The Nitrogen Cycle | 氮循环

📚 The Nitrogen Cycle | 氮循环

All living organisms require nitrogen to synthesise proteins, nucleic acids and other essential biomolecules. Although Earth’s atmosphere is 78% nitrogen gas (N₂), this form is unavailable to most organisms. The nitrogen cycle describes the series of processes by which nitrogen is converted between its various chemical forms, making it accessible to living systems and eventually returning it to the atmosphere. In the CCEA A-Level Biology specification, you are expected to understand the key stages of the cycle, the roles of specific microorganisms, and the importance of nitrogen in ecosystems and agriculture.

所有生物都需要氮来合成蛋白质、核酸和其他必需的生物分子。尽管地球大气中 78% 是氮气 (N₂)，但这种形式的氮大多数生物无法直接利用。氮循环描述了氮在不同化学形态之间转化的一系列过程，使其能被生命系统利用并最终返回大气。在 CCEA A-Level 生物考试大纲中，你需要理解循环的关键阶段、特定微生物的作用，以及氮在生态系统和农业中的重要性。

1. Overview of the Nitrogen Cycle | 氮循环概述

The nitrogen cycle is a biogeochemical cycle that moves nitrogen through the atmosphere, lithosphere, hydrosphere and biosphere. It mainly involves four key transformations: nitrogen fixation, ammonification, nitrification and denitrification. Each step is driven by different groups of microorganisms under specific aerobic or anaerobic conditions. Understanding the cycle is essential for explaining nutrient availability, soil fertility and the impact of human activities such as fertiliser use.

氮循环是一个生物地球化学循环，在大气、岩石圈、水圈和生物圈之间转移氮元素。它主要包含四个关键转化过程：固氮作用、氨化作用、硝化作用和反硝化作用。每一步都由不同微生物群体在特定的有氧或厌氧条件下驱动。理解这一循环对于解释养分有效性、土壤肥力以及施肥等人类活动的影响至关重要。

2. Nitrogen Fixation | 固氮作用

Nitrogen fixation is the process of converting atmospheric nitrogen gas (N₂) into ammonia (NH₃) or ammonium ions (NH₄⁺), which can be assimilated by plants. Since the triple bond in N₂ is extremely strong, this reaction requires a large amount of energy. Fixation can occur through biological, industrial, or atmospheric routes, but in natural ecosystems, biological fixation by prokaryotes dominates.

固氮作用是将大气中的氮气 (N₂) 转化为可被植物吸收的氨 (NH₃) 或铵根离子 (NH₄⁺) 的过程。由于 N₂ 中的三键非常稳固，该反应需要大量能量。固氮可通过生物、工业或大气途径发生，但在自然生态系统中，由原核生物进行的生物固氮占主导地位。

Biological nitrogen fixation is carried out by nitrogen-fixing bacteria. Free-living soil bacteria such as Azotobacter and Clostridium (anaerobic) can fix nitrogen independently. Symbiotic bacteria of the genus Rhizobium invade the root hairs of leguminous plants (e.g., peas, clover) and form root nodules, where they obtain carbohydrates from the plant and supply fixed nitrogen in return. Blue-green algae (cyanobacteria) such as Anabaena can fix nitrogen in aquatic environments and in symbiotic associations with certain ferns.

生物固氮由固氮细菌完成。自由生活的土壤细菌，如 Azotobacter（好氧）和 Clostridium（厌氧），能独立固氮。共生细菌如根瘤菌属 (Rhizobium) 侵入豆科植物（如豌豆、三叶草）的根毛并形成根瘤，它们在根瘤中从植物获取碳水化合物，并反过来提供固定好的氮。蓝绿藻（蓝细菌）如 Anabaena 可在水生环境以及与某些蕨类植物的共生关系中固氮。

The overall reaction catalysed by the enzyme nitrogenase can be summarised as:

N₂ + 8H⁺ + 8e⁻ + 16 ATP → 2NH₃ + H₂ + 16 ADP + 16 Pᵢ

This reaction is highly endergonic and requires ATP derived from respiration or photosynthesis, along with electrons supplied by reduced ferredoxin or flavodoxin.

由固氮酶催化的总反应可概括为：

N₂ + 8H⁺ + 8e⁻ + 16 ATP → 2NH₃ + H₂ + 16 ADP + 16 Pᵢ

该反应高度吸能，需要呼吸作用或光合作用提供的 ATP，以及由还原态铁氧还蛋白或黄素氧还蛋白供应的电子。

3. Ammonification | 氨化作用

Ammonification, also known as decomposition, is the process by which organic nitrogen in dead organisms, faeces and urea is converted into ammonium ions (NH₄⁺) by decomposers. Saprobiotic bacteria and fungi secrete extracellular enzymes to break down proteins, nucleic acids and other nitrogenous compounds. The amino acids released are further deaminated, liberating ammonia. In soil solution, ammonia reacts with hydrogen ions to form ammonium, which can be taken up by plant roots or used by nitrifying bacteria.

