Final answer:
In the nitrogen cycle, bacteria in the geosphere convert atmospheric nitrogen into ammonia, which is then transformed into nitrates for use by plants and animals.
Step-by-step explanation:
To complete the passage about the nitrogen cycle, during nitrogen fixation, specialized bacteria living in Earth's geosphere convert nitrogen taken from the atmosphere into ammonia. Another set of bacteria convert the ammonia to compounds called nitrates that plants and animals can use.
In the nitrogen cycle, nitrogen-fixing bacteria convert atmospheric nitrogen into ammonia, a process known as ammonification. Then, nitrification by other bacteria transforms ammonia into nitrites and further into nitrates, which plants can assimilate. Plants use these nitrates to produce important organic compounds such as DNA and proteins. Eventually, denitrifying bacteria in the soil convert the nitrates back into gaseous nitrogen, releasing it back into the atmosphere.