The Nitrogen Cycle | 氮循环考点精讲

📚 The Nitrogen Cycle | 氮循环考点精讲

Nitrogen is an essential element for all living organisms, forming the backbone of amino acids, proteins, and nucleic acids such as DNA and RNA. Although the atmosphere is about 78% nitrogen gas (N₂), this abundant form is unavailable to most organisms due to the strength of the triple covalent bond. The nitrogen cycle describes the series of processes by which nitrogen is converted between its various chemical forms, moving through the atmosphere, soil, water, and living organisms. Understanding these transformations—nitrogen fixation, nitrification, assimilation, ammonification, and denitrification—is a core requirement for A-Level WJEC Biology, highlighting the crucial role of microorganisms in maintaining ecosystem fertility.

氮是所有生物必需的营养元素，是氨基酸、蛋白质和 DNA、RNA 等核酸的基本组分。尽管大气中约有 78% 的氮气（N₂），但由于其键能较高的三键结构，绝大多数生物无法直接利用这种丰富的气体。氮循环描述了一系列将氮在不同化学形态之间转化的过程，氮元素在大气、土壤、水体和生物体内循环。理解固氮作用、硝化作用、同化作用、氨化作用和反硝化作用等转化过程是 A-Level WJEC 生物学的重要考点，突显出微生物在维持生态系统肥力中的关键作用。

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

The nitrogen cycle can be viewed as a closed loop connecting four main reservoirs: atmospheric N₂, ammonium ions (NH₄⁺), nitrate ions (NO₃⁻), and organic nitrogen in living organisms. The key stages are fixation (conversion of N₂ to ammonia/ammonium), nitrification (oxidation of ammonium to nitrite then nitrate), assimilation (uptake of ammonium or nitrate by producers to synthesise organic molecules), ammonification (decomposition of organic nitrogen back to ammonium), and denitrification (reduction of nitrate to N₂ gas). Each step is mediated by specific groups of bacteria and fungi, many of which are free‑living in soil or form symbiotic relationships with plants.

氮循环可视为连接四个主要库的闭合回路：大气中的 N₂、铵根离子（NH₄⁺）、硝酸根离子（NO₃⁻）以及生物体内的有机氮。关键阶段包括固氮作用（将 N₂ 转化为氨/铵）、硝化作用（铵氧化为亚硝酸盐再氧化为硝酸盐）、同化作用（生产者吸收铵或硝酸盐以合成有机分子）、氨化作用（有机氮分解回到铵）和反硝化作用（硝酸盐还原为 N₂ 气体）。每一步都由特定的细菌和真菌类群介导，其中许多自由生活在土壤中或与植物形成共生关系。

2. Nitrogen Fixation | 固氮作用

Nitrogen fixation is the reduction of unreactive atmospheric nitrogen gas to ammonia (NH₃), which rapidly ionises to ammonium (NH₄⁺) in soil water. The overall reaction catalysed by the enzyme nitrogenase is N₂ + 8 H⁺ + 8 e⁻ → 2 NH₃ + H₂. This demanding process requires a large input of ATP and an anaerobic environment to protect the oxygen‑sensitive nitrogenase enzyme. Fixation can occur through high‑energy natural events, such as lightning strikes, which provide enough energy to break the N≡N bond, forming nitrogen oxides that dissolve in rainwater as dilute nitric acid. However, the vast majority of fixation is biological, carried out by diazotrophs—free‑living soil bacteria such as Azotobacter and symbiotic bacteria like Rhizobium in root nodules of legumes.

