📚 IB & AQA Biology: The Nitrogen Cycle – Key Points | IB 与 AQA 生物：氮循环考点精讲
Whether you are studying IB Biology or AQA A-level Biology, mastering the nitrogen cycle is essential. This article breaks down the key processes, bacteria, equations, and exam tips to help you get top marks.
无论你学习 IB 生物还是 AQA A-level 生物，掌握氮循环都是必不可少。本文分解关键过程、细菌、方程式和考试技巧，助你获得高分。
1. Why Nitrogen Matters | 为什么氮元素至关重要
Nitrogen is a fundamental component of amino acids, proteins, nucleic acids (DNA and RNA), ATP, and chlorophyll.
氮是氨基酸、蛋白质、核酸（DNA 与 RNA）、ATP 和叶绿素的基本组成元素。
Although the atmosphere contains about 78% nitrogen gas (N₂), most organisms cannot use it directly because the triple bond in N₂ is extremely stable.
尽管大气中约含 78% 的氮气（N₂），但由于 N₂ 的三键极其稳定，绝大多数生物无法直接利用。
The nitrogen cycle transforms inert atmospheric nitrogen into biologically available forms such as ammonium (NH₄⁺) and nitrate (NO₃⁻).
氮循环将惰性的大气氮转化为生物可利用的形式，例如铵离子（NH₄⁺）和硝酸根离子（NO₃⁻）。
2. Overview of the Nitrogen Cycle | 氮循环概览
The nitrogen cycle consists of five main processes: nitrogen fixation, ammonification, nitrification, assimilation, and denitrification.
氮循环包括五个主要过程：固氮作用、氨化作用、硝化作用、同化作用和反硝化作用。
Microorganisms play a central role in all conversions except assimilation by plants and animals.
除植物和动物的同化作用外，微生物在所有的转化过程中都扮演着核心角色。
The cycle links the atmosphere, soil, water bodies, and living organisms, maintaining nitrogen balance in ecosystems.
该循环将大气、土壤、水体和生物体联系起来，维持生态系统中的氮平衡。
3. Nitrogen Fixation: Converting N₂ to Ammonia | 固氮作用：将氮气转化为氨
Nitrogen fixation is the reduction of atmospheric N₂ to ammonia (NH₃), which is then protonated to ammonium (NH₄⁺) in the soil.
固氮作用是将大气氮 N₂ 还原为氨（NH₃），氨在土壤中进一步质子化形成铵离子（NH₄⁺）。
Biological fixation is carried out by nitrogen‑fixing bacteria using the enzyme nitrogenase. The reaction requires 16 ATP per N₂ molecule.
生物固氮由固氮菌利用固氮酶完成。每固定一个 N₂ 分子需消耗 16 个 ATP。
Free‑living bacteria such as Azotobacter are found in soil, while symbiotic bacteria such as Rhizobium live in root nodules of legumes.
自由生活的细菌如固氮菌属存在于土壤中，而共生细菌如根瘤菌则生活在豆科植物的根瘤内。
Industrial fixation via the Haber process and lightning‑driven fixation also contribute but to a lesser extent in natural ecosystems.
通过哈伯法的工业固氮和闪电固氮也有贡献，但在自然生态系统中所占比例较小。
4. Symbiotic Nitrogen Fixation: Rhizobium and Legumes | 共生固氮：根瘤菌与豆科植物
Rhizobium bacteria infect legume roots, causing the formation of root nodules that provide an anaerobic environment for nitrogenase.
根瘤菌侵染豆科植物根系，诱导形成根瘤，为固氮酶提供厌氧环境。
The plant supplies carbohydrates to the bacteria, and in return receives fixed nitrogen as ammonium ions.
植物为细菌提供碳水化合物，作为回报获得固定态的铵离子。
Leghaemoglobin, a pink pigment in nodules, binds oxygen tightly, maintaining low O₂ concentration while still supporting bacterial respiration.
根瘤中的豆血红蛋白是一种粉红色色素，能紧密结合氧气，在维持低氧浓度的同时保障细菌呼吸。
This mutualistic relationship improves soil fertility and reduces the need for artificial fertilisers.
这种互惠关系提高了土壤肥力，减少了对人工化肥的需求。
5. Ammonification: Organic Nitrogen to Ammonium | 氨化作用：有机氮转化为铵离子
When organisms die or excrete waste, saprobiontic bacteria and fungi decompose organic nitrogen compounds (proteins, nucleic acids, urea).
当生物死亡或排泄废物时，腐生细菌和真菌分解有机含氮化合物（蛋白质、核酸、尿素）。
The decomposition releases ammonium ions (NH₄⁺) into the soil. This process is also called mineralisation.
