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.

Decomposers directly convert ammonium into nitrogen gas.

Explanation:

The living beings such as plants and animals exhibit biomolecules like nucleic acids and proteins in which the important ingredient called as nitrogen is present.  When these organisms die the dead remains of the organisms are degraded and decomposed by the soil microbes. The organic matter generated after the decomposition process consists of ammonium which is converted into nitrogen gas by the soil microbes. This way the nitrogen fixed by the soil bacteria in the soil into various forms reverts back to the atmosphere by the decomposition process.

"griffith's experiment discovering the "transforming principle" in pneumococcus bacteria. griffith's experiment, reported in 1928 by frederick griffith, was the first experiment suggesting that bacteria are capable of transferring genetic information through a process known as transformation." - griffith's experiment.