Abstract: Xanthium spinosum is a malignant invasive plant in Ili Valley. Studying the effects of nitrogen deposition on seedling growth and biomass allocation will help to understand the response of this species to nitrogen deposition. We examined the effects of nitrogen deposition on morphological characteristics, module’s biomass and biomass allocation of X. spinosum seedlings, with four nitrogen deposition levels (0, 0.18, 0.72, 2.88 g N·m^-2·week^-1). The results showed that: (1) Moderate nitrogen deposition (0.72 g N·m^-2·week^-1) increased taproot length, plant height, number of leaves, basal diameter, and number of thorns, which enhanced the invasive ability. Compared with the control group and low nitrogen deposition, taproot length increased under high nitrogen deposition (2.88 g N·m^-2·week^-1), but plant height, number of leaves, basal diameter and number of thorns significantly decreased, which was not conducive to seedling growth. (2) Under high nitrogen deposition, the biomass of roots, stems and leaves significantly decreased, while the biomass ratio of roots and the ratio of root to shoot increased significantly, indicating that high nitrogen deposition was not conducive to the allocation of biomass to aboveground and seedling growth was inhibited. (3) The response of seedling growth to different nitrogen deposition levels varied over time. The seedlings under medium nitrogen treatment grew slowly in the middle stage of sampling, and grew rapidly at the end of the sampling period, while under the high nitrogen treatment, the growth was rapid in the middle stage and slow in the end stage. These results indicated that moderate level of nitrogen deposition promoted the growth of X. spinosum seedlings and improved their invasive ability, but high level of nitrogen deposition was not conducive to invasion.

Key words: Xanthium spinosum, nitrogen deposition, module, growth, biomass allocation.