分解过程向土壤释放铵离子（NH₄⁺），该过程也称为矿化作用。
Ammonification returns nitrogen from dead biomass to the soil, making it available for nitrifying bacteria.
氨化作用将死亡的生物质中的氮归还土壤，使其可为硝化细菌所利用。
It occurs under both aerobic and anaerobic conditions, though the rate may vary.
该过程在好氧和厌氧条件下均可发生，但速率会有所不同。
6. Nitrification: Oxidation of Ammonium to Nitrate | 硝化作用：铵氧化为硝酸盐
Nitrification is a two‑step aerobic process performed by chemolithoautotrophic bacteria. It oxidises ammonium to nitrite, then to nitrate.
硝化作用是一个两步好氧过程，由化能自养型细菌完成。它将铵氧化为亚硝酸盐，再氧化为硝酸盐。
The first step is carried out by Nitrosomonas: 2NH₄⁺ + 3O₂ → 2NO₂⁻ + 4H⁺ + 2H₂O.
第一步由亚硝化单胞菌进行：2NH₄⁺ + 3O₂ → 2NO₂⁻ + 4H⁺ + 2H₂O。
The second step is carried out by Nitrobacter: 2NO₂⁻ + O₂ → 2NO₃⁻.
第二步由硝化杆菌进行：2NO₂⁻ + O₂ → 2NO₃⁻。
These bacteria obtain energy from the oxidation of inorganic nitrogen and fix CO₂ for their carbon source.
这些细菌从无机氮的氧化中获得能量，并固定 CO₂ 作为碳源。
Nitrification requires well‑aerated soils; waterlogged soils slow the process and favour denitrification.
硝化作用需要通气良好的土壤；积水土壤会减慢该过程，有利于反硝化。
7. Assimilation: Uptake of Nitrogen by Plants | 同化作用：植物对氮的吸收
Plants absorb nitrogen predominantly in the form of nitrate (NO₃⁻) through their roots, though some ammonium can also be taken up.
植物主要通过根系吸收硝酸盐（NO₃⁻），也可吸收部分铵离子。
Inside plant cells, nitrate is reduced to ammonium and then incorporated into amino acids, proteins, and nucleic acids.
在植物细胞内，硝酸盐被还原为铵，进而参与合成氨基酸、蛋白质和核酸。
Animals obtain their nitrogen by consuming plants or other animals, linking assimilation to feeding relationships.
动物通过取食植物或其他动物获得氮，将同化作用与食物链联系起来。
The assimilation step completes the transfer of nitrogen from inorganic forms in the soil to organic molecules in living tissues.
同化步骤完成了氮从土壤无机物到生物组织中有机分子的转移。
8. Denitrification: Returning N₂ to the Atmosphere | 反硝化作用：氮气回归大气
Denitrification is the reduction of nitrate (NO₃⁻) back to nitrogen gas ( N₂), carried out by denitrifying bacteria under anaerobic conditions.
反硝化作用是硝酸盐（NO₃⁻）被还原为氮气（N₂）的过程，由反硝化细菌在厌氧条件下完成。
Common denitrifiers include Pseudomonas and Thiobacillus. They use nitrate as an alternative electron acceptor in respiration.
常见的反硝化细菌包括假单胞菌和硫杆菌。它们利用硝酸盐作为呼吸作用的替代电子受体。
The overall equation is: 2NO₃⁻ + 10e⁻ + 12H⁺ → N₂ + 6H₂O.
总反应式为：2NO₃⁻ + 10e⁻ + 12H⁺ → N₂ + 6H₂O。
Denitrification leads to loss of available nitrogen from soil, which can reduce agricultural productivity if not managed.
反硝化导致土壤中可利用氮的损失，若不加以管理，会降低农业生产力。
Waterlogged fields and compacted soils favour denitrification, making drainage an important farm practice.
积水的田块和紧实的土壤有利于反硝化，因此排水是一项重要的农业措施。
9. Key Equations and Energy Requirements | 关键方程式与能量需求
The nitrogen cycle involves several redox reactions that can be summarised with balanced chemical equations. Below are the core equations students must memorise.
