📚 Nitrogen Cycle: Key Concepts Explained for GCSE Edexcel | GCSE Edexcel 生物：氮循环 考点精讲
The nitrogen cycle is a fundamental biogeochemical cycle that describes how nitrogen moves between the atmosphere, soil, plants, animals, and microorganisms. Nitrogen is an essential element for life, forming proteins, DNA, and other vital molecules. In GCSE Edexcel Biology, understanding the key processes—fixation, nitrification, assimilation, ammonification, and denitrification—and the specific roles of bacteria is crucial for success. This revision guide breaks down every step, highlights common exam questions, and provides clear explanations to help you master this topic.
氮循环是基本的生物地球化学循环，描述了氮在大气、土壤、植物、动物和微生物之间的流动。氮是生命必需的元素，构成蛋白质、DNA 等重要分子。在 GCSE Edexcel 生物学中，理解固氮、硝化、同化、氨化和反硝化等关键过程以及细菌的具体作用是成功的关键。本复习指南将分解每个步骤，突出常见考题，并提供清晰的解释，帮助你掌握这一主题。
1. Introduction to the Nitrogen Cycle | 氮循环简介
The nitrogen cycle is the series of processes by which nitrogen and its compounds are interconverted in the environment and in living organisms. It is a closed system on a global scale, with the atmosphere containing about 78% nitrogen gas (N₂), which is unavailable to most living things.
氮循环是氮及其化合物在环境和生物体中相互转化的一系列过程。在全球范围内它是一个封闭系统，大气中含有约 78% 的氮气（N₂），但大多数生物无法直接利用这种形式的氮。
Living organisms require nitrogen in a reactive form, such as ammonium ions (NH₄⁺) or nitrate ions (NO₃⁻), to synthesise proteins, nucleic acids, and other nitrogen-containing compounds.
生物体需要活性形式的氮，如铵离子（NH₄⁺）或硝酸根离子（NO₃⁻），以合成蛋白质、核酸和其他含氮化合物。
The cycle involves several microbial-driven transformations that make nitrogen available to plants and then to animals through food chains.
该循环涉及几个由微生物驱动的转化过程，使氮可供植物利用，然后通过食物链传递给动物。
2. Why is Nitrogen Important? | 氮的重要性
Nitrogen is a key component of amino acids, the building blocks of proteins, and of nucleotides, the monomers of DNA and RNA. Without a steady supply of nitrogen, organisms cannot grow or reproduce.
氮是氨基酸（蛋白质的构建模块）和核苷酸（DNA 和 RNA 的单体）的关键组分。没有稳定的氮供应，生物体就无法生长或繁殖。
Plants obtain nitrogen from the soil in the form of nitrate or ammonium ions, and animals obtain nitrogen by consuming plants or other animals.
植物以硝酸根离子或铵离子的形式从土壤中获取氮，动物通过取食植物或其他动物获得氮。
Deficiency of nitrogen in the soil leads to stunted growth, yellowing of leaves, and reduced crop yields, which is why nitrogen-rich fertilisers are used in agriculture.
土壤缺氮会导致植物生长迟缓、叶片发黄、作物减产，这就是农业中使用富氮肥料的原因。
3. Overview of the Nitrogen Cycle Processes | 氮循环过程概述
The nitrogen cycle consists of five main stages: nitrogen fixation (converting N₂ into ammonia/ammonium), nitrification (converting ammonium to nitrite then nitrate), assimilation (plants and animals incorporate nitrogen into organic molecules), ammonification (decomposers convert organic nitrogen back to ammonium), and denitrification (converting nitrate back to N₂ gas).
氮循环由五个主要阶段组成：固氮作用（将 N₂ 转化为氨/铵）、硝化作用（将铵转化为亚硝酸盐再转化为硝酸盐）、同化作用（动植物将氮结合到有机分子中）、氨化作用（分解者将有机氮转化回铵）和反硝化作用（将硝酸盐转化回 N₂ 气体）。
Each stage is carried out by specific groups of microorganisms, primarily bacteria and fungi, living in the soil.
每个阶段都由特定的微生物群来完成，主要是生活在土壤中的细菌和真菌。
4. Nitrogen Fixation | 固氮作用
Nitrogen fixation is the process of converting inert atmospheric nitrogen gas (N₂) into ammonia (NH₃) or ammonium ions (NH₄⁺), which can then be used by plants. This process requires a great deal of energy because the triple bond in N₂ is very strong.
固氮作用是将惰性的大气氮气（N₂）转化为氨（NH₃）或铵离子（NH₄⁺）的过程，然后植物才能利用这些形式的氮。由于 N₂ 中的三键非常强，因此该过程需要大量能量。
There are three main ways nitrogen fixation occurs: (1) biological fixation by free-living soil bacteria such as Azotobacter or symbiotic bacteria like Rhizobium in root nodules of legumes (peas, beans, clover); (2) industrial fixation via the Haber process to manufacture fertilisers; (3) atmospheric fixation caused by lightning providing the energy to break N₂ bonds, allowing nitrogen to react with oxygen to form soluble nitrates that are washed into the soil by rain.