固氮作用是将不活泼的大气氮气还原为氨（NH₃），氨在土壤水中迅速离子化为铵（NH₄⁺）。由固氮酶催化的总反应为 N₂ + 8 H⁺ + 8 e⁻ → 2 NH₃ + H₂。该过程需要大量 ATP 输入，并且必须在厌氧环境中进行，以保护对氧敏感的固氮酶。高能量的自然事件，如闪电放电，能够提供足够能量断裂 N≡N 三键，形成氮氧化物，这些氮氧化物溶于雨水成为稀硝酸。但绝大多数的固氮是生物固氮，由固氮菌完成——包括自由生活的土壤细菌如 Azotobacter，以及豆科植物根瘤中的共生细菌如 Rhizobium。

3. Nitrification | 硝化作用

Nitrification is the two‑step aerobic oxidation of ammonium ions to nitrate ions, carried out by chemolithotrophic bacteria in the soil. The first step is performed by nitrifying bacteria such as Nitrosomonas, which oxidise ammonium to nitrite:

2 NH₄⁺ + 3 O₂ → 2 NO₂⁻ + 2 H₂O + 4 H⁺

硝化作用是铵离子经两步好氧氧化生成硝酸根离子的过程，由土壤中的化能自养细菌完成。第一步由 Nitrosomonas 等硝化细菌执行，将铵氧化为亚硝酸盐：

2 NH₄⁺ + 3 O₂ → 2 NO₂⁻ + 2 H₂O + 4 H⁺

The second step is completed by Nitrobacter, which oxidise nitrite to nitrate:

2 NO₂⁻ + O₂ → 2 NO₃⁻

第二步由 Nitrobacter 完成，将亚硝酸盐氧化为硝酸盐：

2 NO₂⁻ + O₂ → 2 NO₃⁻

Nitrification requires well‑aerated soils because both groups are obligate aerobes. The product nitrate is highly soluble and readily leached from soil, but it is also the main form of nitrogen absorbed by plant roots.

硝化作用需要通气良好的土壤，因为这两类细菌都是专性好氧菌。产物硝酸盐极易溶解，易从土壤中淋失，但也是植物根系吸收的主要氮形式。

4. Assimilation | 同化作用

Assimilation refers to the process by which primary producers absorb inorganic nitrogen compounds—mainly nitrate ions (NO₃⁻) but also some ammonium ions (NH₄⁺)—through their root systems and incorporate them into organic molecules. Inside plant cells, nitrate is first reduced back to ammonium via nitrate and nitrite reductases before being combined with carbon skeletons to synthesise amino acids, nucleotides, chlorophyll, and other nitrogenous compounds. Consumers obtain their organic nitrogen by feeding on plants or other animals. This stage effectively transfers nitrogen from the abiotic reservoir into the living biomass of ecosystems.

同化作用是指初级生产者通过根系吸收无机氮化物（主要是硝酸根离子 NO₃⁻，也有部分铵离子 NH₄⁺），并将其掺入有机分子的过程。在植物细胞内，硝酸盐经硝酸还原酶和亚硝酸还原酶还原为铵，随后与碳骨架结合，合成氨基酸、核苷酸、叶绿素和其他含氮化合物。消费者通过取食植物或其他动物获得有机氮。这一阶段将氮从非生物库转移到生态系统的生物量中。

5. Ammonification | 氨化作用

Ammonification is the decomposition of organic nitrogen from dead organisms, animal waste, and shed plant material back into ammonium ions. Saprobiotic microorganisms—mainly bacteria and fungi—release extracellular enzymes that break down proteins, nucleic acids, and urea. Deamination of amino acids removes the amino group as ammonia, which dissolves in soil water to form NH₄⁺. This process recycles nitrogen within the ecosystem, making it available again for nitrification or direct uptake by plants and microorganisms. Ammonification occurs under both aerobic and anaerobic conditions and is accelerated by warm, moist environments.

