Abstract: The increase of plant biomass caused by nitrogen (N) addition is one of the important factors to enhance carbon sequestration potential of plants. At present, the effects of direct and indirect interactions between plants, soil, and microorganisms under N addition on the above and below-ground carbon sequestration (C) function of forests are unclear. We used structural equation models to analyze the direct and indirect effects of leaf and fine root chemical traits, soil microbial community and soil physicochemical factors on above and below-ground biomass increase of poplar 107 (Populus euramericana cv. ‘74/76’) saplings under four N input levels (no N addition, additions of 50, 100, and 200 kg·hm^-2·a^-1 urea). The results showed that with the increases of exogenous N input, the increase of aboveground biomass was greater than that of belowground biomass, resulting in a decrease of root-shoot ratio. The increase of N input significantly increased leaf N concentration and decreased leaf C concentration, resulting in the decrease of leaf C∶N, but the effect on the chemical traits of fine roots was limited. The increase of N input did not affect rhizosphere soil microbial community. The structural equation modeling showed that the direct and indirect factors affecting above and below-ground biomass were consistent, i.e., exogenous N input, leaf C and leaf N had direct effects, while NH₄⁺-N, NO₃^--N content, and leaf N had indirect effects. Our results clarified the direct or indirect interactions among plants, soil and microorganisms, providing a scientific basis for evaluating the effects of N addition on the above and below-ground carbon sequestration of poplar plantations.

Key words: nitrogen addition, poplar, plant traits, biomass, structural equation model, effects