Explanation

Nitrogen fixation, denitrification, and ammonification are the main steps in the nitrogen cycle. Nitrogen fixation converts nitrogen gas from air into forms plants can use. Bacteria in soil perform this important transformation.

Ammonification breaks down dead organisms and waste to release ammonia back into soil. Denitrification returns nitrogen to the atmosphere as gas, completing the cycle. These processes keep nitrogen moving between air, soil, and living things.

Respiration and transpiration are parts of water and carbon cycles, not nutrient cycling. Decomposition, erosion, and weathering are physical processes that affect nutrients but are not specific nutrient cycle steps. Only nitrogen fixation, denitrification, and ammonification specifically cycle nutrients through ecosystems.