📚 The Nitrogen Cycle | 氮循环考点精讲
Nitrogen is essential for all living organisms because it forms a key component of proteins and DNA. Despite being abundant in the atmosphere as N₂ gas (about 78%), most organisms cannot use it directly. The nitrogen cycle describes how nitrogen is converted between different chemical forms so it can move between the atmosphere, soil, plants and animals. For GCSE AQA Biology, you need to know the main processes, microorganisms involved and the roles of different nitrogen compounds.
氮是所有生物体必不可少的元素，因为它是蛋白质和DNA的关键成分。尽管大气中氮气（N₂）含量丰富（约占78%），但大多数生物无法直接利用。氮循环描述了氮在不同化学形式之间转化，从而在大气、土壤、植物和动物之间移动的过程。在GCSE AQA生物考试中，你需要掌握主要过程、涉及的微生物以及不同氮化合物的作用。
1. The Importance of Nitrogen | 氮的重要性
Nitrogen atoms are needed to build amino acids, which link together to form proteins. Proteins are vital for growth, repair and enzymes. Nitrogen is also needed to make DNA and other nucleic acids. In plants, nitrogen is a component of chlorophyll, the pigment required for photosynthesis.
氮原子用于构建氨基酸，氨基酸连接在一起形成蛋白质。蛋白质对生长、修复和酶活动至关重要。氮还用于制造DNA和其他核酸。在植物中，氮是叶绿素的组成成分，叶绿素是进行光合作用所需的色素。
Plants absorb nitrogen in the form of nitrate ions (NO₃⁻) from the soil. Animals obtain nitrogen by eating plants or other animals. When organisms die, the nitrogen in their tissues is recycled back into the environment. The nitrogen cycle ensures that this limited supply of usable nitrogen is constantly reused.
植物以硝酸根离子（NO₃⁻）的形式从土壤中吸收氮。动物通过吃植物或其他动物获取氮。当生物死亡时，组织中的氮被循环回环境。氮循环确保有限的可用氮供应不断被重复利用。
2. Overview of the Nitrogen Cycle | 氮循环总览
The nitrogen cycle consists of several key transformations: nitrogen fixation, nitrification, assimilation, ammonification and denitrification. These processes are driven mainly by bacteria and other decomposers. A simplified diagram usually shows nitrogen gas in the air being fixed into ammonia or nitrates in the soil, taken up by plants, passed along food chains and eventually returned to the soil as ammonium compounds when organisms excrete waste or die. Bacteria then convert these back into nitrates or return nitrogen gas to the atmosphere.
氮循环包括几个关键转化过程：固氮作用、硝化作用、同化作用、氨化作用以及反硝化作用。这些过程主要由细菌和其他分解者驱动。简化示意图通常显示空气中的氮气被固定为土壤中的氨或硝酸盐，被植物吸收，沿食物链传递，最终当生物排泄废物或死亡时以铵化合物的形式返回土壤。然后细菌将这些铵化合物重新转化为硝酸盐，或将氮气返回大气。
It is helpful to remember the sequence: Nitrogen gas → ammonia/ammonium → nitrite → nitrate → plant protein → animal protein → waste/dead matter → ammonia → (back to nitrite/nitrate) or back to nitrogen gas. GCSE exams often ask you to label a diagram or describe the role of named bacteria.
记住顺序很有帮助：氮气 → 氨/铵 → 亚硝酸盐 → 硝酸盐 → 植物蛋白 → 动物蛋白 → 废物/死亡生物质 → 氨 → （回到亚硝酸盐/硝酸盐）或回到氮气。GCSE考试经常要求你给示意图贴标签或描述指定细菌的作用。
3. Nitrogen Fixation – Making Nitrogen Usable | 固氮作用——让氮变得可用
Nitrogen fixation converts unreactive nitrogen gas (N₂) from the air into ammonia (NH₃), which dissolves in soil water to form ammonium ions (NH₄⁺). This can then be used by plants after further conversion. Fixation occurs in three main ways: lightning, the Haber process (industrial) and biological fixation by certain bacteria.
固氮作用将空气中不活泼的氮气（N₂）转化为氨（NH₃），氨溶于土壤水中形成铵离子（NH₄⁺）。铵离子经过进一步转化后即可被植物利用。固氮主要通过三种方式发生：闪电、哈勃法（工业合成氨）以及特定细菌的生物固氮。
Lightning provides enough energy to make nitrogen and oxygen react to form nitrogen oxides, which dissolve in rain and enter the soil as nitrates. However, this contributes only a small fraction. The Haber process produces ammonia for fertilisers on an industrial scale. In nature, biological fixation is far more important.
