Which cycle includes processes such as nitrogen fixation, nitrification, and denitrification?

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Nitrogen moves through ecosystems in a cycle of transformations that connect the air, soil, water, and living organisms. The processes described—nitrogen fixation, nitrification, and denitrification—are key steps in this cycle. Nitrogen fixation is performed by certain bacteria and some cyanobacteria, converting the inert atmospheric N2 into ammonia or ammonium that organisms can use. Nitrification involves microbes oxidizing ammonia first to nitrite and then to nitrate, which plants readily take up. Denitrification then completes the loop by microbes reducing nitrate back to nitrogen gas, releasing it back into the atmosphere. These steps together explain how nitrogen essential for proteins and DNA moves from the air into living systems and back again. Other cycles, like the water, carbon, and sulfur cycles, involve different processes and pathways.