What process in the nitrogen cycle is responsible for returning nitrogen to the atmosphere?

The process responsible for returning nitrogen to the atmosphere is denitrification. In the nitrogen cycle, denitrification occurs when certain bacteria convert nitrate (NO3) and nitrite (NO2) back into nitrogen gas (N2), which is then released into the atmosphere. This process is a crucial part of the nitrogen cycle, as it helps to maintain a balance of nitrogen in the ecosystem.

Decomposition does contribute to the nitrogen cycle by breaking down organic materials, which can release nitrogen compounds in the soil. However, it does not specifically return nitrogen gas to the atmosphere. Nitrogen fixation is the process where nitrogen gas is converted into ammonia by certain bacteria, while assimilation refers to the incorporation of nitrogen into organic molecules by plants. Nitrification involves the conversion of ammonia into nitrites and then nitrates but also does not return nitrogen to the atmosphere.

Thus, the correct process for returning nitrogen to the atmosphere is denitrification, highlighting the importance of specific bacteria in completing the nitrogen cycle and maintaining ecosystem health.