Influence of vegetation type and madrone soil inoculum on associative nitrogen fixation in Douglas-fir rhizospheres

Abstract

Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) seedlings grown on a site cleared of whiteleaf manzanita (Arctostaphylosviscida Parry) and an adjacent, cleared, annual grass meadow were either inoculated with 100–120 mL per seedling of pasteurized or unpasteurized soil from a nearby Pacific madrone (Arbutusmenziesii Pursh) stand or left uninoculated. After one growing season, Douglas-fir seedling whole-plant soil systems were assayed for nitrogenase activity by the acetylene reduction method. The rate of acetylene reduction in rhizospheres of uninoculated seedlings from the manzanita site (1.40 ± 0.44 nmol•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: pasteurized, madrone soil did not have a significant effect. No acetylene was reduced in soil without seedlings. Azospirillum sp., a microaerophilic nitrogen (N2) fixing bacterium, was isolated from within the mycorrhizae of inoculated seedlings harvested from the manzanita site. These results suggest that early successional ectomycorrhizal shrubs and hardwood trees may be important in maintaining mycorrhizal fungi and associated N2 fixers after severe disturbance.