GCSE CIE Biology: The Nitrogen Cycle | GCSE CIE 生物：氮循环考点精讲

📚 GCSE CIE Biology: The Nitrogen Cycle | GCSE CIE 生物：氮循环考点精讲

The nitrogen cycle is a fundamental biogeochemical process that recycles nitrogen between the atmosphere, soil, water and living organisms. For your GCSE CIE Biology exam, you must be able to name the key bacterial groups, describe the conversions between different nitrogen compounds, and explain how human activities can upset this natural balance.

氮循环是一个至关重要的生物地球化学过程，使氮在大气、土壤、水和生物体之间循环。在 GCSE CIE 生物考试中，你必须能够命名关键的细菌群，描述不同含氮化合物之间的转化，并解释人类活动如何扰乱这种自然平衡。

1. The Nitrogen Cycle – An Overview | 氮循环概述

The nitrogen cycle is a closed loop that moves nitrogen atoms from the unreactive N₂ gas in the air, into compounds that plants can absorb, through food chains, and eventually back to the atmosphere. The four main stages are nitrogen fixation, ammonification, nitrification and denitrification.

氮循环是一个闭合的循环，将不活泼的空气中 N₂ 气体中的氮原子，转化为植物可以吸收的化合物，经过食物链，最终返回大气。四个主要阶段是固氮作用、氨化作用、硝化作用和反硝化作用。

Although nitrogen gas (N₂) makes up about 78% of the air, plants and animals cannot use it directly. It must first be ‘fixed’ into ammonia (NH₃) or ammonium ions (NH₄⁺) and then often converted to nitrates (NO₃⁻).

尽管氮气占空气的 78%，但动植物无法直接利用。它必须先被“固定”为氨或铵离子，然后通常被转化为硝酸盐。

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2. Why Nitrogen is Essential for Life | 氮对生命的重要性

Nitrogen is a key component of amino acids, which join together to form proteins. Proteins are needed for growth, repair and building enzymes. Nitrogen is also found in the bases of DNA, in ATP (the energy-carrier molecule) and in chlorophyll, the pigment for photosynthesis.

氮是氨基酸的关键组成部分，氨基酸连接起来形成蛋白质。蛋白质对于生长、修复和构建酶是必需的。氮还存在于 DNA 的碱基中、ATP 分子中以及用于光合作用的叶绿素中。

Without an adequate supply of nitrogen in the soil, plants show stunted growth, yellowing of older leaves (chlorosis) and reduced crop yields. Therefore, understanding the nitrogen cycle is directly linked to food production and ecosystem health.

如果土壤中供氮不足，植物会表现出生长迟缓、老叶黄化和作物减产。因此，理解氮循环直接关系到粮食生产和生态系统的健康。

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3. Nitrogen Fixation – Converting N₂ into Usable Forms | 固氮作用——将氮气转化为可用形式

Nitrogen fixation is the conversion of inert nitrogen gas (N₂) into ammonia (NH₃) or ammonium ions (NH₄⁺). This can occur through three main routes: atmospheric fixation by lightning, biological fixation by microorganisms and industrial fixation via the Haber process.

固氮作用是将惰性氮气转化为氨或铵离子的过程。这可以通过三种主要途径发生：闪电的大气固氮、微生物的生物固氮和通过哈伯法的工业固氮。

Industrial fixation uses the Haber–Bosch process: N₂ + 3H₂ ⇌ 2NH₃. This reaction is carried out at high temperature and pressure with an iron catalyst to manufacture nitrogen fertilisers, which are widely used in agriculture.

工业固氮采用哈伯–博斯法：N₂ + 3H₂ ⇌ 2NH₃。该反应在高温高压和铁催化剂下进行，以制造氮肥，广泛应用于农业。

Natural fixation does not require high energy inputs from fossil fuels; instead, it relies on lightning or specialised nitrogen-fixing bacteria.

自然固氮不需要化石燃料的高能投入，而是依靠闪电或专门的固氮菌。

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4. Role of Lightning in Nitrogen Fixation | 闪电在固氮中的作用

Lightning provides the extreme energy needed to break the very strong triple bond in N₂ molecules. Inside a lightning strike, nitrogen and oxygen react to form nitrogen oxides (NO and NO₂). These dissolve in rainwater to form nitric acid, which then falls to the ground and produces nitrate ions (NO₃⁻) in the soil.

