Abstract: To understand the variations of carbon (C), nitrogen (N), and phosphorus (P) stoichiometry in poplar plantation and the effects of stand development on the element cycle between plant and soil, we measured C, N, and P concentrations and their ratios in different organs (leaf, stem, and root), leaf litter and soil in young and mature poplar plantations. N and P resorption efficiencies were calculated, and the correlations of C, N, and P between plant and soil were explored. The results showed that N and P concentrations decreased in the order of leaf>root>branch for young trees, whereas they were higher in leaves and branches than roots in mature trees. C∶N, C∶P, and N∶P in leaves and branches, and N and P concentrations in roots were higher, but N concentrations in branches, P concentrations in leaves and branches, and C∶N and C∶P in roots were lower in young trees than in mature trees. Leaf litter N and P concentrations of young trees were lower than those of mature trees, while C∶N and C∶P of young trees were higher than those of mature trees. N resorption efficiency was higher but P resorption efficiency was lower in young trees than in mature trees. Soil C and N concentrations increased but soil P concentrations decreased at 0-40 cm layer after poplar afforestation. N and P concentration in both branch and root was positively correlated with soil N and P concentration, respectively. These results indicated that young poplar trees tended to supply more N and P to roots, and decrease the loss of N and P in leaf litter, which would contribute to rapid growth. However, mature trees allocated more N and P to leaves and branches, and increased the return of N and P from leaf litter to soil, so as to promote the nutrient cycle between trees and soil. The growth of young poplar trees was mainly restricted by N, while that of mature trees was mainly restricted by P in Horqin Sandy Land.

Key words: nutrient limitation, organ, seedling, mature tree, nitrogen and phosphorus cycling, shelter forest