How does nitrogen typically return from the soil to the atmosphere?

The process by which nitrogen returns from the soil to the atmosphere is known as denitrification. Soil bacteria play a crucial role in this process. Specifically, certain types of bacteria, which are often referred to as denitrifying bacteria, convert nitrates, which are forms of nitrogen that plants can use, back into nitrogen gas (N₂). This nitrogen gas then re-enters the atmosphere.

In the nitrogen cycle, this conversion is essential because it helps to maintain the balance of nitrogen in the ecosystem. While plants can absorb nitrogen in the form of nitrates and ammonium, the atmospheric nitrogen needs to be cycled back to prevent soil depletion of nitrogen. Thus, the correct answer highlights the significant contribution of soil bacteria in facilitating this natural process.

The other options offer alternative processes that do not describe the correct pathway from soil to atmosphere. For example, decomposers primarily break down organic material and convert it into simpler compounds but do not directly convert ammonium into nitrogen gas. Similarly, while plants do assimilate nitrites, the conversion into nitrogen gas is not part of their biochemical processes; instead, it's the role of bacteria. Next, nitrogen-fixing bacteria specifically convert nitrogen gas into usable forms for plants, rather than the other way around.