Final answer:
Plants obtain nitrogen through nitrogen fixation, which involves converting atmospheric nitrogen into a usable form. Legumes have a mutualistic symbiotic relationship with nitrogen-fixing bacteria in their root nodules, while free-living bacteria can also fix nitrogen in the soil.
Step-by-step explanation:
Plants obtain nitrogen through a process called nitrogen fixation. This process involves the conversion of atmospheric nitrogen (N2) into a form that plants can use, such as ammonia (NH3) or nitrates (NO3-). There are two main ways in which plants acquire nitrogen through nitrogen fixation.
The first way is through mutualistic symbiotic relationships with nitrogen-fixing bacteria. Legumes, such as clovers, peas, and beans, have specialized root nodules that house nitrogen-fixing bacteria called rhizobia. These bacteria convert atmospheric nitrogen into ammonia, which the plants can then use to synthesize organic compounds.
The second way is through free-living nitrogen-fixing bacteria in the soil. These bacteria, such as Azotobacter, are independent of plant roots but can still convert atmospheric nitrogen into ammonia. The ammonia can then be absorbed by nearby plants.