闪电提供了断裂 N₂ 分子中极强三键所需的极端能量。在闪电内部，氮和氧反应生成氮氧化物。它们溶于雨水形成硝酸，然后落到地面，在土壤中产生硝酸根离子。

This source of natural nitrate is relatively small but can be significant in regions with regular thunderstorms. It is an example of a non-biological way to fix nitrogen.

这种天然硝酸盐来源相对较少，但在常有雷暴的地区可能很重要。这是非生物固氮的一个例子。

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5. Biological Nitrogen Fixation by Bacteria | 细菌的生物固氮作用

Biological fixation is carried out by nitrogen-fixing bacteria that possess the enzyme nitrogenase. These bacteria can be free-living in the soil, such as Azotobacter, or they can form mutualistic symbiotic relationships with the roots of leguminous plants, such as Rhizobium.

生物固氮由拥有固氮酶的固氮菌进行。这些细菌可以是土壤中自由生活的，如固氮菌，或与豆科植物的根形成互惠共生关系，如根瘤菌。

In the symbiosis, bacteria invade the root hairs and stimulate the plant to form root nodules. Inside these nodules, the bacteria receive carbohydrates from the plant and, in return, fix N₂ into ammonium, which the plant can use to make amino acids. This relationship is a classic example of mutualism.

在共生关系中，细菌侵入根毛，刺激植物形成根瘤。在这些根瘤内，细菌从植物获得碳水化合物，作为回报，将 N₂ 固定为铵，供植物制造氨基酸。这种关系是互利共生的典型例子。

Type	Example	Habitat
Free-living	Azotobacter	Soil
Symbiotic	Rhizobium	Root nodules of legumes

农民利用豆科植物进行轮作，以减少对合成氮肥的依赖。

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6. Nitrification – From Ammonium to Nitrates | 硝化作用——从铵离子到硝酸盐

Nitrification is a two-step aerobic process carried out by nitrifying bacteria in the soil. In the first step, Nitrosomonas bacteria oxidise ammonium ions (NH₄⁺) into nitrite ions (NO₂⁻). This reaction releases some energy, which the bacteria use.

硝化作用是一个两步的好氧过程，由土壤中的硝化细菌完成。第一步，亚硝化单胞菌将铵离子氧化成亚硝酸根离子。该反应释放出一些能量，供细菌利用。

NH₄⁺ → NO₂⁻

In the second step, Nitrobacter bacteria oxidise the nitrite ions (NO₂⁻) into nitrate ions (NO₃⁻). Nitrate is the main form in which nitrogen is taken up by plant roots.

第二步，硝化杆菌将亚硝酸根离子氧化成硝酸根离子。硝酸盐是植物根系吸收氮的主要形式。

NO₂⁻ → NO₃⁻

Both groups of nitrifying bacteria require well-oxygenated soil; therefore, ploughing and good soil structure promote nitrification.

两种硝化细菌都需要充氧良好的土壤；因此，翻耕和良好的土壤结构会促进硝化作用。

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7. Ammonification – Decomposition Releases Ammonium | 氨化作用——分解释放铵离子

When plants and animals die, or when animals excrete waste (such as urea and faeces), the organic nitrogen in proteins, DNA and other compounds is returned to the soil. Decomposers – mainly bacteria and fungi – break down these organic remains and release ammonium ions into the soil. This process is called ammonification, or sometimes decomposition in the nitrogen context.

当动植物死亡，或动物排泄废物时，蛋白质、DNA 和其他化合物中的有机氮返回土壤。分解者主要是细菌和真菌，它们分解这些有机残体，向土壤释放铵离子。这个过程称为氨化作用，有时在氮循环语境中就称作分解作用。

Ammonification is essential because it keeps nitrogen cycling within the ecosystem. Without decomposers, nitrogen would be locked up in dead matter and unavailable to plants.