氨化作用是将死生物、动物排泄物和凋落物中的有机氮分解回铵离子的过程。腐生微生物——主要是细菌和真菌——释放胞外酶分解蛋白质、核酸和尿素。氨基酸的脱氨基作用将氨基以氨的形式移除，氨溶于土壤水形成 NH₄⁺。这一过程在生态系统内循环氮素，使其可再次用于硝化作用或被植物和微生物直接吸收。氨化作用在好氧和厌氧条件下均可发生，温暖潮湿的环境会加速其进程。

6. Denitrification | 反硝化作用

Denitrification is the anaerobic reduction of nitrate (NO₃⁻) to nitrogen gas (N₂), which returns to the atmosphere and completes the cycle. This process is carried out by facultative anaerobic bacteria, notably species of Pseudomonas and Thiobacillus, which use nitrate as a terminal electron acceptor in respiration when oxygen is scarce. A simplified reaction is:

2 NO₃⁻ + 10 e⁻ + 12 H⁺ → N₂ + 6 H₂O

反硝化作用是在厌氧条件下将硝酸盐（NO₃⁻）还原为氮气（N₂）的过程，氮气返回大气完成循环。该过程由兼性厌氧细菌完成，尤其是 Pseudomonas 和 Thiobacillus 属的种类，它们在氧气匮乏时用硝酸盐作为呼吸作用的末端电子受体。简化的反应为：

2 NO₃⁻ + 10 e⁻ + 12 H⁺ → N₂ + 6 H₂O

Denitrification occurs in waterlogged soils, poorly drained fields, and deep aquatic sediments. While it prevents the accumulation of nitrate, excessive denitrification can deplete soil fertility and contribute to the loss of biologically available nitrogen from agricultural land.

反硝化作用发生在渍水土壤、排水不良的田地和深层水体沉积物中。它既能防止硝酸盐过度积累，但过度的反硝化也会耗尽土壤肥力，导致农田中有效氮的流失。

7. Symbiotic Nitrogen Fixation in Legumes | 豆科植物的共生固氮

A highly efficient form of biological nitrogen fixation occurs in the root nodules of leguminous plants such as peas, beans, and clover. The gram‑negative bacterium Rhizobium infects root hairs and induces the formation of nodules, where the bacteria differentiate into bacteroids capable of nitrogenase activity. The plant provides the bacteria with carbohydrates as an energy source and oxygen‑scavenging leghaemoglobin, which maintains a low oxygen tension essential for nitrogenase function. In return, the bacteria supply the plant with fixed nitrogen in the form of ammonia. This mutualistic relationship is of great agricultural importance: farmers often rotate crops with legumes or plough them in as green manure to replenish soil nitrogen naturally, reducing the need for synthetic fertilisers.

在豌豆、大豆和三叶草等豆科植物的根瘤中发生着一种极为高效的生物固氮形式。革兰氏阴性菌 Rhizobium 侵染根毛并诱导根瘤形成，细菌在根瘤内分化为能够进行固氮酶活动的类菌体。植物为细菌提供碳水化合物作为能源，并合成除氧的豆血红蛋白，维持固氮酶所必需的低氧分压。作为回报，细菌以氨的形式为植物提供固定态氮。这种互利共生关系在农业上有重要意义：农民常与豆科植物轮作或将其翻入土中作为绿肥，自然补充土壤氮素，减少合成肥料的使用。

8. Key Microorganisms in the Nitrogen Cycle | 氮循环关键微生物

The table below summarises the major groups of microbes driving the nitrogen cycle, their roles, and the conditions they require.

下表总结了驱动氮循环的主要微生物类群、它们的角色及所需条件。

Process	Microorganisms	Conditions	Key Reaction
Nitrogen fixation	Rhizobium (symbiotic), Azotobacter (free‑living)	Aerobic / microaerobic; low O₂ in nodules	N₂ → NH₃/NH₄⁺
Nitrification	Nitrosomonas, Nitrobacter	Aerobic	NH₄⁺ → NO₂⁻ → NO₃⁻
Ammonification	Saprophytic bacteria and fungi	Aerobic / anaerobic	Organic N → NH₄⁺
Denitrification	Pseudomonas, Thiobacillus	Anaerobic	NO₃⁻ → N₂

These microorganisms are crucial for maintaining the balance of nitrogen in the environment. Their activities are influenced by soil pH, temperature, oxygen concentration, and the availability of organic matter.