氨化作用，亦称分解作用，是指将死亡生物体、粪便和尿素中的有机氮通过分解者转化为铵根离子 (NH₄⁺) 的过程。腐生细菌和真菌分泌胞外酶来分解蛋白质、核酸及其他含氮化合物。释放出的氨基酸进一步脱氨基，释放出氨。在土壤溶液中，氨与氢离子反应生成铵根离子，铵根离子可被植物根系吸收或被硝化细菌利用。

The rate of ammonification depends on soil temperature, moisture, oxygen availability and the carbon-to-nitrogen ratio of the organic matter. This process is crucial because it recycles nitrogen trapped in dead biomass back into the soil, making it available for plants and other organisms.

氨化作用的速率取决于土壤温度、湿度、氧气供应以及有机物质的碳氮比。这一过程至关重要，因为它将死亡生物质中固定的氮重新释放回土壤，使其可被植物和其他生物利用。

4. Nitrification | 硝化作用

Nitrification is the aerobic oxidation of ammonium ions (NH₄⁺) to nitrite (NO₂⁻) and then to nitrate (NO₃⁻). It is performed by two specialised groups of chemoautotrophic bacteria that derive energy from the oxidation of inorganic nitrogen compounds.

硝化作用是在有氧条件下将铵根离子 (NH₄⁺) 氧化为亚硝酸根 (NO₂⁻)，进而氧化为硝酸根 (NO₃⁻) 的过程。它由两类专门的化能自养细菌完成，这些细菌通过氧化无机氮化合物获取能量。

Ammonia-oxidising bacteria such as Nitrosomonas convert ammonium to nitrite:
2 NH₄⁺ + 3 O₂ → 2 NO₂⁻ + 2 H₂O + 4 H⁺ + energy
Nitrite-oxidising bacteria such as Nitrobacter convert nitrite to nitrate:
2 NO₂⁻ + O₂ → 2 NO₃⁻ + energy

对应的中文：

氨氧化细菌，如 Nitrosomonas，将铵根转化为亚硝酸根：
2 NH₄⁺ + 3 O₂ → 2 NO₂⁻ + 2 H₂O + 4 H⁺ + 能量
亚硝酸氧化细菌，如 Nitrobacter，将亚硝酸根转化为硝酸根：
2 NO₂⁻ + O₂ → 2 NO₃⁻ + 能量

Nitrification requires well-aerated soils because both groups are obligate aerobes. Nitrate is the most readily absorbed form of nitrogen for most plants, so nitrification is vital for soil fertility. However, nitrate is highly soluble and can leach into groundwater, causing environmental problems such as eutrophication.

硝化作用需要通气良好的土壤，因为这两类细菌均为专性好氧菌。硝酸根是大多数植物最容易吸收的氮形态，因此硝化作用对土壤肥力至关重要。但硝酸根极易溶解，可能淋溶进入地下水，引起富营养化等环境问题。

5. Denitrification | 反硝化作用

Denitrification is the anaerobic reduction of nitrate (NO₃⁻) to nitrogen gas (N₂), which returns to the atmosphere. This process is carried out by facultative anaerobic bacteria, such as Pseudomonas and Thiobacillus, which use nitrate as a terminal electron acceptor in respiration when oxygen is absent.

反硝化作用是在厌氧条件下将硝酸根 (NO₃⁻) 还原为氮气 (N₂) 并返回大气的过程。它由兼性厌氧细菌完成，如 Pseudomonas 和 Thiobacillus，它们在无氧条件下利用硝酸根作为呼吸作用的最终电子受体。

The denitrification pathway involves several intermediates: NO₃⁻ → NO₂⁻ → NO → N₂O → N₂. While the final product is harmless nitrogen gas, the intermediate nitrous oxide (N₂O) is a potent greenhouse gas.

反硝化途径包含多个中间体：NO₃⁻ → NO₂⁻ → NO → N₂O → N₂。虽然最终产物是无害的氮气，但中间产物一氧化二氮 (N₂O) 是一种强效温室气体。

Denitrification tends to occur in waterlogged, compacted soils or deep sediments where oxygen diffusion is limited. It can lead to a loss of soil nitrogen that would otherwise be available to plants, therefore reducing agricultural productivity.

