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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Nitrogen Cycle Stages
The nitrogen cycle is one of Earth's most important nutrient cycles, moving nitrogen between the atmosphere, soil, water, plants, animals and bacteria. Although nitrogen makes up about 78% of our atmosphere, most living things can't use it in its atmospheric form (N₂). The nitrogen cycle transforms nitrogen into forms that organisms can use and then returns it to the atmosphere.
Key Definitions:
Nitrogen is essential for life because it's a key component of:
Without the nitrogen cycle, life as we know it couldn't exist!
Despite being abundant in the atmosphere, nitrogen gas (N₂) has a triple bond between its atoms, making it extremely stable and unusable by most organisms. The nitrogen cycle solves this problem by converting N₂ into forms that plants and animals can use.
This is why the nitrogen cycle is so crucial - it makes an essential element available to living things.
The nitrogen cycle consists of four key stages that transform nitrogen between different forms. Each stage involves specific organisms and chemical processes.
Nitrogen fixation is the conversion of atmospheric nitrogen (N₂) into ammonia (NH₃) or related nitrogen compounds that organisms can use.
These organisms have an enzyme called nitrogenase that can break the triple bond in N₂.
Leguminous plants like peas and beans have a special relationship with Rhizobium bacteria. The bacteria live in nodules on the plant roots, where they fix nitrogen for the plant. In return, the plant provides carbohydrates to the bacteria. This symbiotic relationship is so effective that farmers often rotate crops with legumes to naturally replenish soil nitrogen!
Nitrification is a two-step process where ammonia is converted first to nitrite and then to nitrate by nitrifying bacteria in the soil.
Bacteria such as Nitrosomonas convert ammonia (NH₃) to nitrite (NO₂⁻):
NH₃ → NO₂⁻
This is an oxidation reaction that releases energy which the bacteria use.
Bacteria such as Nitrobacter convert nitrite (NO₂⁻) to nitrate (NO₃⁻):
NO₂⁻ → NO₃⁻
Nitrate is the form of nitrogen most easily used by plants. It dissolves easily in water and can be absorbed by plant roots.
Ammonification is the process where decomposers break down nitrogen-containing organic compounds from dead organisms and waste, converting them back to ammonia.
Several types of microorganisms are involved in ammonification:
These organisms are essential for recycling nitrogen in ecosystems.
Denitrification is the process where nitrates (NO₃⁻) are converted back to atmospheric nitrogen (N₂), completing the cycle.
Denitrifying bacteria convert nitrates through several steps:
NO₃⁻ → NO₂⁻ → NO → N₂O → N₂
This process occurs in anaerobic (oxygen-poor) conditions, such as waterlogged soils and sediments.
These bacteria use nitrate instead of oxygen for respiration in anaerobic conditions, releasing nitrogen gas as a by-product.
When excess nitrogen from fertilisers washes into lakes and rivers, it can cause eutrophication - a process where nutrient enrichment leads to excessive algal growth. As these algae die and decompose, oxygen in the water is used up, creating "dead zones" where fish and other aquatic life cannot survive. A famous example is the Baltic Sea, where agricultural runoff has created one of the world's largest dead zones. This shows how human disruption of the nitrogen cycle can have serious environmental consequences.
Human activities have dramatically altered the nitrogen cycle, more than doubling the amount of fixed nitrogen entering the biosphere.
These activities add large amounts of reactive nitrogen to soils and water.
The nitrogen cycle is vital for maintaining healthy ecosystems and supporting all forms of life. Understanding this cycle helps us appreciate the delicate balance of nature and the potential consequences of human interference.
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