这些微生物对于维持环境中的氮平衡至关重要。它们的活性受土壤 pH、温度、氧浓度和有机物可利用性的影响。

9. Human Impact: Eutrophication and Nitrogen Pollution | 人类影响：富营养化与氮污染

Human activities have dramatically altered the nitrogen cycle, primarily through the manufacture and application of inorganic nitrogenous fertilisers and the combustion of fossil fuels. When excess nitrate‑based fertilisers are applied to agricultural land, heavy rainfall can leach the soluble nitrates into nearby rivers and lakes, leading to eutrophication. The sequence of events includes:

Rapid algal bloom stimulated by high nitrate and phosphate levels.
Algae block sunlight, causing submerged aquatic plants to die.
When the algae die, their decomposition by aerobic saprobionts consumes dissolved oxygen in the water.
Reduced oxygen concentration (hypoxia) leads to the death of fish and other aerobic organisms.
Anaerobic bacteria thrive, producing toxic by‑products such as hydrogen sulfide.

人类活动已显著改变了氮循环，主要是通过无机氮肥的生产施用以及化石燃料的燃烧。当过量的硝酸盐基肥料施用于农田后，强降雨可将可溶性硝酸盐淋溶进入附近河流和湖泊，引发富营养化。事件序列包括：

高浓度硝酸盐和磷酸盐刺激藻类大量繁殖（藻华）。
藻类遮蔽阳光，导致沉水植物死亡。
藻类死亡后，好氧腐生生物分解它们，消耗水中溶解氧。
溶解氧浓度下降（缺氧）导致鱼类和其他好氧生物死亡。
厌氧细菌大量繁殖，产生硫化氢等有毒副产物。

Furthermore, denitrification in waterlogged fertilised soils can release nitrous oxide (N₂O), a potent greenhouse gas. Managing the nitrogen cycle sustainably is therefore a major environmental challenge.

此外，在渍水且施用过化肥的土壤中，反硝化作用会释放氧化亚氮（N₂O），一种强效温室气体。因此，可持续管理氮循环是一项重大的环境挑战。

10. Key Terms and Definities | 关键术语与定义

Nitrogen fixation – Conversion of atmospheric N₂ into ammonia/ammonium by lightning or diazotrophic bacteria.

固氮作用 – 通过闪电或固氮细菌将大气中的 N₂ 转化为氨/铵。

Nitrification – Two‑stage aerobic oxidation of ammonium to nitrite (by Nitrosomonas) and nitrite to nitrate (by Nitrobacter).

硝化作用 – 铵经两步好氧氧化为亚硝酸盐（Nitrosomonas），再氧化为硝酸盐（Nitrobacter）。

Assimilation – Uptake of nitrate or ammonium by plants and incorporation into organic compounds.

同化作用 – 植物吸收硝酸盐或铵，并将其转化为有机化合物。

Ammonification – Decomposition of organic nitrogen to ammonium by saprobionts.

氨化作用 – 腐生生物将有机氮分解为铵。

Denitrification – Anaerobic reduction of nitrate to N₂ gas by bacteria such as Pseudomonas.

反硝化作用 – 在厌氧条件下，Pseudomonas 等细菌将硝酸盐还原为氮气。

Leghaemoglobin – Oxygen‑binding protein in legume root nodules that maintains low O₂ concentration for nitrogenase.

豆血红蛋白 – 豆科植物根瘤中的携氧蛋白，为固氮酶维持低氧环境。

Eutrophication – Nutrient enrichment of water bodies leading to algal blooms and oxygen depletion.

富营养化 – 水体营养盐富集导致藻华和缺氧的现象。

Mastering these terms and the connections between processes will ensure excellent performance on WJEC nitrogen‑cycle questions, which often require linking microbial actions to ecosystem functioning and agricultural impacts.

掌握这些术语及过程间的联系，能够确保在 WJEC 氮循环考题中表现出色，此类题目常要求将微生物活动与生态系统功能和农业影响相联系。

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The Nitrogen Cycle | 氮循环考点精讲