Draw a neat, labeled diagram of nitrogen cycle in nature. Which natural phenomenon occurring in the sky is responsible for nitrogen fixation?

The nitrogen cycle illustrates how nitrogen circulates through the atmosphere, soil, living organisms, and back to the atmosphere in a continuous biogeochemical cycle. The cycle begins with nitrogen fixation, where atmospheric nitrogen gas (N₂) is converted into ammonia by nitrogen-fixing bacteria (Rhizobium in root nodules and free-living Azotobacter) and by lightning strikes. The ammonia produced is then converted to nitrites and nitrates through nitrification by soil bacteria (Nitrosomonas and Nitrobacter). Plants absorb these nitrates from soil through their roots and incorporate nitrogen into plant proteins and nucleic acids. When animals consume plants, they utilize plant proteins to build their own proteins and other nitrogen-containing compounds.

When plants and animals die, decomposer bacteria and fungi break down the organic nitrogen compounds in dead matter, returning ammonia to the soil through a process called ammonification. Some soil bacteria perform denitrification, converting nitrates back to nitrogen gas, which is released into the atmosphere, completing the cycle. This continuous cycling ensures nitrogen availability for all living organisms despite plants' inability to use atmospheric nitrogen directly.

Lightning is the natural atmospheric phenomenon responsible for nitrogen fixation in the sky. During lightning strikes, the extreme temperatures (up to 30,000°C) and electrical energy break the strong triple bonds in nitrogen molecules (N₂) and oxygen molecules (O₂), allowing nitrogen and oxygen atoms to combine and form nitrogen oxides (NO and NO₂). These nitrogen oxides dissolve in atmospheric moisture and rainwater, forming dilute nitric acid that falls to Earth during rainfall. Once in soil, nitric acid dissociates into nitrate ions that plants can absorb and use. While biological nitrogen fixation by microorganisms contributes the majority of fixed nitrogen, lightning accounts for approximately 5-8% of global nitrogen fixation, playing a measurable role in maintaining soil fertility and supporting the nitrogen cycle.