固氮作用主要有三种方式：(1) 生物固氮，由自由生活的土壤细菌（如固氮菌）或共生细菌（如豆科植物根瘤中的根瘤菌）完成；(2) 工业固氮，通过哈伯法生产化肥；(3) 大气固氮，由闪电提供能量打断 N₂ 键，使氮与氧反应生成可溶性硝酸盐，随雨水冲入土壤。
In GCSE exams, you are expected to name specific bacteria: Rhizobium in root nodules of legumes and Azotobacter as free-living nitrogen-fixing bacteria in soil.
在 GCSE 考试中，需要能说出具体的细菌：豆科植物根瘤中的根瘤菌和土壤中自由生活的固氮菌（如固氮菌属）。
The enzyme nitrogenase is used by these bacteria to catalyse the reduction of N₂ to ammonia.
这些细菌利用固氮酶催化 N₂ 还原为氨。
5. Nitrification | 硝化作用
Nitrification is a two-step oxidation process carried out by nitrifying bacteria in the soil. First, ammonium ions (NH₄⁺) are oxidised to nitrite ions (NO₂⁻) by bacteria such as Nitrosomonas. Then, nitrite ions are oxidised to nitrate ions (NO₃⁻) by bacteria such as Nitrobacter.
硝化作用是由土壤中的硝化细菌进行的两步氧化过程。首先，铵离子（NH₄⁺）被亚硝化单胞菌（Nitrosomonas）等细菌氧化为亚硝酸根离子（NO₂⁻）。然后，亚硝酸根离子被硝化杆菌（Nitrobacter）等细菌氧化为硝酸根离子（NO₃⁻）。
Both steps require oxygen, so nitrification occurs in well-aerated soils. This process increases the availability of nitrogen in a form (nitrate) that is readily absorbed by plant roots.
两个步骤都需要氧气，因此硝化作用发生在通气良好的土壤中。该过程增加了可供植物根系直接吸收的氮形式（硝酸盐）的可用性。
Nitrifying bacteria are chemolithotrophs, meaning they obtain energy from these oxidation reactions.
硝化细菌是化能无机营养菌，意味着它们从这些氧化反应中获取能量。
6. Assimilation and Food Chains | 同化作用与食物链
Assimilation is the process by which plants absorb nitrate ions (and to a lesser extent ammonium) from the soil through their roots and use them to synthesise plant proteins, nucleic acids, and chlorophyll.
同化作用是植物通过根部从土壤中吸收硝酸根离子（以及少量铵离子），并利用它们合成植物蛋白质、核酸和叶绿素的过程。
When animals consume plants, they digest plant proteins and use the amino acids to build their own proteins. This transfers nitrogen up the food chain.
动物取食植物时，消化植物蛋白质，利用氨基酸构建自身蛋白质，从而使氮沿着食物链传递。
Nitrogen is returned to the soil through animal waste (urine, faeces) and when organisms die, starting the process of decomposition.
氮通过动物排泄物（尿液、粪便）以及生物死亡后返回土壤，从而启动分解过程。
7. Ammonification (Decomposition) | 氨化作用（分解）
Ammonification is the conversion of organic nitrogen compounds from dead organisms and animal waste into ammonium ions (NH₄⁺) by decomposers, mainly saprobiotic bacteria and fungi.
氨化作用是指分解者（主要是腐生细菌和真菌）将死生物体和动物排泄物中的有机氮化合物转化为铵离子（NH₄⁺）的过程。
Enzymes released by decomposers break down proteins, DNA, and urea into amino acids, which are then deaminated to release ammonia (NH₃), which dissolves in soil water to form ammonium ions.
分解者释放的酶将蛋白质、DNA 和尿素分解为氨基酸，然后脱氨基释放出氨（NH₃），氨溶于土壤水分形成铵离子。
This process is essential because it recycles nitrogen within the ecosystem, making it available for nitrification and plant uptake once more.
这一过程至关重要，因为它使氮在生态系统中循环，再次可用于硝化作用和植物吸收。
8. Denitrification | 反硝化作用
Denitrification is the anaerobic process by which denitrifying bacteria convert nitrate ions (NO₃⁻) in the soil back into nitrogen gas (N₂), which returns to the atmosphere.
反硝化作用是反硝化细菌在厌氧条件下将土壤中的硝酸根离子（NO₃⁻）转化回氮气（N₂）的过程，氮气返回大气。
This occurs in waterlogged or compacted soils where oxygen is limited. Common denitrifying bacteria include Pseudomonas and Thiobacillus.
这发生在缺氧的积水或紧实土壤中。常见的反硝化细菌包括假单胞菌（Pseudomonas）和硫杆菌（Thiobacillus）等。
While denitrification completes the cycle by balancing nitrogen fixation, excessive denitrification can lead to loss of soil fertility, which is a concern in agriculture.
反硝化作用通过平衡固氮作用完成循环，但过度的反硝化会导致土壤肥力流失，这在农业中是一个问题。
9. Bacteria in the Nitrogen Cycle – Summary Table | 氮循环中的细菌总结表
| Bacteria / Microorganism | Process | Substrate → Product | Conditions |
|---|---|---|---|
| Rhizobium (symbiotic in legume root nodules) | Nitrogen fixation | N₂ → NH₄⁺ | 更多咨询请联系16621398022（同微信）
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