闪电提供足够能量使氮气与氧气反应生成氮氧化物，氮氧化物溶于雨水中，以硝酸盐形式进入土壤。但这仅贡献一小部分。哈勃法在工业规模上生产用于肥料的氨。在大自然中，生物固氮远为重要。
Biological nitrogen fixation is carried out by nitrogen-fixing bacteria. Some, such as Rhizobium, live inside root nodules of leguminous plants (e.g. peas, beans, clover). They have a mutualistic relationship: the bacteria receive carbohydrates from the plant and provide the plant with ammonium. Other free-living nitrogen-fixing bacteria, like Azotobacter, live freely in the soil.
生物固氮由固氮细菌完成。有些固氮菌，如根瘤菌（Rhizobium），生活在豆科植物（如豌豆、豆类、三叶草）的根瘤内。它们与植物形成互利关系：细菌从植物获取碳水化合物，并为植物提供铵。其他自由生活的固氮菌，如固氮菌属（Azotobacter），在土壤中独立生活。
N₂ + 8H⁺ + 8e⁻ → 2NH₃ + H₂ (simplified biological fixation)
N₂ + 8H⁺ + 8e⁻ → 2NH₃ + H₂ (简化的生物固氮反应)
4. Nitrification – Ammonia to Nitrates | 硝化作用——氨转变为硝酸盐
Nitrification is the oxidation of ammonia/ammonium ions to nitrites and then to nitrates. This is carried out by nitrifying bacteria in the soil. It is a two-step process requiring oxygen (aerobic conditions).
硝化作用是将氨/铵离子氧化为亚硝酸盐，再氧化为硝酸盐的过程。由土壤中的硝化细菌完成。这是一个需要在有氧条件下进行的两步过程。
First, ammonium ions (NH₄⁺) are converted to nitrite ions (NO₂⁻) by bacteria such as Nitrosomonas. Then, nitrite ions are converted to nitrate ions (NO₃⁻) by bacteria such as Nitrobacter. Nitrates are the form that plants can readily absorb through their roots by active transport. Nitrifying bacteria are crucial because ammonia itself is toxic to many plants at high concentrations, so it must be converted quickly.
首先，铵离子（NH₄⁺）被诸如亚硝化单胞菌（Nitrosomonas）之类的细菌转化为亚硝酸根离子（NO₂⁻）。然后，亚硝酸根离子被诸如硝化杆菌（Nitrobacter）之类的细菌转化为硝酸根离子（NO₃⁻）。硝酸盐是植物可以通过根部主动吸收的形式。硝化细菌至关重要，因为高浓度的氨本身对许多植物有毒，因此必须迅速转化。
NH₄⁺ → NO₂⁻ (by Nitrosomonas) → NO₃⁻ (by Nitrobacter)
NH₄⁺ → NO₂⁻ (由亚硝化单胞菌完成) → NO₃⁻ (由硝化杆菌完成)
Well-aerated, well-drained soils favour nitrification. Waterlogged, compacted soils have less oxygen, slowing down this process and leading to buildup of ammonium, which reduces soil fertility.
通气良好、排水良好的土壤有利于硝化作用。积水、板结的土壤含氧量较低，会减慢这一过程，导致铵积累，降低土壤肥力。
5. Assimilation – Incorporating Nitrogen into Biomass | 同化作用——将氮纳入生物量
Assimilation is the process by which plants and animals take up nitrogen compounds and use them to build their own organic molecules. Plants absorb nitrate ions from the soil through their roots, then combine them with carbohydrates produced in photosynthesis to form amino acids. These amino acids are then used to synthesise proteins needed for growth and enzymes. Animals then consume these plants (or other animals) and digest the proteins to obtain the amino acids they need.
同化作用是植物和动物吸收含氮化合物，并利用它们构建自身有机分子的过程。植物通过根从土壤中吸收硝酸根离子，然后将其与光合作用产生的碳水化合物结合，形成氨基酸。这些氨基酸进而用于合成生长和酶所需的蛋白质。动物随后吃掉这些植物（或其他动物），消化蛋白质以获得所需的氨基酸。
Essentially, assimilation moves nitrogen from the abiotic environment (soil nitrate) into the biotic chains (producers, consumers). Decomposers also assimilate nitrogen when they break down organic material to grow and reproduce.
