatment had the highest growth rate. Total biomass (TM) in the N₁₁₅F₄ treatment reached 33.9 and 45.5 t·hm^-2 in 2011 and 2012, respectively, which were 42% and 49% significantly higher than that in CK (P < 0.05), respectively. Variance in NR could significantly affect the TM of stand. In 2011, the TM under N₁₁₅ was significantly higher than that under N₂₃₀ and N₃₄₅ (P < 0.05), while in 2012 it was significantly higher than N₃₄₅ (P<0.05) and not significantly higher than N₂₃₀(P >0.05). NF showed almost no effect on biomass accumulation, but higher NF (F₄) resulted in significantly higher average annual N uptake (P <0.05). The relationship between total biomass and NR could be well fitted (P0.01, R²=0.482) with the quadratic regression equation. The optimal NR calculated from the equation was 192 kg·hm^-2a^-1, and its 90% confidence interval was 148-236 kg·hm^-2a^-1. [Conclusion] To sum up, under drip fertigation, the traditional NR (N₃₄₅) in P. tomentosa plantation did not bring biomass yield benefits, and adopting drip fertigation technique could decrease the fertilizer application rate relative to the traditional fertilization technique. NF had no effect on biomass, but higher NF could be applied to increase N uptake. Thus, when apply N fertigation to 3-5 years old P. tomentosa plantation under drip irrigation, the recommended NR is 192 kg·hm^-2a^-1, which should be applied in four times within a year.

Key words: fertigation, Populus tomentosa, biomass, N uptake