The Nitrogen Cycle | 氮循环考点精讲

📚 The Nitrogen Cycle | 氮循环考点精讲

For AQA A-Level Biology, the nitrogen cycle is a core topic that explains how nitrogen moves between the atmosphere, soil, and living organisms. This article breaks down every key process, the essential microorganisms involved, and common exam pitfalls.

在 AQA A-Level 生物课程中，氮循环是阐释氮元素如何在空气、土壤与生物体之间迁移的核心主题。本文拆解每一个关键过程、涉及的关键微生物，以及常见考试失分点。

1. Why Nitrogen is Essential | 为什么氮是必需的

Nitrogen is required to synthesise proteins, nucleic acids (DNA and RNA), ATP, and chlorophyll. Despite making up 78% of the atmosphere as N₂ gas, this form is inert and inaccessible to most organisms. Living cells need nitrogen in a ‘fixed’ form – as ammonium (NH₄⁺) or nitrate (NO₃⁻) – to build biological molecules.

氮元素是合成蛋白质、核酸（DNA 和 RNA）、ATP 及叶绿素的必需成分。尽管大气中 78% 都是氮气（N₂），这种形态却高度稳定，大多数生物无法直接利用。活细胞需要“固定态”的氮——即铵离子（NH₄⁺）或硝酸根（NO₃⁻）——才能构建生物分子。

The cycling of nitrogen between these forms depends on a series of microbial transformations, making the nitrogen cycle a perfect illustration of nutrient recycling in ecosystems.

氮在不同形态间的循环依赖一系列微生物转化过程，这使氮循环成为生态系统中营养物再循环的完美例证。

2. Overview of the Nitrogen Cycle | 氮循环概览

The nitrogen cycle comprises five main processes: nitrogen fixation, nitrification, assimilation, ammonification, and denitrification. These processes convert nitrogen between atmospheric N₂, ammonium, nitrite (NO₂⁻), nitrate, and organic nitrogen in biomass.

氮循环包含五个主要过程：固氮作用、硝化作用、同化作用、氨化作用和反硝化作用。这些过程将氮在大气 N₂、铵、亚硝酸盐（NO₂⁻）、硝酸盐以及生物质中的有机氮之间进行转化。

In AQA exams, you must be able to describe each transformation, name the responsible microorganisms, and state whether conditions are aerobic or anaerobic.

在 AQA 考试中，你必须能够描述每一种转化过程，说出负责的微生物名称，并指明条件是需氧还是厌氧。

3. Nitrogen Fixation | 固氮作用

Nitrogen fixation is the conversion of nitrogen gas (N₂) into ammonia (NH₃) or ammonium ions (NH₄⁺). This can occur through biological fixation, industrial fixation, or atmospheric fixation (lightning).

固氮作用是将氮气（N₂）转化为氨（NH₃）或铵离子（NH₄⁺）的过程。它可以经由生物固氮、工业固氮或大气固氮（闪电）三种途径实现。

Biological fixation is carried out by free-living soil bacteria such as Azotobacter, and by symbiotic bacteria like Rhizobium, which live inside root nodules of leguminous plants. These bacteria possess the enzyme nitrogenase, which catalyses the reduction of N₂ to NH₃. This reaction requires ATP and works best under anaerobic conditions – legume root nodules contain a pinkish oxygen-binding protein called leghaemoglobin to maintain low O₂ levels.

生物固氮由自由生活的土壤细菌（如固氮菌 Azotobacter）以及共生细菌（如根瘤菌 Rhizobium，它们生活在豆科植物根瘤内）完成。这些细菌含有固氮酶，可催化 N₂ 还原为 NH₃。该反应需要 ATP，且在厌氧条件下效率最高——豆科植物根瘤中含有淡红色的氧结合蛋白——豆血红蛋白，以维持低氧环境。

Industrial nitrogen fixation (the Haber process) produces ammonia fertilisers from N₂ and H₂ at high temperature and pressure. Lightning provides enough energy to combine N₂ and O₂ to form nitrogen oxides, which dissolve in rainwater to produce nitrates.

