The Nitrogen Cycle | 氮循环考点精讲

📚 The Nitrogen Cycle | 氮循环考点精讲

In IGCSE OCR Biology, the nitrogen cycle is one of the most important nutrient cycles you need to master. It describes how nitrogen moves between the atmosphere, soil, plants and animals through a series of microbial transformations. A solid understanding of the key stages, the bacteria involved and the environmental conditions required is essential for exam success.

在 IGCSE OCR 生物中，氮循环是你必须掌握的最重要的物质循环之一。它描述了氮如何通过一系列微生物转化作用，在大气、土壤、植物和动物之间转移。牢固掌握关键阶段、涉及的细菌种类以及所需的环境条件，是考试取得好成绩的关键。

1. Introduction to the Nitrogen Cycle | 氮循环概述

About 78% of the Earth’s atmosphere is nitrogen gas (N₂), yet most living organisms cannot use it directly. Nitrogen is a vital element for building proteins, nucleic acids (DNA and RNA) and ATP. The nitrogen cycle converts inert nitrogen gas into compounds like ammonium (NH₄⁺) and nitrate (NO₃⁻) that plants can absorb and incorporate into organic molecules. The cycle comprises five main processes: nitrogen fixation, nitrification, assimilation, ammonification and denitrification, all driven by specialised microorganisms.

地球大气中约 78% 是氮气（N₂），但大多数生物无法直接利用它。氮是构建蛋白质、核酸（DNA 和 RNA）以及 ATP 的关键元素。氮循环将惰性的氮气转化为植物可吸收并有机化的化合物，如铵根（NH₄⁺）和硝酸根（NO₃⁻）。这个循环包括五个主要过程：固氮作用、硝化作用、同化作用、氨化作用和反硝化作用，所有这些过程都由特定的微生物驱动。

---

2. Nitrogen Fixation | 固氮作用

Nitrogen fixation is the conversion of atmospheric nitrogen gas (N₂) into ammonia (NH₃) or ammonium ions (NH₄⁺). Because the triple bond in N₂ is extremely stable, this process requires a lot of energy. In nature, most fixation is carried out by nitrogen-fixing bacteria, but small amounts can also occur via lightning strikes that provide enough energy to combine nitrogen and oxygen, resulting in nitrates that enter soil with rain. Industrially, the Haber process artificially fixes nitrogen to produce fertilisers. For your exam, focus on biological fixation.

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。由于 N₂ 中的三键非常稳定，这个过程需要大量能量。在自然界中，大部分固氮由固氮细菌完成，但也有少量通过闪电提供能量使氮与氧结合，生成硝酸盐随雨水进入土壤。工业上，哈伯法人工固定氮来生产化肥。考试中重点关注生物固氮。

---

3. Types of Nitrogen-Fixing Bacteria | 固氮菌的类型

There are two main groups of nitrogen-fixing bacteria: symbiotic and free-living. Symbiotic bacteria, such as Rhizobium, live inside root nodules of leguminous plants like peas, beans and clover. They receive carbohydrates from the plant and in return supply fixed nitrogen as ammonium. Free-living bacteria, including Azotobacter and Clostridium, carry out nitrogen fixation independently in the soil. Blue-green algae (cyanobacteria) in aquatic environments also contribute to fixation. Knowing the names and associations is a common exam requirement.

固氮细菌主要有两大类：共生固氮菌和自生固氮菌。共生细菌如根瘤菌属（Rhizobium）生活在豆科植物（如豌豆、菜豆和三叶草）的根瘤中。它们从植物获得碳水化合物，反过来为植物提供固定好的铵态氮。自生固氮菌如固氮菌属（Azotobacter）和梭菌属（Clostridium）在土壤中独立进行固氮。水生环境中的蓝绿藻（蓝细菌）也能固氮。记住这些名称和关联是常见的考点。

---

4. Nitrification | 硝化作用

Nitrification is a two-step aerobic process that converts ammonium ions into nitrates, the form most easily taken up by plants. First, nitrifying bacteria such as Nitrosomonas oxidise ammonium (NH₄⁺) to nitrite (NO₂⁻). Then, another group such as Nitrobacter further oxidises nitrite to nitrate (NO₃⁻). Both steps require oxygen, so nitrification occurs in well-aerated soils. The overall conversions are often summarised as:

硝化作用是一个两步需氧过程，将铵离子转化为硝酸盐——植物最容易吸收的氮形式。首先，硝化细菌如亚硝化单胞菌属（Nitrosomonas）将铵（NH₄⁺）氧化为亚硝酸盐（NO₂⁻）。然后，另一类细菌如硝化杆菌属（Nitrobacter）再将亚硝酸盐氧化为硝酸盐（NO₃⁻）。这两步都需要氧气，因此硝化作用发生在通气良好的土壤中。整体转化可概括为：

NH₄⁺ → NO₂⁻ → NO₃⁻

A common exam question asks you to identify which bacteria carry out each step and why oxygen is essential. Oxygen acts as the final electron acceptor in their respiration and is directly required for the oxidation reactions.

