Nitrogen Fixation
Nitrogen: important nutrient, often limiting
Atmospheric N2: most abundant N resource but plants do not possess enzymes to fix N2
Root nodules: symbiosis with nitrogen-fixing bacteria (Rhizobium) in some plants (legumes)
Controlled by exchange of chemicals; suppressed in N-rich soil
Specific symbioses: each plant has a different Rhizobium species in nodules