Final answer:
Plants acquire nitrogen needed for amino acids and DNA from the soil, where it is available as nitrate or ammonia produced by nitrogen-fixing bacteria such as Rhizobium in symbiosis with legumes, and by free-living bacteria like Azotobacter.
Step-by-step explanation:
Plants require nitrogen to synthesize essential biomolecules such as amino acids and DNA. However, they cannot directly utilize atmospheric nitrogen (N₂), which is abundant but inert due to its triple covalent bonds. Instead, plants acquire nitrogen from the soil, primarily in the form of either nitrate (NO₃-) or ammonia (NH₃), which are produced through the action of nitrogen-fixing bacteria.
Nitrogen Fixation and Plant Utilization
Some of these bacteria, such as Rhizobium, form symbiotic relationships with legumes by living in their root nodules. The bacteria fix atmospheric nitrogen into ammonia, which plants then use to build nucleic acids and proteins. This symbiotic relationship is crucial for the nitrogen cycle, where nitrogen is continuously transformed and recycled in ecosystems. Cyanobacteria and other free-living nitrogen-fixing bacteria like Azotobacter also contribute to converting atmospheric nitrogen into a form that plants can uptake and use.
Through these biological processes, nitrogen becomes part of the soil as organic compounds, providing an accessible source of nitrogen for plants. This natural phenomenon is essential not only for plant growth but also for agricultural practices, as it enhances soil fertility and reduces the need for chemical fertilizers.