工业固氮（哈伯法）在高温高压下由 N₂ 和 H₂ 合成氨肥。闪电提供足够能量使 N₂ 与 O₂ 结合形成氮氧化物，这些氧化物溶于雨水产生硝酸盐。

4. Nitrification | 硝化作用

Nitrification is a two-stage aerobic process that oxidises ammonium ions to nitrite and then to nitrate. It is performed by chemoautotrophic bacteria that obtain energy from these oxidation reactions.

硝化作用是一个两步需氧过程，将铵离子氧化为亚硝酸盐，再氧化为硝酸盐。它由化能自养细菌完成，它们从这些氧化反应中获取能量。

First, Nitrosomonas bacteria oxidise ammonium to nitrite:

首先，亚硝酸细菌（Nitrosomonas）将铵氧化成亚硝酸盐：

2NH₄⁺ + 3O₂ → 2NO₂⁻ + 2H₂O + 4H⁺

Second, Nitrobacter bacteria oxidise nitrite to nitrate:

接着，硝酸细菌（Nitrobacter）将亚硝酸盐氧化成硝酸盐：

2NO₂⁻ + O₂ → 2NO₃⁻

Both steps require oxygen. This is why well-aerated soils favour nitrification, and waterlogged soils limit it. In an exam, you must name both genera – mixing them up is a common mistake.

两个步骤都需要氧气。这就是通气良好的土壤有利于硝化作用的原因，而淹水土壤会抑制该过程。考试中你必须正确说出两个属的名称——把二者混淆是常见错误。

5. Assimilation | 同化作用

Assimilation is the uptake of ammonium ions or nitrate ions by plants from the soil and their incorporation into plant proteins, nucleic acids, and other nitrogen-containing compounds. Nitrate is reduced to ammonium inside plant cells before being used to synthesise amino acids.

同化作用是植物从土壤中吸收铵离子或硝酸根离子，并将其转化进植物蛋白质、核酸等含氮化合物的过程。硝酸盐在植物细胞内先被还原为铵，再用于合成氨基酸。

Animals obtain their nitrogen by consuming plants or other animals – they cannot use inorganic nitrogen directly. This transfers organic nitrogen through the food web.

动物通过取食植物或其他动物获取氮——它们不能直接利用无机氮。有机氮由此沿着食物网传递。

Excess amino acids in animals are deaminated in the liver, producing ammonia, which is quickly converted to urea or uric acid for excretion. These nitrogenous waste products feed into the ammonification pathway.

动物体内多余的氨基酸在肝脏被脱氨，产生氨，氨随即转化为尿素或尿酸排出体外。这些含氮排泄物进入氨化作用途径。

6. Ammonification | 氨化作用

Ammonification is the decomposition of organic nitrogen compounds from dead organisms, faeces, and urine into ammonium ions. Saprobiotic bacteria and fungi (decomposers) release extracellular enzymes that break down proteins and nucleic acids, producing ammonium as a by-product.

氨化作用是将死生物体、粪便和尿液中的含氮有机物分解为铵离子的过程。腐生细菌和真菌（分解者）释放胞外酶分解蛋白质和核酸，产生铵离子作为副产物。

This process recycles nitrogen back into the soil in a form that plants can either assimilate directly or that can be nitrified further. Without ammonification, organic nitrogen would remain locked in dead matter, severely limiting primary productivity.

该过程将氮以植物可以直接同化或可进一步硝化的形式回收到土壤中。没有氨化作用，有机氮就会被固锁在死亡物质中，严重限制初级生产力。

7. Denitrification | 反硝化作用

Denitrification is the anaerobic conversion of nitrate and nitrite back into nitrogen gas (N₂) by denitrifying bacteria such as Pseudomonas. This process reduces soil fertility and returns nitrogen to the atmosphere, completing the cycle.

反硝化作用是反硝化细菌（如假单胞菌 Pseudomonas）在厌氧条件下将硝酸盐和亚硝酸盐还原为氮气（N₂）的过程。它会降低土壤肥力，并使氮返回大气，从而完成循环。

The key condition for denitrification is the absence of oxygen – hence it commonly occurs in waterlogged or compacted soils. Farmers try to prevent denitrification by ensuring good soil drainage and aeration to preserve nitrate for crops.

