📚 The Nitrogen Cycle | 氮循环
Nitrogen is an essential element for all living organisms. It is a key component of proteins, nucleic acids (DNA and RNA), and other important molecules. However, most organisms cannot use nitrogen directly from the atmosphere. The nitrogen cycle describes the series of processes by which nitrogen is converted between its different chemical forms and is made available to living things.
氮是所有生命体必需的元素。它是蛋白质、核酸（DNA 和 RNA）以及其他重要分子的关键组成部分。然而，大多数生物无法直接利用大气中的氮。氮循环描述的正是氮在不同化学形式之间转化的系列过程，以及这些过程如何使氮为生物所利用。
1. Why Nitrogen Matters | 氮为什么重要
Nitrogen is a vital element because it is used to build amino acids, which join together to form proteins. Proteins are needed for growth, repair, enzymes, and many other cellular functions. Nitrogen is also part of the nitrogenous bases in DNA, making it essential for heredity.
氮是一种至关重要的元素，因为它用于构建氨基酸，而氨基酸连接形成蛋白质。蛋白质是生长、修复、酶促反应以及许多细胞功能所必需的。氮也是 DNA 中含氮碱基的组成成分，因此对遗传至关重要。
- Plants need nitrogen to make proteins and DNA.
- Animals get nitrogen by eating plants or other animals.
- Nitrogen deficiency in plants causes stunted growth and yellow leaves.
- 植物需要氮来制造蛋白质和 DNA。
- 动物通过吃植物或其他动物获得氮。
- 植物缺氮会导致生长迟缓、叶片发黄。
2. Where Nitrogen Is Found | 氮在自然界中的存在
Most nitrogen on Earth exists as nitrogen gas (N₂) in the atmosphere, making up about 78% of the air. This form is very stable and cannot be used by most organisms. Nitrogen also exists in the soil as ammonium ions (NH₄⁺), nitrite ions (NO₂⁻), and nitrate ions (NO₃⁻), as well as in organic forms found in dead organisms and waste.
地球上大部分氮以氮气（N₂）的形式存在于大气中，约占空气的 78%。这种形式非常稳定，大多数生物无法利用。土壤中的氮还以铵离子（NH₄⁺）、亚硝酸根离子（NO₂⁻）和硝酸根离子（NO₃⁻）的形式存在，也存在于死亡生物体及排泄物中的有机形式里。
- Atmospheric N₂ is the largest nitrogen reservoir.
- Nitrates (NO₃⁻) are the most easily absorbed form of nitrogen for plants.
- Nitrites (NO₂⁻) are usually toxic and do not accumulate in large amounts.
- 大气中的 N₂ 是最大的氮库。
- 硝酸盐（NO₃⁻）是植物最容易吸收的氮形式。
- 亚硝酸盐（NO₂⁻）通常有毒，不会大量积累。
3. Nitrogen Fixation | 固氮作用
Nitrogen fixation is the process by which atmospheric nitrogen gas (N₂) is converted into ammonia (NH₃) or ammonium ions (NH₄⁺). This can occur by lightning, by industrial processes such as the Haber process, and, most importantly for ecosystems, by nitrogen-fixing bacteria.
固氮作用是将大气中的氮气（N₂）转化为氨（NH₃）或铵离子（NH₄⁺）的过程。这一过程可通过闪电发生，也可通过哈伯法等工业过程实现，而对生态系统最重要的是固氮细菌的作用。
- Rhizobium bacteria live in root nodules of legumes (e.g. peas and beans) and fix nitrogen in exchange for sugars.
- Free-living soil bacteria such as Azotobacter also fix nitrogen.
- Lightning provides enough energy to combine N₂ with oxygen, forming nitrates that dissolve in rain.
- 根瘤菌生活在豆科植物（如豌豆和豆类）的根瘤中，它们固定氮并换取糖类。
- 像固氮菌这样的自生土壤细菌也能固定氮。
- 闪电提供足够能量使 N₂ 与氧气结合，生成硝酸盐并溶于雨水。
N₂ → NH₃ → NH₄⁺
4. Ammonification | 氨化作用
Ammonification is the breakdown of organic nitrogen-containing compounds, such as proteins and nucleic acids, into ammonia (NH₃) or ammonium ions (NH₄⁺). This process is carried out by decomposers, mainly bacteria and fungi.
氨化作用是将有机含氮化合物（如蛋白质和核酸）分解为氨（NH₃）或铵离子（NH₄⁺）的过程。该过程主要由腐生细菌和真菌等分解者完成。
- When organisms die, decomposers break down their proteins.
- Animal waste (urea) is also converted into ammonium ions.
- Ammonification releases nitrogen back into the soil in a form that can be further processed.
- 当生物死亡时，分解者会分解其蛋白质。
- 动物排泄物（尿素）也会转化为铵离子。
- 氨化作用将氮以可进一步转化的形式释放回土壤。
5. Nitrification | 硝化作用
Nitrification is the two-step oxidation of ammonia to nitrate, carried out by nitrifying bacteria under aerobic conditions. First, Nitrosomonas bacteria oxidise ammonium ions to nitrites (NO₂⁻). Then, Nitrobacter bacteria oxidise nitrites to nitrates (NO₃⁻).
