INFLUENCE OF VEGETATION TYPE AND MADRONE SOIL INOCULUM ON ASSOCIATIVE NITROGEN-FIXATION IN DOUGLAS-FIR RHIZOSPHERES

Pseudotsuga menziesii seedlings grown on a site cleared of whiteleaf manzanita Arctostaphylos viscida and an adajcent, cleared, annual grass meadow were either inoculated with 100-120 mL per seedling of pasteurized or unpasteurized soil from a nearby Pacific madrone Arbutus menziesii stand or left uninoculated. After 1 growing season the rate of acetylene reduction in rhizospheres of uninoculated seedlings from the manzanita site (1.40 ± 0.44 mnol.h-1) was significantly higher than that of uninoculated seedlings from the meadow site (0.67 ± 0.15 nmol.h-1). Unpasteurized madrone soil increased the rate of acetylene reduction over 500% for inoculated seedlings grown on the manzanita site, but decreased it by 80% for those grown on the meadow site. The madrone soil influence was apparently biotic. No acetylene was reduced in soil without seedlings. Azospirillum sp., a microaerophilic N2-fixing bacterium, was isolated from within the mycorrhizae of inoculated seedlings harvested from the manzanita site. Early successional ectomycorrhizal shrubs and hardwood trees thus may be important in maintaining mycorrhizal fungi and associated N2 fixers afer severe disturbance. -from Authors