Explain the processes involved in the nitrogen cycle. How would life on Earth be affected if nitrogen were not cycled?
Nitrogen is an essential element for the synthesis of proteins, nucleic acids (DNA and RNA), and other vital biological molecules in all living organisms. The nitrogen cycle involves the following key processes:
(1) Nitrogen Fixation: Atmospheric nitrogen is converted into ammonia by nitrogen-fixing bacteria such as Rhizobium. Lightning also helps convert nitrogen into compounds that enter the soil through rainwater.
(2) Nitrification: Nitrifying bacteria convert ammonia into forms more readily used by plants. Bacteria such as Nitrosomonas convert ammonia (NH₃) into nitrites (NO₂⁻), and Nitrobacter further converts nitrites into nitrates (NO₃⁻), which can be readily absorbed by plant roots.
(3) Assimilation: Plants absorb nitrates and ammonium ions from the soil through their roots and incorporate nitrogen into organic molecules such as proteins and nucleic acids. Animals obtain nitrogen by consuming plants or other animals.
(4) Ammonification: When plants and animals die or excrete waste, decomposers such as bacteria and fungi break down the complex nitrogen-containing organic compounds and return simpler nitrogen compounds, primarily ammonium ions/ammonia, to the soil.
(5) Denitrification: Bacteria such as Pseudomonas convert nitrates in the soil back into nitrogen gas (N₂), which is released into the atmosphere to complete the nitrogen cycle.
If nitrogen were not cycled on Earth, ecosystems would collapse because nitrogen is essential for proteins, nucleic acids, and chlorophyll. Plants would be unable to obtain usable nitrogen forms, leading to stunted growth and death. Without plants, animals would lose their primary food source and fail to build tissues and enzymes. Decomposers could still break down organic matter, but without recycling nitrogen back into the soil, the supply of available nitrogen would be exhausted. As a result, food chains would break down, biodiversity would decline, and Earth would become inhospitable for most life forms.
