EFFECTS OF NITROGEN AND WATER LIMITATION AND ELEVATED ATMOSPHERIC CO2ON ECTOMYCORRHIZA OF LONGLEAF PINE

Longleaf pine (Pinus palustris Mill.) seedlings were exposed to two co ncentrations of atmospheric CO2 (365 or 720 mu mol mol(-1)) and two le vels of N (0.02 or 0.20 mg N g(-1) soil yr(-1)) within open-top chambe rs for 20 months. Seedlings were adequately watered for 19 wk to ensur e seedling establishment, after which two water-stress treatments (tar get values -0.5 or -1.5 MPa xylem pressure potential) were implemented . Fine-root samples were collected in July and November 1993, and in M arch and November 1994. Ectomycorrhizal and non-mycorrhizal short root s per unit length of fine root were quantified. The percentage of ecto mycorrhizal short roots and numbers of ectomycorrhizas per unit root l ength were higher for seedlings grown with elevated CO2, low N and ade quate water. Interactions among main treatment variables demonstrated higher percentages of ectomycorrhizal short roots, fine root length pe r seedling, and total numbers of ectomycorrhizas per seedling for plan ts grown with high CO2 (compared with ambient) or adequate water (comp ared with water stress) only under high N conditions. Increased fine-r oot length and ectomycorrhizal colonization under elevated CO2 resulte d in higher (almost double) numbers of ectomycorrhizas per seedling at each sampling.