Which statement correctly describes how nitrogen in the soil returns to the atmosphere?

Soil bacteria converts usable nitrogen into nitrogen gas. Decomposers directly convert ammonium into nitrogen gas. Plants convert usable nitrogen into nitrogen gas. Nitrogen-fixing bacteria in plant roots convert usable nitrogen into nitrogen gas.

The answer is "Decomposers directly convert ammonium into nitrogen gas".

Living organisms such as plants and animals contains nitrogen in the form of the amino acids and proteins. When they die, the nitrogen in the form of ammonium is decomposed by microorganisms. It is then converted into nitrites and then to nitrates by nitrifying bacteria. The denitrifying bacteria then converts nitrates into nitrogen.