Final answer:
The main forms of nitrogen that plants absorb are nitrate and ammonia, with a preference for nitrate due to its less toxic nature. These nutrients are available via nitrogen fixation and nitrification processes carried out by soil bacteria.
Step-by-step explanation:
The main forms of nitrogen that are taken up by plant roots and useful to plants are nitrate (NO3-) and ammonia (NH3). Plants can absorb these nutrients through their roots. Although both ions can be used by plants, most plants have a preference for nitrate because ammonia can be toxic and must be quickly metabolized.
Ammonia and nitrate are available to plants due to processes such as nitrogen fixation and nitrification. Nitrogen fixation is the conversion of atmospheric nitrogen (N2) into ammonia (NH3), primarily through the action of nitrogen-fixing bacteria that live in the soil or in symbiotic relationships with legumes.
Nitrification involves the conversion of ammonia to nitrite (NO2-) and then to nitrate (NO3-), making nitrogen available in a form that plants can use for growth and development.