What is meant by the term "nutrient cycle"? Explain the nitrogen cycle with the help of a diagram.

Nutrient Cycle

A nutrient cycle refers to the continuous movement and recycling of essential nutrients (such as carbon, nitrogen, phosphorus, and sulfur) through the biosphere, atmosphere, hydrosphere, and lithosphere. These cycles maintain the balance of nutrients in ecosystems, ensuring that living organisms receive the necessary elements for growth and survival. Nutrients are absorbed by plants and animals, then returned to the environment through decomposition, excretion, and other natural processes.

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Nitrogen Cycle

The nitrogen cycle is the process by which nitrogen is converted into various chemical forms, making it accessible to living organisms. Nitrogen is an essential nutrient for all living things, particularly for the synthesis of proteins and nucleic acids. The nitrogen cycle involves several key steps:

Steps of the Nitrogen Cycle

1. Nitrogen Fixation

   • Atmospheric nitrogen ($N_2$) is converted into nitrates ($N{O}_3^{-}$) or ammonia ($N{H}_3$) by:
     • Nitrogen-fixing bacteria (e.g., Rhizobium in legume roots).
     • Lightning (atmospheric fixation).
     • Industrial processes (Haber process).

2. Nitrification

   • Ammonia ($N{H}_3$) is oxidized into nitrites ($N{O}_2^{-}$) by nitrifying bacteria (Nitrosomonas).
   • Nitrites are further oxidized into nitrates ($N{O}_3^{-}$) by Nitrobacter.

3. Assimilation

   • Plants absorb nitrates ($N{O}_3^{-}$) from the soil and convert them into amino acids and proteins.

4. Ammonification

   • When plants and animals die, decomposers (bacteria and fungi) break down organic nitrogen into ammonia ($N{H}_3$).

5. Denitrification

   • Denitrifying bacteria (e.g., Pseudomonas) convert nitrates ($N{O}_3^{-}$) back into nitrogen gas ($N_2$), which is released into the atmosphere.

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The nitrogen cycle ensures that nitrogen is continuously recycled, maintaining ecological balance and supporting life processes. Without this cycle, organisms would lack essential nitrogen compounds for growth.