Final answer:
Bacteria primarily fix nitrogen from the air in the nitrogen cycle. Only about 0.001% of atmospheric nitrogen is used in the cycle each year.
Step-by-step explanation:
The primary role of bacteria in the nitrogen cycle is to fix nitrogen from the air. These nitrogen-fixing bacteria convert atmospheric nitrogen, which is inaccessible to most living organisms, into forms that can be used by plants, such as ammonia and subsequently nitrates. Nitrates can then be taken up by plants and used to build proteins and other essential compounds. Regarding the percentage of atmospheric nitrogen involved in the nitrogen cycle per year, it is very small, approximately 0.001% of the atmospheric nitrogen is transformed by microbial activity each year.
False Statements in the Nitrogen Cycle
Answering the question about which statement about the nitrogen cycle is false, statement c. Nitrification by bacteria converts nitrates (NO3) to nitrites (NO2) is incorrect because nitrification actually refers to the process in which bacteria converts ammonia (NH4+) into nitrites (NO2) and then into nitrates (NO3), not the other way around.