反硝化作用通常发生在渍水、板结的土壤或深层沉积物中，这些环境氧气扩散受限。它可能导致土壤中原本可供植物利用的氮素流失，从而降低农业生产力。

6. Role of Microorganisms in the Nitrogen Cycle | 微生物在氮循环中的角色

Microorganisms are the driving force behind every major transformation in the nitrogen cycle. No eukaryotic organism can fix nitrogen or perform nitrification or denitrification. The table below summarises the key microbial groups and their roles.

微生物是氮循环中每一个主要转化过程的驱动力量。没有任何真核生物能固氮、硝化或反硝化。下表总结了关键的微生物类群及其作用。

Process 过程	Microorganisms 微生物	Conditions 条件
Nitrogen fixation 固氮	Rhizobium, Azotobacter, Anabaena	Aerobic or microaerophilic
Ammonification 氨化	Saprobiotic bacteria and fungi	Aerobic & anaerobic
Nitrification 硝化	Nitrosomonas, Nitrobacter	Aerobic
Denitrification 反硝化	Pseudomonas, Thiobacillus	Anaerobic

The mutualistic relationship between legumes and Rhizobium is a classic example of symbiosis examined in CCEA specifications. The bacteria differentiate into bacteroids within nodule cells and synthesise nitrogenase, while the plant provides organic acids as energy sources and leghemoglobin to buffer oxygen levels.

豆科植物与根瘤菌之间的互利关系是 CCEA 大纲中考察的经典共生案例。细菌在根瘤细胞中分化为类菌体并合成固氮酶，而植物则提供有机酸作为能量来源，并提供豆血红蛋白以调节氧气浓度。

7. The Haber Process and Industrial Fixation | 哈伯法与工业固氮

In addition to biological fixation, humans fix nitrogen industrially through the Haber-Bosch process, in which N₂ and H₂ are reacted at high temperature and pressure with an iron catalyst to produce ammonia: N₂ + 3 H₂ ⇌ 2 NH₃. This industrial ammonia is used to manufacture nitrate fertilisers, urea and other nitrogenous chemicals. The Haber process has dramatically increased global food production, but it has also doubled the amount of reactive nitrogen cycling through ecosystems, causing pollution and biodiversity loss.

除生物固氮外，人类还通过哈伯-博斯法进行工业固氮。该法在高温、高压和铁催化剂作用下，使 N₂ 与 H₂ 反应生成氨：N₂ + 3 H₂ ⇌ 2 NH₃。工业合成的氨用于生产硝酸盐肥料、尿素和其他含氮化学品。哈伯法极大地提高了全球粮食产量，但也使通过生态系统循环的活性氮量翻倍，导致污染和生物多样性丧失。

8. Human Impacts on the Nitrogen Cycle | 人类对氮循环的影响

Human activities have profoundly altered the nitrogen cycle. The main impacts include:

Fertiliser overuse: Excessive nitrate from fertilisers leaches into water bodies, causing eutrophication. Algal blooms deplete dissolved oxygen, creating dead zones.
Fossil fuel combustion: High-temperature burning releases nitrogen oxides (NOₓ), which contribute to acid rain and photochemical smog.
Deforestation and land clearance: Removal of vegetation disrupts the balance between nitrogen uptake and release, increasing nitrate runoff.
Livestock farming: Manure and urine increase local ammonium and nitrate levels, raising the risk of denitrification and N₂O emission.

人类活动深刻改变了氮循环。主要影响包括：

肥料过量使用：肥料中过量的硝酸盐淋溶进入水体，引发富营养化。藻类大量繁殖消耗溶解氧，形成死亡区。
化石燃料燃烧：高温燃烧释放氮氧化物 (NOₓ)，导致酸雨和光化学烟雾。
砍伐森林与土地开垦：植被移除破坏了氮吸收与释放之间的平衡，增加硝酸盐流失。
畜牧业：粪便和尿液使局部环境中铵根和硝酸根浓度升高，增加反硝化作用和 N₂O 排放的风险。

Understanding these impacts is important for evaluating sustainable farming practices, such as crop rotation with legumes to restore soil nitrogen naturally, and precision fertiliser application to minimise runoff.

理解这些影响对于评估可持续农业实践至关重要，例如通过与豆科植物轮作以自然恢复土壤氮素，以及精准施肥以减少径流。

9. The Nitrogen Cycle in Aquatic Systems | 水生系统中的氮循环

In freshwater and marine ecosystems, the nitrogen cycle follows similar transformations but with some distinct features. Cyanobacteria are the primary nitrogen fixers in aquatic environments, especially in oligotrophic lakes and oceans. In the water column, ammonium and nitrate are rapidly assimilated by phytoplankton and aquatic plants. Decomposition and ammonification occur in the water column and sediments, regenerating ammonium. Nitrification takes place in oxygenated waters, while denitrification occurs in anoxic sediments, where organic matter accumulates.

