The Nitrogen Cycle

The idea

The air is 78% nitrogen, and almost nothing can use it. $\mathrm{N}{}_2$ is held together by a triple bond that most living things cannot break.

graph TD
    N2["N2 in the atmosphere"] -->|"nitrogen-fixing bacteria"| NH3["Ammonia / ammonium"]
    N2 -->|lightning| NO3["Nitrates"]
    NH3 -->|nitrifying bacteria| NO3
    NO3 -->|absorbed by roots| PLANTS["Plant proteins"]
    PLANTS -->|eaten| ANIMALS["Animal proteins"]
    ANIMALS -->|"waste and death"| DECOMP["Decomposers"]
    DECOMP --> NH3
    NO3 -->|denitrifying bacteria| N2

Why farmers care

Nitrogen is usually the limiting nutrient: the one that runs out first and caps how much can grow. Three ways to add it:

1. Plant legumes — clover, beans, alfalfa — which host nitrogen-fixing bacteria.
2. Spread manure or compost, returning nitrogen the decomposers released.
3. Apply synthetic fertiliser, made by forcing $\mathrm{N}{}_2$ and hydrogen together at high temperature and pressure.

Too much of a good thing

Fertiliser that washes into a lake feeds algae instead of crops. The algae bloom, die, and decompose — and decomposition consumes the dissolved oxygen that fish need. This is eutrophication, and it is why buffer strips beside waterways are worth the lost field space.

Curriculum connection

B2.2

explain how naturally occurring phenomena, including the cycling of matter and the flow of energy, contribute to the dynamic equilibrium within and between ecosystems

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B2.7

explain how sustainable practices related to the cycling of matter and the flow of energy can be applied in agricultural innovations

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