📚 The Nitrogen Cycle | 氮循环
Nitrogen is an essential element for life, yet most organisms cannot use the abundant nitrogen gas directly from the air. The nitrogen cycle describes the series of processes that convert unreactive atmospheric nitrogen into forms that plants and animals can use, before returning it to the atmosphere. Understanding this cycle is key for IGCSE Biology, as it connects ecosystems, microorganisms and human activities such as farming.
氮是所有生命不可或缺的元素，然而绝大多数生物并不能直接利用空气中丰富的氮气。氮循环所描述的，正是将惰性的大气氮转化为植物和动物可以利用的形态、最终又使其回归大气的一系列过程。理解这一循环对 IGCSE 生物学而言至关重要，因为它把生态系统、微生物以及农业等人类活动紧密地联系在了一起。
1. The Importance of Nitrogen for Living Organisms | 氮对生物体的重要性
Nitrogen atoms are needed to build proteins, DNA, RNA and ATP – all vital molecules for life. Although the atmosphere is about 78% nitrogen gas (N₂), this triple‑bonded molecule is very stable. Plants can only absorb nitrogen in the form of nitrate ions (NO₃⁻) or, in smaller amounts, ammonium ions (NH₄⁺). Animals obtain their nitrogen by feeding on plants or other animals. The nitrogen cycle ensures a continuous supply of these usable forms.
氮原子是构建蛋白质、DNA、RNA 和 ATP 等生命关键分子所必需的。虽然大气中约有 78% 是氮气（N₂），但这种三键分子极为稳定。植物只能吸收硝酸根离子（NO₃⁻）形式的氮，也有少量以铵根离子（NH₄⁺）形式吸收。动物则通过取食植物或其他动物来获得氮。氮循环确保了这些可利用形式的持续供给。
2. Overview of the Nitrogen Cycle | 氮循环概览
The nitrogen cycle involves four main microbial processes: nitrogen fixation, ammonification, nitrification, and denitrification. These processes are driven by different types of bacteria and fungi. In addition, physical events such as lightning and human activities like fertiliser production also add fixed nitrogen to the soil. The cycle moves nitrogen between the atmosphere, soil, water, and living organisms.
氮循环包括四个主要的微生物过程：固氮作用、氨化作用、硝化作用和反硝化作用。这些过程由不同种类的细菌和真菌驱动。此外，闪电等自然现象以及肥料生产等人类活动也会向土壤中添加固定态氮。循环使氮在大气、土壤、水体与生物体之间不断迁移。
3. Nitrogen Fixation – Making Nitrogen Usable | 固氮——让氮变得可用
Nitrogen fixation is the conversion of atmospheric N₂ into ammonia (NH₃) or related compounds. This can happen in three ways: atmospheric fixation by lightning, industrial fixation in the Haber process, and biological fixation by microorganisms. In each case, the strong triple bond of N₂ is broken and nitrogen is combined with hydrogen to form ammonia.
固氮是指将大气中的 N₂ 转化为氨（NH₃）或相关化合物。这可以通过三种方式实现：闪电的大气固氮、哈伯法工业固氮以及微生物的生物固氮。无论哪种方式，都是使 N₂ 的牢固三键断裂，让氮与氢结合生成氨。
4. Lightning and Industrial Fixation | 闪电固氮与工业固氮
Lightning provides enough energy to break N₂ bonds, allowing nitrogen to react with oxygen to form nitrogen oxides. These dissolve in rainwater and enter the soil as dilute nitric acid, which forms nitrates. Industrially, the Haber process combines nitrogen and hydrogen under high temperature and pressure to produce ammonia on a massive scale for fertilisers. Both contribute non‑biological inputs to the nitrogen cycle.
闪电提供了足以断裂 N₂ 键的能量，使氮与氧反应生成氮氧化物。它们溶于雨水，以稀硝酸的形式进入土壤，形成硝酸盐。在工业上，哈伯法在高温高压下将氮与氢结合，大规模生产用于肥料的氨。两者都为氮循环提供了非生物的输入。
5. Biological Nitrogen Fixation | 生物固氮
Certain bacteria possess the enzyme nitrogenase, which catalyses the reduction of N₂ to ammonia at normal temperatures. Free‑living soil bacteria such as Azotobacter fix nitrogen independently. More important in agriculture are the symbiotic bacteria Rhizobium, which live inside root nodules of leguminous plants (peas, beans, clover). The plant supplies carbohydrates; the bacteria supply fixed nitrogen, benefiting both partners.
某些细菌含有固氮酶，可在常温下催化 N₂ 还原为氨。像固氮菌（Azotobacter）这样的自生土壤细菌能独立固氮。在农业上更重要的则是共生细菌根瘤菌（Rhizobium），它们生活在豆科植物（豌豆、菜豆、三叶草）的根瘤内。植物提供碳水化合物，细菌提供固定态氮，双方互利。
6. Nitrification – From Ammonia to Nitrate | 硝化作用——从氨到硝酸盐
Ammonia and ammonium ions in the soil are quickly oxidised by nitrifying bacteria. This aerobic process occurs in two stages. First, Nitrosomonas bacteria oxidise ammonium to nitrite ions (NO₂⁻). Then, Nitrobacter bacteria oxidise nitrite to nitrate ions (NO₃⁻). The overall conversion can be summarised as:
NH₄⁺ → NO₂⁻ → NO₃⁻
土壤中的氨和铵离子会被硝化细菌迅速氧化。这一好氧过程分两步进行。首先，亚硝化单胞菌（Nitrosomonas）将铵氧化为亚硝酸根离子（NO₂⁻）。然后，硝化杆菌（Nitrobacter）将亚硝酸根氧化为硝酸根离子（NO₃⁻）。整个转化过程可概括为：
NH₄⁺ → NO₂⁻ → NO₃⁻
7. Assimilation – Plants Take Up Nitrate | 同化——植物吸收硝酸盐
Nitrate ions dissolved in soil water are actively taken up by plant root hairs. Once inside the plant, nitrate is reduced back to ammonium and then incorporated into amino acids, proteins and nucleic acids. This process is called assimilation. When herbivores eat plants, and carnivores eat herbivores, nitrogen‑containing compounds pass along the food chain, building animal proteins.
