Nutrient Cycles: Nitrogen

This lesson covers the nitrogen cycle, including nitrogen fixation, nitrification, denitrification, ammonification, and the role of microorganisms, as required by the Edexcel A-Level Biology specification (9BI0), Topic 10 -- Ecosystems.

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Why Is Nitrogen Important?

Nitrogen is an essential element for life. It is a component of:

• Amino acids (and therefore proteins)
• Nucleotides (and therefore DNA and RNA)
• ATP (adenosine triphosphate)
• Chlorophyll

Although the atmosphere is approximately 78% nitrogen gas (N2), most organisms cannot use N2 directly because the triple covalent bond (N triple-bond N) is extremely strong and unreactive. Nitrogen must be converted into a usable form (NH4+ or NO3-) before it can be incorporated into biological molecules.

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Overview of the Nitrogen Cycle

flowchart TB
    A["N2 in\natmosphere"] -->|"Nitrogen fixation\n(Rhizobium, Azotobacter,\nlightning)"| B["NH4+\n(ammonium ions\nin soil)"]
    B -->|"Nitrification\n(Nitrosomonas)"| C["NO2-\n(nitrite ions)"]
    C -->|"Nitrification\n(Nitrobacter)"| D["NO3-\n(nitrate ions\nin soil)"]
    D -->|"Absorption by\nplant roots"| E["Nitrogen in\nplant proteins\nand nucleic acids"]
    E -->|"Feeding"| F["Nitrogen in\nconsumer proteins"]
    E -->|"Death /\nexcretion"| G["Dead organic\nmatter / urea"]
    F -->|"Death /\nexcretion"| G
    G -->|"Ammonification\n(decomposers)"| B
    D -->|"Denitrification\n(Pseudomonas)\n(anaerobic conditions)"| A

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Key Processes in the Nitrogen Cycle

1. Nitrogen Fixation