硝化作用是在有氧条件下由硝化细菌完成的两步氧化过程，将氨转化为硝酸盐。首先，亚硝化单胞菌将铵离子氧化为亚硝酸盐（NO₂⁻）；然后，硝化杆菌将亚硝酸盐氧化为硝酸盐（NO₃⁻）。
NH₄⁺ → NO₂⁻ → NO₃⁻
This process requires oxygen. Therefore, well-aerated, warm, and neutral soils promote nitrification. Nitrates are the main form of nitrogen that plants absorb through their roots.
这个过程需要氧气。因此，通气良好、温暖且呈中性的土壤有利于硝化作用。硝酸盐是植物通过根部吸收的主要氮形式。
6. Assimilation | 同化作用
Assimilation is the uptake by organisms of nitrogen compounds and their incorporation into cellular material. Plants absorb nitrate ions (NO₃⁻) through root hair cells and use them to synthesise amino acids, proteins, and DNA. Animals obtain nitrogen by consuming plant tissue or other animals and then build their own proteins.
同化作用是指生物摄取含氮化合物并将其整合到细胞物质中的过程。植物通过根毛细胞吸收硝酸根离子（NO₃⁻），并利用它们合成氨基酸、蛋白质和 DNA。动物通过取食植物组织或其他动物获得氮，并构建自身的蛋白质。
- Nitrates are actively transported into plant roots.
- Amino acids are produced from nitrates and carbohydrates.
- Proteins are used for enzymes, cell membranes, and muscle tissue.
- 硝酸盐通过主动运输进入植物根部。
- 氨基酸由硝酸盐和碳水化合物合成。
- 蛋白质用于制造酶、细胞膜和肌肉组织。
7. Denitrification | 反硝化作用
Denitrification is the conversion of nitrates back into nitrogen gas (N₂) or nitrous oxide (N₂O) by denitrifying bacteria. This process occurs under anaerobic conditions (waterlogged or compacted soil) and reduces the availability of nitrogen in the soil.
反硝化作用是反硝化细菌将硝酸盐还原为氮气（N₂）或一氧化二氮（N₂O）的过程。这一过程在厌氧条件下（例如淹水或板结的土壤）发生，并降低土壤中氮的可利用性。
NO₃⁻ → N₂
- Denitrifying bacteria use nitrate instead of oxygen during respiration.
- Poor drainage and lack of oxygen encourage denitrification.
- This process returns nitrogen to the atmosphere, completing the cycle.
- 反硝化细菌在呼吸时使用硝酸盐代替氧气。
- 排水不良和缺氧会促进反硝化作用。
- 这一过程将氮返回大气，完成整个循环。
8. The Role of the Nitrogen Cycle in Ecosystems | 氮循环在生态系统中的作用
Healthy ecosystems rely on a balanced nitrogen cycle. Fertilisers often add nitrogen in the form of nitrates or ammonium to boost crop growth. However, excessive use of fertilisers can disrupt the natural balance, leading to problems such as eutrophication.
健康的生态系统依赖于氮循环的平衡。化肥常以硝酸盐或铵的形式添加氮以促进作物生长。然而，过量使用化肥会破坏自然平衡，导致水体富营养化等问题。
- Nitrogen from fertilisers can leach into rivers and lakes.
- Increased algae growth blocks light and depletes oxygen.
- Fish and other aquatic organisms may die from oxygen shortage.
- 化肥中的氮可渗入河流和湖泊。
- 藻类大量繁殖会阻挡光照并耗尽氧气。
- 鱼类和其他水生生物可能因缺氧而死亡。
9. The Full Cycle | 完整循环总结
The nitrogen cycle is a continuous system of transformations involving nitrogen fixation, ammonification, nitrification, assimilation, and denitrification. Each stage depends on specific microorganisms and environmental conditions.
氮循环是一个连续转化的系统，包括固氮、氨化、硝化、同化和反硝化。每个阶段都依赖于特定的微生物和环境条件。
| Process | Conversion | Organisms involved |
|---|---|---|
| Nitrogen fixation | N₂ → NH₃ / NH₄⁺ | Rhizobium, Azotobacter, lightning |
| Ammonification | Organic N → NH₄⁺ | Decomposers: bacteria and fungi |
| Nitrification | NH₄⁺ → NO₂⁻ → NO₃⁻ | Nitrosomonas, Nitrobacter |
| Assimilation | NO₃⁻ → amino acids, proteins, DNA | Plants and animals |
| Denitrification | NO₃⁻ → N₂ | Denitrifying bacteria (anaerobic) |
N₂ → NH₄⁺ → NO₂⁻ → NO₃⁻ → proteins → NH₄⁺ → N₂
10. Exam Tips | 考试提示
In IGCSE Science questions, remember the following key points. Read the data table carefully to identify whether the conditions are aerobic or anaerobic. Link each process to the correct microorganism. Know at least one example of a nitrogen-fixing bacterium and a nitrifying bacterium. Also, be able to explain the effects of agricultural practices on the nitrogen cycle.
在 IGCSE 科学考试中，记住以下要点。仔细阅读数据表，判断条件是有氧还是厌氧。将每个过程与正确的微生物对应起来。至少要知道一种固氮细菌和一种硝化细菌的名称。同时，能够解释农业生产活动对氮循环的影响。
- Always write metabolic reactions with correct chemical symbols, e.g. NH₄⁺, NO₃⁻.
- Do not confuse nitrification with denitrification.
- Remember that denitrification requires anaerobic conditions.
- 书写代谢反应时务必使用正确的化学符号，例如 NH₄⁺、NO₃⁻。
- 不要混淆硝化作用与反硝化作用。
- 记住反硝化作用需要厌氧条件。
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