Atmospheric CO 2 enrichment and soil N fertility effects on juvenile ponderosa pine: Growth, ectomycorrhizal development, and xylem water potential

Abstract

Interactive effects of elevated atmospheric CO 2 and soil N fertility on above- and below-ground growth, mycorrhizal colonization, and water relations of juvenile ponderosa pine ( Pinus ponderosa Dougl. ex Laws.) were investigated. One-year-old seedlings were planted in undisturbed field soil within open-top chambers which permitted creation of atmospheres with 700 μl l −1, 525 μl l −1, or ambient CO 2 concentrations. High and medium soil N treatments were imposed by incorporating sufficient (NH 4) 2SO 4 to increase total N by 200 μg g −1 and 100 μg g −1, respectively, while unamended soil, which had a total N concentration of approximately 900 μg g −1, constituted the low N treatment. Following each of two consecutive field growing seasons, whole seedlings of every combination of CO 2 and N treatment were harvested to permit assessment of shoot and root growth and quantification of ectomycorrhizal development. Late in the second growing season, a simulated drought episode was imposed by withholding irrigation during which predawn and midday xylem water potential and soil water potential were measured. The initial harvest revealed that coarse and fine root weights were increased by CO 2 enrichment during the first growing season. This result was most apparent in the 525 μl l −1 CO 2 treatment and high soil N, which produced the greatest root volume as well. Shoot/root ratio decreased with increasing CO 2 at the first harvest. After two growing seasons, elevated CO 2 increased seedling diameter, shoot and root volume, and shoot and coarse root weight, again most prominently in high N. Unlike the initial results, however, stimulation of seedling growth by the 700 μl l −1 CO 2 atmosphere exceeded that in 525 μl l −1 CO 2 after two growing seasons, and shoot/root ratio was unaffected by either CO 2 or N. At both harvests, seedlings grown in the enriched atmospheres generally had higher mycorrhizal counts and greater percentages of colonized root length, but differences among treatments in ectomycorrhizal development were nonsignficant regardless of quantification method. During the imposed drought episode, xylem water potential of seedlings grown in elevated CO 2 descended below that of seedlings grown in the ambient atmosphere as soil water potential decreased, most notably in the predawn measurements. These results suggest that CO 2 enrichment stimulates shoot and root growth of juvenile ponderosa pine under field conditions, a response somewhat dependent on soil N availability. However, below-ground growth is not increased proportionally more than that above ground, which may predispose this species to greater stress when soil water is limited.