在淡水和海洋生态系统中，氮循环经历相似的转化过程，但具有一些独特的特点。蓝细菌是水生环境中主要的固氮生物，特别是在贫营养湖泊和海洋中。在水体中，铵根和硝酸盐被浮游植物和水生植物迅速同化。分解和氨化作用在水体和沉积物中进行，重新生成铵根。硝化作用发生在含氧的水体中，而反硝化作用则发生在缺氧的沉积物中，那里有机物积累。

In estuaries and coastal zones, high nutrient inputs from rivers can lead to severe eutrophication. An understanding of aquatic nitrogen cycling helps explain the formation of hypoxic zones, fisheries decline, and harmful algal blooms.

在河口和沿海区域，河流带来的高营养盐输入可导致严重的富营养化。了解水生氮循环有助于解释低氧区形成、渔业衰退和有害藻华等现象。

10. Nitrogen Assimilation and Fate in Plants | 植物对氮的同化与去向

Plants absorb nitrogen mainly in the form of nitrate (NO₃⁻) through their roots, although ammonium (NH₄⁺) can also be taken up directly. Once inside the root cells, nitrate is reduced to nitrite by nitrate reductase and then to ammonium by nitrite reductase. The ammonium produced, along with any directly absorbed ammonium, is quickly incorporated into amino acids via the glutamine synthetase–glutamate synthase (GS-GOGAT) pathway to avoid toxicity.

植物主要通过根系吸收硝酸根 (NO₃⁻) 形式的氮，但也可直接吸收铵根 (NH₄⁺)。进入根细胞后，硝酸根在硝酸还原酶作用下还原为亚硝酸根，再由亚硝酸还原酶还原为铵根。生成的铵根以及直接吸收的铵根通过谷氨酰胺合成酶-谷氨酸合酶 (GS-GOGAT) 途径迅速掺入氨基酸以避免毒性。

Excess nitrate can be stored in vacuoles or transported to shoots. When plants are consumed by herbivores, the organic nitrogen enters the food chain. Upon death and excretion, the nitrogen is returned to the soil via ammonification, closing the cycle.

多余的硝酸根可储存在液泡中或转运至地上部分。当植物被植食动物取食时，有机氮进入食物链。在死亡和排泄后，氮通过氨化作用回归土壤，完成循环。

11. Nitrogen Cycle in Soil Fertility and Crop Rotation | 氮循环与土壤肥力及作物轮作

Soil nitrogen content directly influences plant growth. Farmers manage the nitrogen cycle through practices like crop rotation, cover cropping and the application of organic manures. Leguminous plants are often included in rotations because their symbiosis with Rhizobium replenishes soil nitrogen naturally, reducing the need for synthetic fertilisers.

土壤氮含量直接影响植物生长。农民通过作物轮作、覆盖作物种植和施用有机肥料来管理氮循环。豆科植物通常被纳入轮作中，因为它们与根瘤菌的共生关系可自然补充土壤氮素，减少对合成肥料的需求。

Crop rotation sequences such as maize → soybean → wheat break pest cycles, improve soil structure and balance the removal and addition of nitrogen. Cover crops like clover or vetch are ploughed back as green manure, releasing ammonium through decomposition for subsequent crops.

轮作序列如玉米 → 大豆 → 小麦可打破病虫害周期、改善土壤结构，并平衡氮素的移除与补充。三叶草或野豌豆等覆盖作物作为绿肥翻压入土，通过分解为后续作物释放铵根。

12. Key Exam Points Summary | 考试要点总结

Be able to name and describe the four main stages: nitrogen fixation, ammonification, nitrification, denitrification.
Know the specific bacteria involved in each stage and whether their metabolism is aerobic or anaerobic.
Understand the Haber process as an industrial parallel to biological fixation and its environmental consequences.
Explain how human activities (fertilisers, combustion, agriculture) disrupt the nitrogen cycle and lead to eutrophication and acid rain.
Link the nitrogen cycle to productivity, food webs and sustainability.

能够说出并描述四个主要阶段：固氮、氨化、硝化、反硝化。
了解每个阶段涉及的具体细菌及其代谢是有氧还是厌氧。
理解哈伯法作为生物固氮的工业对应过程及其环境后果。
解释人类活动（肥料、燃烧、农业）如何干扰氮循环并导致富营养化和酸雨。
将氮循环与生产力、食物网和可持续性联系起来。

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The Nitrogen Cycle | 氮循环