Nitrogen-fixing bacteria can convert ammonia into nitrogen?

In biology, symbiotic nitrogen-fixing bacteria invade the root hairs of host plants where increment or multiply  and stimulate  formation of root nodules,  enlargements of plant cells and bacteria in intimate association.  The  bacteria   that change nitrogen gas from the atmosphere into solid nitrogen usable by plants are called nitrogen-fixing  bacteria. These bacteria are found both in the soil and in symbiotic relationship with plants.

Explanation:

layer 3 is younger than layer 6.