Abstract: To clarify the response of poplar to nitrogen deposition and water enhancement, Populus ×xiaozhuanica cv. Zhangwu seedlings were grown under a full-factorial experiment of two factors (nitrogen and water addition),i.e., four N fertilization levels (0, 5, 10 and 15 g N·m^-2·a^-1) and three levels of water addition (natural precipitation, 50% increase in precipitation, 100% increase in precipitation) with randomized block design. We measured the concentrations of C, N, and P in different organs (leaf, branch, stem, coarse root and fine root) and analyzed the relationships between element concentrations in different organs. The results showed that with increasing N addition level, N concentration and N∶P in coarse roots and fine roots and P concentration in fine roots showed upward trends, whereas C∶N in coarse roots and fine roots and C∶P in fine roots showed downward trends. With increasing water addition levels, leaf and fine root C concentrations reduced, stem and coarse root C concentrations remained constant, whereas N∶P in coarse roots increased. Nitrogen addition and water addition interacted to affect C∶N∶P stoichiometry in the poplar seedlings. With increasing nitrogen addition, under natural precipitation treatment, no significant variations were found in C concentrations in branches, stems, coarse roots and fine roots, and N and P concentrations in leaves, branches and stems. Under 50% increase in precipitation, upward trends were found in C concentrations in leaves, branches and fine roots, N concentrations in all organs, and P concentrations in leaves, stems and coarse roots. Under 100% increase in precipitation, C and P concentrations in leaves, stems and coarse roots remained unchanged with increased nitrogen addition. Significant positive relationships were found among different organs within the same element (C, N, P) under water and nitrogen addition. Nitrogen concentrations showed significantly positive correlation with P concentrations in leaves and fine roots. Therefore, nitrogen addition only affected underground part N and P distribution of poplar seedlings under natural precipitation, whereas it promoted C, N, and P distribution among organs under 50% increase in precipitation. However, the effects of nitrogen addition were diminished under 100% increase in precipitation.

Key words: water and nitrogen coupling, organ differentia, nutrient allocation, climate change, Horqin sandy land.