Abstract: By using growth chamber, Betula albosinensis seedlings were planted with two densities (28 and 84 plants·m^-2) and treated with 350 and 700 μmol CO₂·mol^-1 to study the effects of elevated CO₂ on the growth and nutrient contents of the seedlings. The results showed that under elevated CO₂, the plant height, leaf area index, net assimilation rate and relative growth rate of B. albosinensis seedlings with both of the two planting densities increased, while the leaf-mass ratio and specific leaf area decreased. Elevated CO₂ increased the dry mass of stem, branch, leaf and root and the uptake of N and P per plant, but decreased the N and P contents of each organ. It also benefited the use efficiency and accumulation rate of N and P. The decrease of N and P contents in each organ was mainly resulted from the dilution effect of the rapid accumulation of biomass, while the increase of N and P use efficiency might be helpful to the alleviation of the impact of soil nutrients deficiency, a limiting factor on the productivity of subalpine and alpine mountain forests owing to the elevated CO₂ environment in the future. The increment of plant growth under elevated CO₂ decreased with increasing planting density, and the more enhancement of nutrient uptake per plant was observed under low than under high planting density, mainly because the increase of planting density significantly decreased the dry mass of B. albosinensis seedlings.

Key words: Wheat-cotton double cropping, Cotton, Rhizosphere and non-rhizosphere zones, Soil enzyme activity, Soil nutrient content