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How do denitrifying bacteria return nitrogen to the atmosphere?

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Wispy cirrus clouds stretch across a bright blue sky above trees and a river at Pinawa Dam Provincial Park in Manitoba, Canada, with sunlight illuminating the scene. — Atmosphere The atmospheres of planets in the solar system are composed of various gases, particulates, and liquids. They are also dynamic places that redistribute heat and other forms of energy. On Earth, the atmosphere provides critical ingredients for living things. Here, feathery cirrus clouds drift across the deep blue sky over Pinawa Dam Provincial Park, near Pinawa, Manitoba, Canada.

Denitrifying bacteria return nitrogen to the atmosphere through a process known as denitrification, which is a critical component of the nitrogen cycle.12

These microorganisms, which include species such as Thiobacillus denitrificans, Pseudomonas aeruginosa, and Achromobacter, thrive in anaerobic (oxygen-free) environments, such as water-logged or swampy soils. In these conditions, the bacteria metabolize nitrates that have accumulated in the soil or water. Through their metabolic activity, they reduce these nitrates—often by converting them into nitrites and subsequently into free atmospheric nitrogen gas ( $N_{2}$ ).123

This process effectively completes the nitrogen cycle by releasing nitrogen back into the atmosphere, preventing the excessive accumulation of soluble nitrates in aquatic ecosystems. While this action depletes soil fertility for agricultural purposes, it is essential for maintaining the global balance of nitrogen in the environment.142