氮循环涉及若干氧化还原反应，可用配平的化学方程式概括。以下是学生必须记忆的核心方程式。
Biological nitrogen fixation: N₂ + 8H⁺ + 8e⁻ + 16ATP → 2NH₃ + H₂ + 16ADP + 16Pi
生物固氮：N₂ + 8H⁺ + 8e⁻ + 16ATP → 2NH₃ + H₂ + 16ADP + 16Pi
Nitrification step 1: 2NH₄⁺ + 3O₂ → 2NO₂⁻ + 4H⁺ + 2H₂O (Nitrosomonas)
硝化第一步：2NH₄⁺ + 3O₂ → 2NO₂⁻ + 4H⁺ + 2H₂O （亚硝化单胞菌）
Nitrification step 2: 2NO₂⁻ + O₂ → 2NO₃⁻ (Nitrobacter)
硝化第二步：2NO₂⁻ + O₂ → 2NO₃⁻ （硝化杆菌）
Denitrification: 2NO₃⁻ + 10e⁻ + 12H⁺ → N₂ + 6H₂O (anaerobic)
反硝化：2NO₃⁻ + 10e⁻ + 12H⁺ → N₂ + 6H₂O （厌氧）
Note that ATP is only required for nitrogen fixation; nitrification releases energy for the bacteria.
注意只有固氮过程需要 ATP；硝化过程为细菌释放能量。
10. Human Impact and Eutrophication | 人类活动影响与富营养化
The widespread use of nitrogen‑based fertilisers and the burning of fossil fuels have dramatically altered the global nitrogen cycle.
氮肥的广泛使用和化石燃料的燃烧极大改变了全球氮循环。
Excess nitrate leaches from agricultural land into water bodies, causing eutrophication: algal blooms deplete oxygen and kill aquatic life.
过量的硝酸盐从农田淋溶进入水体，引发富营养化：藻华大量消耗氧气，导致水生生物死亡。
Nitrous oxide (N₂O) released from denitrification and fertiliser application is a potent greenhouse gas.
反硝化及施肥释放的氧化亚氮（N₂O）是一种强效温室气体。
Reducing fertiliser overuse, creating buffer zones, and using crop rotation with legumes can mitigate these effects.
减少化肥过量使用、建立缓冲带以及采用豆科作物轮作可以缓解这些影响。
Understanding the nitrogen cycle is essential for evaluating sustainable farming practices and environmental policies.
理解氮循环对于评估可持续农业实践和环境政策至关重要。
11. Exam Tips and Common Misconceptions | 考试技巧与常见误区
- Many students confuse nitrification with denitrification. Remember: nitrification is aerobic and produces nitrate; denitrification is anaerobic and produces N₂ gas.
- 许多学生会混淆硝化与反硝化。记住：硝化好氧、产硝酸盐；反硝化厌氧、产氮气。
- Do not state that plants absorb nitrogen gas directly from the atmosphere. They take up mineral ions (NO₃⁻, NH₄⁺) from the soil.
- 切勿声称植物直接从大气中吸收氮气。它们从土壤中吸收矿质离子（NO₃⁻、NH₄⁺）。
- Nitrogen‑fixing bacteria are not the same as nitrifying bacteria. Fixers convert N₂ → NH₃/NH₄⁺; nitrifiers oxidise NH₄⁺ → NO₂⁻ → NO₃⁻.
- 固氮菌不同于硝化菌。固氮菌转化 N₂ → NH₃ / NH₄⁺；硝化菌氧化 NH₄⁺ → NO₂⁻ → NO₃⁻。
- Ammonification releases NH₄⁺, not N₂. It is decomposition, not combustion.
- 氨化作用释放 NH₄⁺，而非 N₂。它属于分解过程，不是燃烧。
- Use specific bacterial names (Rhizobium, Nitrosomonas, Nitrobacter) to demonstrate precise knowledge and gain extra marks.
- 使用具体的细菌名称（根瘤菌、亚硝化单胞菌、硝化杆菌）来展示精准知识，赢得额外分数。
12. Summary Table of Processes and Bacteria | 过程与细菌总结表
The following table provides a concise revision overview of the key nitrogen cycle processes, conditions, and microorganisms involved.
下表简明扼要地复习了氮循环的关键过程、条件及涉及的微生物。
| Process | Conversion | Key Organisms | Conditions |
|---|---|---|---|
| Nitrogen fixation | N₂ → NH₃ / NH₄⁺ | Rhizobium, Azotobacter | Anaerobic (root nodules) or aerobic (free-living); requires ATP |
| Ammonification | Organic N → NH₄⁺ | Saprobiontic bacteria and fungi | Aerobic and anaerobic |
| Nitrification | NH₄⁺ → NO₂⁻ → NO₃⁻ | Nitrosomonas, Nitrobacter | Aerobic, well-drained soil |
| Assimilation | NO₃⁻ / NH₄⁺ → organic N | Plants, then animals | Any (requires energy in plants) |
| Denitrification | NO₃⁻ → N₂ | Pseudomonas, Thiobacillus | Anaerobic, waterlogged soil |
Memorising this table will allow you to quickly answer structured questions comparing the different bacterial roles in the cycle.
记住此表格，你就能快速回答比较细菌在循环中不同作用的结构化问题。
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