What is the first step in the nitrogen cycle?

The first step in the nitrogen cycle is nitrogen fixation. This process involves converting atmospheric nitrogen gas (N₂), which is not directly usable by most organisms, into forms that can be utilized by living organisms. Nitrogen fixation is primarily carried out by certain bacteria, including those in the root nodules of legumes or free-living bacteria in the soil. During this process, nitrogen is transformed into ammonia (NH₃) or related compounds, making it accessible for plants to absorb and use to synthesize essential biological molecules like amino acids and nucleotides.

This foundational step is crucial because it initiates the flow of nitrogen through the ecosystem, enabling not just plants but also herbivores and, subsequently, carnivores to benefit from the nitrogen cycle. Without nitrogen fixation, there would be a limited supply of bioavailable nitrogen, greatly restricting the productivity of ecosystems. The processes that follow, such as nitrification, denitrification, and ammonification, depend on the initial availability of organic nitrogen sources that begin with fixation.