biological nitrogen fixation
31.3 Nutritional Adaptations of Plants Atmospheric nitrogen is the largest pool of available nitrogen in terrestrial ecosystems. However, plants cannot use this nitrogen because they do not have the necessary enzymes. Biological nitrogen fixation BNF is the conversion of atmospheric nitrogen to ammonia. The most important source of BNF is the symbiotic interaction between soil bacteria and legumes. The bacteria form nodules on the legumes roots in which nitrogen fixation takes place. Fungi form symbiotic associations mycorrhizae with plants, becoming integrated into the physical structure of the root. Through mycorrhization, the plant obtains minerals from the soil and the fungus obtains photosynthate from the plant root. Ectomycorrhizae form an extensive dense sheath around the root, while endomycorrhizae are embedded within the root tissue. Some plantsparasites, saprophytes, symbionts, epiphytes, and insectivoreshave evolved adaptations to obtain their organic or mineral nutrition from various sources.