Final answer:
Plants absorb nitrogen compounds during the step of assimilation in the nitrogen cycle. Nitrogen fixation, often carried out by bacteria in the soil or on legume roots, is the process that converts atmospheric nitrogen into nitrates and ammonium, which plants can then take up through their roots.
Step-by-step explanation:
In the nitrogen cycle, plants absorb nitrogen compounds during the step known as assimilation. This occurs after nitrogen has been "fixed" into forms that are usable by plants, such as nitrate ions (NO3-) and ammonium ions (NH4+).
Nitrogen fixation is a crucial process carried out by symbiotic and free-living nitrogen-fixing bacteria, which convert atmospheric nitrogen gas (N2) into forms that plants can use. The fixed nitrogen, in the form of nitrates and ammonia, provides essential nutrients that help in the synthesis of proteins and nucleic acids crucial for plant growth. Following this, plants can take up these compounds through their roots from the soil, thus incorporating nitrogen into the biological cycle.
It's noteworthy that legumes, such as peas and beans, have a symbiotic relationship with rhizobia bacteria that live in nodules on their roots. These bacteria fix nitrogen directly from the atmosphere and make it available to the plant in a readily absorbable form, mostly as ammonia which the plants convert to amino acids and other nitrogenous compounds.