本质上，同化作用将氮从非生物环境（土壤硝酸盐）转移到生物链中（生产者、消费者）。分解者在分解有机物质以生长和繁殖时，也在进行同化作用。
6. Ammonification – Recycling Nitrogen from Waste | 氨化作用——从废弃物中回收氮
Ammonification is the process by which decomposers (bacteria and fungi) break down proteins from dead organisms, faeces and urea into ammonia (NH₃) or ammonium ions (NH₄⁺). This is a crucial stage that returns nitrogen from organic compounds back into the soil in an inorganic form.
氨化作用是分解者（细菌和真菌）将来自死亡生物体、粪便和尿素的蛋白质分解为氨（NH₃）或铵离子（NH₄⁺）的过程。这是一个关键阶段，它将有机化合物中的氮以无机形式重新归还到土壤中。
When organisms die, their bodies contain proteins and nucleic acids. Saprobiotic decomposers secrete enzymes that break these molecules down extracellularly, releasing ammonium ions into the soil. Urea from animal urine is also broken down by urease enzymes into ammonia. The ammonium ions can then either be directly absorbed by some plants, be held on soil clay particles, or proceed to nitrification.
当生物死亡时，它们的身体含有蛋白质和核酸。腐生分解者分泌酶，将这些分子在细胞外分解，释放铵离子到土壤中。动物尿液中的尿素也被脲酶分解成氨。铵离子随后可直接被一些植物吸收，或被吸附在土壤粘土颗粒上，或进入硝化作用。
Proteins in dead matter → amino acids → ammonia/ammonium ions (by decomposers)
死亡物质中的蛋白质 → 氨基酸 → 氨/铵离子 (由分解者完成)
7. Denitrification – Returning Nitrogen to the Air | 反硝化作用——将氮归还大气
Denitrification converts nitrates in the soil back into nitrogen gas (N₂), which is released into the atmosphere. This is carried out by denitrifying bacteria that thrive in anaerobic (low oxygen) conditions, such as waterlogged soils. Denitrification can reduce soil fertility because the useful nitrate ions plants need are lost to the air.
反硝化作用将土壤中的硝酸盐重新转化为氮气（N₂），释放到大气中。由反硝化细菌完成，这些细菌在厌氧（低氧）条件下茁壮生长，例如积水土壤中。反硝化作用会降低土壤肥力，因为植物所需的有用硝酸根离子散失到了空气中。
While denitrification seems wasteful from an agricultural perspective, it is essential for balancing the global nitrogen cycle. Without it, all atmospheric nitrogen would eventually become fixed into soil and oceans, and the atmosphere’s composition would change. In farming practice, farmers try to minimise denitrification by improving soil drainage and avoiding over-irrigation.
尽管从农业角度看来反硝化作用似乎是一种浪费，但它对于平衡全球氮循环至关重要。如果没有它，所有大气中的氮最终都会固定在土壤和海洋中，大气的成分将会改变。在农业实践中，农民通过改善土壤排水和避免过度灌溉来尽量减少反硝化作用。
NO₃⁻ → NO₂⁻ → NO → N₂O → N₂ (facilitated by denitrifying bacteria, no oxygen)
NO₃⁻ → NO₂⁻ → NO → N₂O → N₂ (由反硝化细菌促进，无氧条件)
8. Key Microorganisms Table | 关键微生物总结表
| Process | Bacteria involved | Conditions |
|---|---|---|
| Nitrogen fixation | Rhizobium (root nodules), Azotobacter (free-living) | Aerobic or microaerophilic |
| Nitrification | Nitrosomonas (step 1), Nitrobacter (step 2) | Aerobic |
| Ammonification | Decomposers (various bacteria and fungi) | Aerobic/anaerobic |
| Denitrification | Denitrifying bacteria (e.g. Pseudomonas) | Anaerobic |
过程与细菌对应关系：固氮 – 根瘤菌/固氮菌；硝化 – 亚硝化单胞菌/硝化杆菌；氨化 – 分解者；反硝化 – 反硝化细菌。有氧/无氧条件清晰呈现，考试常要求写出细菌名称和条件。
9. Soil Fertility and Farming Practices | 土壤肥力与农业实践
The nitrogen cycle directly affects soil fertility. Farmers need to maintain high levels of nitrate for crop growth. Continuous harvesting removes nitrogen from the land, so fertilisers (natural like manure, or synthetic like ammonium nitrate) must be added. Legumes (like clover) are often grown in crop rotation because their root nodules host nitrogen-fixing bacteria, which add ammonia to the soil, reducing the need for artificial fertilisers.
