The nitrogen in plants comes from the soil. Bacteria in the soil take nitrogenous wastes and convert it into forms of nitrogen that plants can use. Plants then take up nitrogen through their roots.

Only two options here actually lead to the conversion of fixed nitrogen to atmospheric nitrogen, volatilization and denitrification of which denitrification is a relatively rapid process carried out by numerous denitrifying microbes thus making it the greater contributor to the return of nitrogen to the atmosphere from fixed nitrogen.

Nitrogen fixation is mostly done by bacteria living in the soil. Plants need nitrogen to grow, but they cannot use it straight from the atmosphere or as ammonia from the soil.

Humans and animals largely obtain their necessary nitrogen by consuming plants, and do not fix nitrogen or rely directly on bacteria for the process.

Volatilization is a process by which fixed nitrogen is released back into the atmosphere as gas.

Condensation is not part of the nitrogen cycle. It is part of the water cycle, during which water molecules condense together in the atmosphere to form clouds.

Since the nitrogen gas that composes 78% of the atmosphere is not immediately usable to all organisms except for nitrogen-fixing organisms the nitrogen that composes the Earth's major nitrogen reserve is not immediately usable to most organisms.

Like carbon, nitrogen is one the most abundant elements in biotic factors. Nitrogen gas is highly abundant in our atmosphere, however it cannot be utilized by humans and plants while in its gaseous state because of the very strong triple bond between the two nitrogen atoms. For plants to use nitrogen, they must have it converted to ammonium or nitrate by bacteria found in the soil and roots. The process of converting nitrogen gas to ammonium is called nitrogen fixation. Decomposition of plants and animals also releases ammonium into the ground. This ammonium can be further converted to nitrate with the help of nitrifying bacteria. Returning nitrogen back to the atmosphere is called denitrification. This process is carried out by some bacteria found in lakes and swamps. These bacteria are anaerobic, so they use the nitrate and release nitrogen gas into the air.

The atmosphere is composed of 78% nitrogen gas and while by mass the atmosphere is less massive than the all the other choices the other choices are not primarily composed of nitrogen and contain relatively little compared to the nitrogen in the atmosphere.

Nitrification is the process by which nitrite (NO2-) is converted to nitrate (NO3-). This is the final step required in the processes used to oxidize nitrogen wastes (ammonia) to usable nitrate ions.

The conversion of gaseous nitrogen to ammonia (N2 to NH3) describes nitrogen fixation, and is usually done by nitrogen-fixing bacteria.

The conversion of nitrate to plant matter (NO3- to plants) describes the process of assimilation.

The conversion of nitrate to gaseous nitrogen (NO3- to N2) describes denitrification, and is performed by denitrification bacteria.

The conversion of animal waste to NH3 describes ammonification, and is accomplished by saprobiotic (decomposing) bacteria.