常见的考试问题是让你辨别每一步是哪类细菌完成的，以及为什么氧气至关重要。氧气既是细菌呼吸的最终电子受体，也是氧化反应直接需要的反应物。

---

5. Assimilation | 同化作用

Assimilation refers to the uptake and incorporation of nitrogen compounds into living organisms. Plants absorb nitrate ions from the soil through their root hairs by active transport. They then use these nitrates to synthesise amino acids, proteins, chlorophyll and nucleic acids. When primary consumers eat plants, the nitrogen becomes part of animal proteins. At each trophic level, nitrogen is recycled within the organism’s body until it dies or excretes waste.

同化作用指的是生物体摄取氮化合物并将其转化为自身成分的过程。植物通过根毛主动运输吸收土壤中的硝酸根离子，然后用这些硝酸盐合成氨基酸、蛋白质、叶绿素和核酸。当初级消费者吃掉植物时，氮成为动物蛋白质的一部分。在每个营养级内，氮一直在生物体内循环利用，直到生物死亡或排出废物。

---

6. Ammonification | 氨化作用

When plants and animals die, or when animals produce urine and faeces, the organic nitrogen in their tissues and waste products is broken down by decomposers. Saprobiotic bacteria and fungi secrete enzymes that digest proteins, nucleic acids and urea externally. This decomposition releases ammonium ions (NH₄⁺) into the soil. The process is called ammonification, and it returns nitrogen to the soil in a form that can be used again by nitrifying bacteria.

当动植物死亡，或者动物排出尿液和粪便时，它们组织和废物中的有机氮会被分解者分解。腐生细菌和真菌分泌酶，在体外消化蛋白质、核酸和尿素。这种分解作用向土壤中释放铵离子（NH₄⁺）。这个过程称为氨化作用，它将氮以铵的形式归还土壤，可被硝化细菌再次利用。

Without ammonification, the nitrogen locked in dead organic matter would remain unavailable. The speed of this process depends on temperature, moisture and soil pH, making it an important topic in both ecology and agriculture.

如果没有氨化作用，死亡有机质中的氮将一直无法被利用。这一过程的速率取决于温度、湿度和土壤 pH，使其成为生态学和农业中的重要话题。

---

7. Denitrification | 反硝化作用

Denitrification is the conversion of nitrates in the soil back into nitrogen gas (N₂), which is released into the atmosphere. This process is carried out by denitrifying bacteria, such as Pseudomonas and Thiobacillus, under anaerobic (oxygen-poor) conditions. These bacteria use nitrate as an alternative electron acceptor for respiration, thereby reducing NO₃⁻ to N₂. The simplified conversion:

反硝化作用是将土壤中的硝酸盐转化为氮气（N₂）并释放到大气中的过程。该过程由反硝化细菌（如假单胞菌属 Pseudomonas 和硫杆菌属 Thiobacillus）在厌氧（缺氧）条件下完成。这些细菌利用硝酸盐作为呼吸作用的替代电子受体，将 NO₃⁻ 还原为 N₂。简化的转化式为：

NO₃⁻ → N₂

Denitrification reduces soil fertility because it removes plant-available nitrogen. Waterlogged soils, compaction and poor aeration encourage denitrification. In the exam, you must link denitrifying bacteria to anaerobic conditions and explain how they negatively affect crop growth.

反硝化作用会降低土壤肥力，因为它去除了植物可利用的氮。涝渍、板结和通气不良的土壤会促进反硝化作用。在考试中，你必须把反硝化细菌与厌氧条件联系起来，并解释它们如何对作物生长产生负面影响。

---

8. Role of Leguminous Plants | 豆科植物的作用

Leguminous plants, such as peas, beans and alfalfa, play a special role in the nitrogen cycle because of their symbiotic relationship with Rhizobium bacteria. The plant provides a low-oxygen environment inside root nodules that allows nitrogenase enzymes to fix nitrogen, while supplying the bacteria with carbohydrates. In return, the plant receives a direct supply of ammonium. This mutualism improves soil nitrogen content naturally, reducing the need for synthetic fertilisers.