溶解在土壤水中的硝酸根离子被植物根毛主动吸收。进入植物体后，硝酸盐被重新还原为铵，然后进一步合成氨基酸、蛋白质和核酸。这一过程称为同化。当食草动物取食植物，食肉动物又捕食食草动物时，含氮化合物便沿食物链传递，构成动物蛋白质。
8. Ammonification – Recycling Nitrogen from Dead Matter | 氨化作用——从死亡有机体回收氮
When plants and animals die, and when animals excrete urea and faeces, decomposers such as fungi and saprophytic bacteria break down the organic nitrogen in these materials. The process releases ammonium ions (NH₄⁺) into the soil. This is ammonification. It ensures that nitrogen locked up in dead biomass is recycled back into a form that can re‑enter the cycle.
当植物和动物死亡，以及动物排泄尿素和粪便时，真菌和腐生细菌等分解者会将这些材料中的有机氮分解。这一过程向土壤释放铵根离子（NH₄⁺），这就是氨化作用。它确保被固定在死亡生物量中的氮得以回收，以可重新进入循环的形式归还土壤。
9. Denitrification – Returning Nitrogen to the Atmosphere | 反硝化作用——氮回归大气
In waterlogged, anaerobic soils, denitrifying bacteria (e.g. Pseudomonas) use nitrate as an alternative electron acceptor during respiration. They convert nitrate all the way back to nitrogen gas (N₂), which diffuses into the atmosphere. While denitrification completes the cycle, it represents a loss of fixed nitrogen from the soil, which can reduce soil fertility unless fertilisers or fixation replenish it.
在渍水的厌氧土壤中，反硝化细菌（如假单胞菌 Pseudomonas）在呼吸作用中用硝酸盐作为替代电子受体。它们将硝酸盐一路还原为氮气（N₂），后者扩散到大气中。反硝化虽然使循环得以闭合，但也意味着土壤中固定态氮的损失；除非通过施肥或固氮加以补充，否则会降低土壤肥力。
10. Summary of Microorganism Roles | 微生物作用总结
| Process | Microorganisms involved | Conversion |
|---|---|---|
| Nitrogen fixation | Rhizobium (symbiotic), Azotobacter (free‑living) | N₂ → NH₃ / NH₄⁺ |
| Ammonification | Saprophytic bacteria and fungi | Organic N → NH₄⁺ |
| Nitrification | Nitrosomonas, Nitrobacter | NH₄⁺ → NO₂⁻ → NO₃⁻ |
| Denitrification | Denitrifying bacteria (e.g. Pseudomonas) | NO₃⁻ → N₂ |
以下为各过程及其微生物的中文对照：
| 过程 | 参与的微生物 | 转化 |
|---|---|---|
| 固氮作用 | 根瘤菌（共生）、固氮菌（自生） | N₂ → NH₃ / NH₄⁺ |
| 氨化作用 | 腐生细菌和真菌 | 有机氮 → NH₄⁺ |
| 硝化作用 | 亚硝化单胞菌、硝化杆菌 | NH₄⁺ → NO₂⁻ → NO₃⁻ |
| 反硝化作用 | 反硝化细菌（如假单胞菌） | NO₃⁻ → N₂ |
11. Human Impact and Eutrophication | 人类影响与富营养化
Farmers often add nitrate or ammonium fertilisers to increase crop yields. If more fertiliser is applied than plants can take up, excess nitrates are washed (leached) into rivers and lakes. Here they stimulate explosive growth of algae, known as an algal bloom. The algae block sunlight, causing underwater plants to die. As decomposers break down the dead plant material, they use up oxygen, leading to hypoxia and the death of fish and other aquatic organisms. This sequence is called eutrophication. Sewage discharge can have a similar effect by adding phosphates and nitrates.
农民常常施加硝酸盐或铵态肥料来提高作物产量。如果施肥量超过植物的吸收能力，多余的硝酸盐就会被冲刷（淋溶）进入河流和湖泊。在水体中，它们刺激藻类爆发性增长，即水华。藻类遮蔽阳光，导致水下植物死亡。分解者在分解死亡的植物材料时大量耗氧，造成水体缺氧，最终导致鱼类和其他水生生物死亡。这一连串事件称为富营养化。排放污水由于带入磷酸盐和硝酸盐，也会产生类似的影响。
12. Key Points and Exam Tips | 要点与考试提示
In the IGCSE Edexcel exam, you should be able to name the four main processes and state the bacteria involved. Be clear that plants absorb nitrogen mainly as nitrate, not as ammonium or N₂. You may be asked to interpret a diagram of the nitrogen cycle or to explain how over‑use of fertilisers leads to eutrophication. Always link processes to the correct microorganism and specify whether conditions are aerobic or anaerobic, particularly for nitrification (aerobic) and denitrification (anaerobic).
在 IGCSE Edexcel 考试中，你应当能够说出四个主要过程的名称，并指出所涉及的细菌。要明确植物吸收氮的主要形式是硝酸盐，而不是铵或 N₂。考试可能会要求你解读氮循环图，或解释过量使用肥料如何导致富营养化。务必把各个过程与正确的微生物关联起来，并说明反应条件是需氧还是厌氧，特别是硝化作用（需氧）和反硝化作用（厌氧）的条件。
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