Final answer:
Free nitrogen refers to atmospheric nitrogen (N₂), which is not directly usable by plants. Nitrogen fixation by bacteria converts this into usable forms such as ammonia (NH₃), which then can be taken up by plants.
Step-by-step explanation:
When we talk about "free nitrogen", we refer to the atmospheric nitrogen (N₂) that is the largest pool of nitrogen in terrestrial ecosystems. However, this form of nitrogen is not directly usable by plants because they lack the enzymes necessary to fix nitrogen. Nitrogen fixation is the process where nitrogen-fixing bacteria convert this inert atmospheric nitrogen into ammonia (NH₃), which plants can then take up and use for growth.
Bacteria such as Rhizobium live in a symbiotic relationship with legumes, forming root nodules where this fixation occurs. The fixed nitrogen becomes part of the soil nitrogen cycle, contributing to plant nutrition and thus supporting the entire ecosystem.