豆科植物（如豌豆、菜豆和苜蓿）因与根瘤菌的共生关系在氮循环中扮演特殊角色。植物在根瘤内提供低氧环境，使固氮酶能固定氮，同时为细菌提供碳水化合物。作为回报，植物获得直接的铵供应。这种互利关系能自然增加土壤氮含量，减少对合成化肥的需求。

Farmers exploit this by practising crop rotation, alternating legumes with other crops, or by ploughing leguminous plants back into the soil as green manure. Exam questions often ask you to explain how planting legumes can replenish soil nitrogen for the next crop.

农民通过轮作（将豆科植物与其他作物交替种植）或将豆科植物翻压作为绿肥来利用这一点。考题经常要求你解释种植豆科植物如何为下一茬作物补充土壤氮。

---

9. Environmental Importance | 环境重要性

A balanced nitrogen cycle is essential for healthy ecosystems and sustainable agriculture. However, human activities can disrupt it. Overuse of nitrate-based fertilisers can lead to leaching, where nitrates wash into water bodies, causing eutrophication – algal blooms that deplete oxygen and kill aquatic life. Burning fossil fuels also releases nitrogen oxides, contributing to acid rain. Understanding the cycle helps scientists design strategies to reduce pollution, manage soil fertility and protect natural habitats.

平衡的氮循环对健康的生态系统和可持续农业至关重要。然而，人类活动可能破坏它。过量使用硝酸盐肥料会导致淋溶，硝酸盐被冲刷进入水体，引起富营养化——藻华爆发耗尽水中氧气，杀死水生生物。燃烧化石燃料还会释放氮氧化物，导致酸雨。理解氮循环有助于科学家设计减少污染、管理土壤肥力和保护自然栖息地的策略。

In the exam, you may be given data on fertiliser use and nitrate concentrations in rivers, and asked to evaluate the environmental impact. Make sure you can describe the sequence of eutrophication clearly: fertiliser run-off → algal bloom → blocking light → plants die → decomposers use up oxygen → fish death.

在考试中，你可能会遇到关于肥料使用和河流中硝酸盐浓度的数据，并被要求评价环境影响。确保你能清晰描述富营养化的过程：肥料径流→ 藻华→ 遮蔽光照→ 水生植物死亡→ 分解者耗氧→ 鱼类死亡。

---

10. Key Exam Points & Common Misconceptions | 考点与常见误区

To score high marks, avoid these common mistakes: confusing nitrogen fixation with nitrification (fixation makes ammonia from N₂; nitrification converts ammonia to nitrate); forgetting that nitrification needs oxygen while denitrification occurs when oxygen is absent; mixing up Nitrosomonas (ammonium → nitrite) and Nitrobacter (nitrite → nitrate); or thinking animals can use atmospheric nitrogen directly. Also, note that denitrifying bacteria are not the same as nitrifying bacteria – they have opposite roles.

要拿高分，请避免以下常见错误：混淆固氮作用与硝化作用（固氮是将 N₂ 转变为氨；硝化是将氨转变为硝酸盐）；忘记硝化需氧而反硝化在无氧时发生；弄混亚硝化单胞菌（NH₄⁺ → NO₂⁻）和硝化杆菌（NO₂⁻ → NO₃⁻）；或者以为动物可直接利用大气中的氮。还要注意反硝化细菌不同于硝化细菌——它们的作用相反。

Use the table below to recap the four key bacterial groups and their roles, a favourite in OCR mark schemes:

使用下表总结四类关键细菌及其作用，这是 OCR 评分方案中的常客：

Process | 过程	Bacteria | 细菌	Conversion | 转化	Condition | 条件
Nitrogen fixation | 固氮	Rhizobium, Azotobacter	N₂ → NH₄⁺	Aerobic / Anaerobic (depends on species)
Nitrification | 硝化	Nitrosomonas, Nitrobacter	NH₄⁺ → NO₂⁻ → NO₃⁻	Aerobic
Denitrification | 反硝化	Pseudomonas	NO₃⁻ → N₂	Anaerobic
Ammonification | 氨化	Saprobiotic fungi and bacteria | 腐生真菌和细菌	Organic N → NH₄⁺	Aerobic / Anaerobic

Finally, always use precise biological terminology: say ‘nitrate ions’ not ‘nitrogen’; refer to ‘ammonium ions’ rather than ‘ammonia’ where appropriate; and clearly distinguish between the different bacterial genera. With these details, you will be well prepared for any nitrogen cycle question on the OCR IGCSE Biology exam.

最后，务必使用准确的生物学术语：说“硝酸根离子”而不是“氮”；在适当地方提到“铵离子”而非“氨”；清楚区分不同的细菌属名。掌握这些细节，你将能从容应对 OCR IGCSE 生物考试中的任何氮循环题目。

Published by TutorHao | Biology Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）