Growth and ectomycorrhizal colonization of ponderosa pine seedlings supplied different levels of atmospheric CO 2 and soil N and P

Abstract

Individual and interactive effects of atmospheric CO 2 enrichment and soil N and P fertility on above- and below-ground growth and mycorrhizal colonization of juvenile ponderosa pine ( Pinus ponderosa Dougl. ex Laws.) were examined. Seedlings were grown from seed in atmospheres with 700 μl l −1, 525 μl l −1, or ambient CO 2 concentrations. High and low soil N treatments were created by adding sufficient (NH 4) 2SO 4 to an infertile soil mixture to establish total N concentrations of 500 μg g −1 and 400 μg g −1, respectively, while high and low P treatments consisted of 68 μg g −1 and 43 μg g −1 concentrations, respectively, of extractable P created by additions of CaHPO 4. All seedlings were inoculated with the mycobiont Pisolithus tinctorius (Pers.) Coker and Couch shortly after emergence. Three whole-seedling harvests at four month intervals permitted assessment of treatment effects on shoot and root growth and ectomycorrhizal development. Initially, 525 μl l −1 CO 2 and high N and P were all influential factors in above-ground growth, with each of these treatments increasing shoot weight while the latter increased height, diameter and volume. Stimulation of root growth was evident in dry weight and length measurements at the first harvest, with N and P main treatment effects again evident, but the response to elevated CO 2 was most pronounced in the 700 μl l −1 atmosphere. After eight months, soil P was of little consequence above- or below-ground, but high N increased shoot dimensions, volume, and weight and root weight and length. Furthermore, the 525 μl l −1 CO 2 treatment emerged as the dominant stimulatory atmosphere both above- and below-ground, as seedlings grown in intermediate CO 2 exhibited the largest shoot diameters, greatest shoot and root weights, and the longest root systems at the second harvest. At the final harvest, interactive effects of 525 μl l −1 CO 2 and high N were prominent, as this treatment combination produced the largest shoot dimensions, volume and weight, and the greatest root volume and coarse and fine root weights. Intermediate CO 2 also produced the longest root systems after 12 months. Shoot/root ratios were lowered by growth in 700 μl l −1 CO 2 after four months and by both enriched atmospheres after eight months, but this effect was no longer evident at the final harvest. Greater numbers of mycorrhizae were formed by seedlings grown in 700 μl l −1 CO 2 after four months and by those grown in 525 μl l −1 CO 2 after eight months. Both enriched atmospheres increased mycorrhizal counts after 12 months, and seedlings grown in high CO 2 and low N exhibited the highest percentage of total root system length colonized at the final harvest as well. Overall, these results indicate that CO 2 enrichment stimulates shoot and root growth of juvenile ponderosa pine, a response dependent on soil N rather than P availability, and that the magnitude of the growth increase is greater in 1.5× ambient than in 2× ambient CO 2.