Final answer:
Bacteria contribute to the nitrogen cycle through nitrogen fixation, ammonification, nitrification, and denitrification, which involve converting atmospheric nitrogen into compounds useful for plants and recycling it back into the atmosphere.
Step-by-step explanation:
Bacteria participate in the nitrogen cycle primarily through four key mechanisms: nitrogen fixation, decomposition or ammonification, nitrification, and denitrification. In nitrogen fixation, nitrogen-fixing bacteria, such as those living in the root nodules of legumes or free-living in soil, convert atmospheric nitrogen (N₂) into organic compounds that plants can utilize. During ammonification, decomposer organisms like bacteria and fungi break down organic nitrogen of dead matter and waste products into ammonia (NH₃), which they then convert into ammonium ions (NH₄+). Nitrification is a two-step process where nitrifying bacteria first convert ammonium ions into nitrites (NO₂-) and then further oxidize them into nitrates (NO₃-), which can be absorbed by plants. Lastly, denitrification is performed by denitrifying bacteria that convert nitrates back into nitrogen gas, closing the nitrogen cycle.