氮循环直接影响土壤肥力。农民需要维持高水平的硝酸盐以促进作物生长。持续收获会从土地中带走氮，因此必须添加肥料（天然的如粪肥，或合成的如硝酸铵）。豆类（如三叶草）常在轮作中种植，因为它们的根瘤寄生固氮细菌，可以增加土壤中的氨，减少对人造肥料的需求。
However, excess fertiliser can run off into water bodies, causing eutrophication – a rapid growth of algae that depletes oxygen and kills aquatic life. Also, waterlogged fields promote denitrification, so good drainage is important to preserve nitrates. AQA questions often test understanding of how farming choices affect the nitrogen cycle.
然而，过量的肥料会流入水体，导致富营养化——藻类迅速繁殖，耗尽氧气，杀死水生生物。此外，积水田地会促进反硝化作用，因此良好排水对于保持硝酸盐至关重要。AQA考题经常测试对农业选择如何影响氮循环的理解。
10. Exam Tips and Common Questions | 考试技巧与常见问题
For AQA GCSE Biology, you are expected to know the names of the specific bacteria and what they convert. Typical 4–6 mark questions ask you to ‘Describe the nitrogen cycle’ or ‘Explain how nitrogen from the air becomes available to plants’. You should structure your answer logically: start with nitrogen gas, mention fixation by lightning, Haber process and Rhizobium; then decomposition and ammonification; followed by nitrification to nitrates; then uptake by roots; and finally denitrification. Always include the roles of Nitrosomonas and Nitrobacter for nitrification marks.
对于AQA GCSE生物，你需要知道特定细菌的名称及其转化内容。典型的4-6分题目会要求你“描述氮循环”或“解释空气中的氮如何成为植物可用的”。你应该有逻辑地组织答案：从氮气开始，提到通过闪电、哈勃法和根瘤菌固氮；然后是分解与氨化作用；接着是硝化作用生成硝酸盐；然后是被根部吸收；最后是反硝化作用。务必在硝化作用部分提到亚硝化单胞菌和硝化杆菌以获取分数。
Graph-based questions may show concentrations of nitrate and ammonium over time in different soils. You might need to deduce whether waterlogged conditions reduced nitrification or increased denitrification. Also, be prepared to evaluate the benefits of crop rotation and use of natural fertilisers.
基于图表的题目可能显示不同土壤中硝酸盐和铵的浓度随时间变化。你可能需要推断积水条件是否减少了硝化作用或增加了反硝化作用。此外，要准备好评估轮作和使用天然肥料的好处。
11. Common Misconceptions | 常见误区
One common mistake is confusing nitrification with nitrogen fixation. Remember: fixation turns N₂ into ammonia/ammonium; nitrification turns ammonium into nitrites then nitrates. Another misconception is that denitrification happens everywhere – it specifically needs anaerobic conditions, which is why it occurs in waterlogged soils. Also, plants do not absorb nitrogen directly as gas; they absorb nitrates (and sometimes ammonium). Students sometimes forget that decomposers release ammonium (ammonification), not nitrates directly.
一个常见错误是混淆硝化作用和固氮作用。记住：固氮将N₂转化为氨/铵；硝化作用将铵转化为亚硝酸盐，然后转化为硝酸盐。另一个误区是认为反硝化作用到处发生——它特别需要厌氧条件，这就是为什么它发生在积水土壤中。此外，植物不直接以气体形式吸收氮；它们吸收硝酸盐（有时是铵）。学生有时会忘记分解者释放的是铵（氨化作用），而不是直接释放硝酸盐。
12. Putting It All Together | 综合回顾
The nitrogen cycle demonstrates the interdependence of organisms and the crucial role of microorganisms in recycling nutrients. By understanding each stage – fixation, nitrification, assimilation, ammonification and denitrification – you can see how nitrogen is constantly cycled between the atmosphere, soil, plants, animals and decomposers. Master this topic by drawing your own labeled diagram and testing yourself on the exact roles of each bacterial group. This will set you up perfectly for any GCSE AQA Biology nitrogen cycle question.
氮循环展示了生物之间的相互依存关系，以及微生物在回收养分中的关键作用。通过理解每个阶段——固氮、硝化、同化、氨化和反硝化——你可以看到氮是如何在大气、土壤、植物、动物和分解者之间不断循环的。通过自己绘制标注示意图，并测试自己对每种细菌群确切作用的理解，来掌握这个主题。这将为你在GCSE AQA生物考试中应对任何氮循环问题做好充分准备。
Published by TutorHao | GCSE Biology Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导