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Nitrogen Stores: Organic and Inorganic

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2.3.17 The nitrogen cycle contains organic and inorganic stores.

Organic nitrogen stores in ecosystems consist of proteins and other nitrogenous carbon compounds in living organisms and in dead organic matter. Inorganic stores consist of nitrogen in the atmosphere, as well as ammonia and other nitrogen compounds (nitrites and nitrates) in soil and water.

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Nitrogen makes up approximately 78% of the atmosphere, yet it is often a limiting factor for plant growth. This is because atmospheric nitrogen exists as dinitrogen (N₂), a highly stable molecule with a triple bond that most organisms cannot use directly.

Nitrogen is stored in ecosystems in two main forms:

Organic nitrogen stores are found in carbon-containing molecules associated with living or once-living organisms:

• Proteins and amino acids in living organisms (e.g. the protein in a soybean crop or in the muscle tissue of a grazing cow)
• Nitrogenous carbon compounds in dead organic matter (e.g. decomposing leaf litter on the floor of a Scottish pine forest, or salmon carcasses in Pacific Northwest rivers after spawning)

Inorganic nitrogen stores are not associated with living organisms:

• Atmospheric dinitrogen (N₂) – the largest store, approximately 3.9 × 10⁹ Tg
• Ammonia (NH₃) and ammonium (NH₄⁺) in soil and water
• Nitrites (NO₂⁻) in soil and water
• Nitrates (NO₃⁻) in soil and water (e.g. dissolved nitrates in the water of Lake Erie or the River Thames)

Chemical formulae key:

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1. Explain why atmospheric nitrogen (N₂) is unavailable to most organisms, even though it makes up 78% of the atmosphere.

2. Sort the following into organic or inorganic nitrogen stores: protein in a leaf, atmospheric N₂, nitrate dissolved in soil water, amino acids in a dead rabbit, ammonium ions in a river.

3. Explain why nitrogen can be a limiting factor for plant growth in boreal forests despite the vast atmospheric store.

The Role of Bacteria in the Nitrogen Cycle

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2.3.18 Bacteria have essential roles in the nitrogen cycle.

Nitrogen fixation is conversion of nitrogen from the atmosphere into ammonia. Nitrification is conversion of ammonia to nitrates. Denitrification is conversion of nitrates to nitrogen. Decomposition is the conversion of amino acids into ammonium. Conversion of ammonium and nitrites to nitrates takes place. Chemical details of reactions are not required.

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Bacteria play essential roles in converting nitrogen between its different forms. There are four key processes to understand. Chemical details of the reactions are not required.

1. Nitrogen fixation: N₂ → NH₃

The conversion of atmospheric dinitrogen into ammonia. This is the only natural process that makes atmospheric nitrogen available to living organisms.

• Carried out by nitrogen-fixing bacteria such as Rhizobium (in root nodules of legumes), free-living Azotobacter in soil, and cyanobacteria such as Anabaena in rice paddies and freshwater ecosystems
• Lightning also contributes a small proportion (roughly 5–8% of natural fixation) by providing the energy to split N₂ molecules

2. Nitrification: NH₃/NH₄⁺ → NO₂⁻ → NO₃⁻

The conversion of ammonia to nitrites and then to nitrates. This is important because nitrate (NO₃⁻) is the form of nitrogen most easily absorbed by plant roots.

• Carried out by nitrifying bacteria: Nitrosomonas converts ammonia to nitrites; Nitrobacter converts nitrites to nitrates
• Requires aerobic (oxygen-rich) conditions
• Real-world example: nitrifying bacteria are used in sewage treatment works, such as the Beckton Sewage Treatment Works in London (one of Europe's largest), to convert toxic ammonia in wastewater into less harmful nitrates before discharge

3. Denitrification: NO₃⁻ → N₂

The conversion of nitrates back into atmospheric nitrogen gas completes the cycle.

• Carried out by denitrifying bacteria
• Only occurs in anaerobic conditions (see section below)

4. Decomposition and ammonification: organic N → NH₄⁺

The conversion of amino acids and other organic nitrogen compounds in dead organisms and waste products into ammonium.

• Carried out by decomposer bacteria and fungi
• Real-world example: after spawning, millions of Pacific salmon die in rivers of the Pacific Northwest (USA and Canada). Decomposer organisms break down the salmon carcasses, releasing ammonium into the soil and water of riverside ecosystems. This marine-derived nitrogen significantly enriches the surrounding forest ecosystem.

Summary of bacterial processes:

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1. State the four key bacterial processes in the nitrogen cycle.
2. In which form do most plants absorb nitrogen from the soil?
3. Explain why nitrification is important for plant growth.
4. Distinguish between nitrogen fixation and nitrification.
5. Explain why the decomposition of salmon carcasses in Pacific Northwest rivers is an example of ammonification.