氨化作用至关重要，因为它保持了生态系统中氮的循环。没有分解者，氮将被锁在死亡物质中，无法被植物利用。

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8. Denitrification – Returning Nitrogen to the Atmosphere | 反硝化作用——氮返回大气

Denitrification is the process by which nitrate ions (NO₃⁻) are converted back into nitrogen gas (N₂), which escapes into the atmosphere. This is carried out by denitrifying bacteria, such as Pseudomonas, under anaerobic conditions (in the absence of oxygen).

反硝化作用是将硝酸根离子重新转化为氮气并逸散到大气中的过程。这由反硝化细菌，如假单胞菌，在厌氧条件下完成。

NO₃⁻ → N₂

Waterlogged, compacted or poorly drained soils favour denitrification because oxygen concentrations are low. While denitrification completes the cycle by returning N₂ to the air, it reduces the amount of nitrate available for plant growth, so farmers often drain fields to minimise this loss.

渍水、压实或排水不良的土壤会促进反硝化作用，因为氧气浓度低。虽然反硝化作用通过将 N₂ 送回空气使循环闭合，但它减少了可供植物利用的硝酸盐数量，所以农民通常要排水以减少这种损失。

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9. How Plants and Animals Fit into the Cycle | 植物和动物如何参与氮循环

Plants absorb nitrate ions (and a small amount of ammonium) from the soil through their root hairs by active transport. They use these ions to synthesise amino acids, proteins, nucleic acids and chlorophyll. Nitrogen therefore enters the living component of the ecosystem at the producer level.

植物通过根毛以主动运输的方式从土壤吸收硝酸根离子。它们利用这些离子合成氨基酸、蛋白质、核酸和叶绿素。因此，氮在生产者层次进入生态系统的生物组分。

Animals obtain their nitrogen by eating plants or other animals. They digest proteins and use the amino acids to build their own proteins. Excess amino acids are broken down in the liver to form urea, which is excreted in urine. When organisms die, their tissues are broken down by decomposers, restarting the ammonification stage.

动物通过吃植物或其他动物获得氮。它们消化蛋白质，利用氨基酸构建自身的蛋白质。多余的氨基酸在肝脏中分解形成尿素，随尿液排出。当生物死亡时，它们的组织被分解者分解，重新启动氨化作用阶段。

The feeding relationships therefore move nitrogen through food chains, but the cycle remains driven by microbes at every stage.

因此，捕食关系使氮通过食物链流动，但循环的每一个阶段仍然由微生物驱动。

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10. Human Influences and Eutrophication | 人类影响与富营养化

Human activities can drastically alter the nitrogen cycle. The excessive application of nitrogen-rich fertilisers or untreated manure can lead to the build-up of nitrates in soil. These nitrates are highly soluble and easily leached (washed) out of the soil into rivers, lakes and groundwater.

人类活动可以极大地改变氮循环。过量施用富含氮的化肥或未经处理的粪肥会导致土壤中硝酸盐积聚。硝酸盐极易溶解，容易被淋溶出土壤，进入河流、湖泊和地下水。

In aquatic ecosystems, excess nitrates cause eutrophication. They stimulate rapid growth of algae (algal bloom). The algae block sunlight from reaching submerged plants, which die. Decomposers then break down the dead plants and algae, using up oxygen in the process. This leads to anoxic conditions that kill fish and other aquatic life.

在水生生态系统中，过量的硝酸盐引起富营养化。它们刺激藻类快速生长。藻类阻挡阳光到达沉水植物，导致它们死亡。分解者随后分解死亡的植物和藻类，消耗水中的氧气。这导致缺氧环境，使鱼类和其他水生生物死亡。

To reduce such impacts, farmers can use crop rotation with legumes, apply fertilisers only when crops are actively growing, and maintain buffer strips of vegetation near watercourses to absorb excess nitrates.

为了减少这些影响，农民可以采用豆科植物轮作，仅在作物活跃生长时施肥，并在水道附近保持植被缓冲带以吸收多余的硝酸盐。

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11. Summary Table of Key Bacteria and Their Functions | 关键细菌及其功能汇总

The table below brings together the main groups of microorganisms responsible for driving the nitrogen cycle. Remember, exam questions often ask you to name

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