反硝化作用的关键条件是缺氧——因此常见于淹水或紧实的土壤。农民通过保证良好的土壤排水和通气，避免反硝化作用，以保留作物可利用的硝酸盐。

2NO₃⁻ → 2NO₂⁻ → 2NO → N₂O → N₂

You do not need to memorise all intermediates, but you should know that the end product is N₂ gas, which is lost to the atmosphere.

你不必记住所有中间产物，但要知道最终产物是 N₂ 气体，它们散逸到大气中。

8. Symbiotic Nitrogen Fixation in Root Nodules | 根瘤中的共生固氮作用

Leguminous plants (e.g., peas, beans, clover) form a mutualistic relationship with Rhizobium bacteria. The plant roots secrete flavonoids that attract Rhizobium, which then trigger the formation of root nodules. Inside these nodules, Rhizobium differentiates into bacteroids and fixes nitrogen.

豆科植物（如豌豆、菜豆、三叶草）与根瘤菌（Rhizobium）形成互利共生关系。植物根部分泌类黄酮吸引根瘤菌，根瘤菌继而诱发根瘤形成。在根瘤内，根瘤菌分化为类菌体并进行固氮。

The plant supplies the bacteria with carbohydrates and a low‑oxygen environment maintained by leghaemoglobin. In return, the bacteria provide the plant with ammonium. This symbiosis is significant for sustainable agriculture as it reduces reliance on nitrogen fertilisers.

植物为细菌提供碳水化合物以及由豆血红蛋白维持的低氧环境。作为回报，细菌为植物提供铵离子。这种共生关系对可持续农业意义重大，因为它减少了对氮肥的依赖。

9. Human Impact: Eutrophication | 人类影响：富营养化

Excessive application of nitrate and phosphate fertilisers on farmland can leach into nearby water bodies through run-off. This leads to eutrophication, a sequence of events that AQA examiners regularly test.

农田过量施用硝酸盐和磷酸盐肥料可经径流沥入附近水体，导致富营养化，这是 AQA 考官经常考查的一系列事件。

First, nutrient enrichment stimulates a rapid growth of algae and cyanobacteria (algal bloom). This dense surface mat blocks sunlight, killing submerged aquatic plants. When the algae die, their decomposition by aerobic saprobiotic bacteria greatly increases the biochemical oxygen demand (BOD). Dissolved oxygen in the water becomes depleted, leading to the death of fish and other aerobic organisms. The waterway eventually turns into a dead zone.

首先，营养物质富集刺激藻类和蓝绿藻迅速生长（藻华）。密集的表面藻层遮挡阳光，杀死沉水植物。藻类死亡后，好氧腐生细菌分解它们，大幅增加生化需氧量（BOD）。水中溶解氧耗竭，导致鱼类及其他好氧生物死亡。水体最终变成死水区。

In your answer, you must connect each step logically – from fertiliser run‑off to low dissolved oxygen – to score full marks.

在你的答案中，必须逻辑清晰地串联每一步——从肥料径流到低溶解氧——才能取得满分。

10. Mycorrhizae and Nitrogen Uptake | 菌根与氮吸收

Many plants form mutually beneficial associations with fungi called mycorrhizae. These fungal hyphae extend far into the soil, vastly increasing the surface area for water and mineral absorption, including nitrate and ammonium ions. In exchange, the plant provides the fungi with sugars.

许多植物与真菌形成互利的菌根联合体。真菌菌丝深入土壤，极大地增加了吸收水分和矿物质（包括硝酸根和铵离子）的表面积。作为交换，植物为真菌提供糖类。

This relationship is especially important in phosphorus and nitrogen uptake in nutrient-poor soils. While not exclusively a nitrogen cycle process, it integrates closely with assimilation and often appears in context-based exam questions.

这种关系在贫瘠土壤中对于磷和氮的吸收尤为重要。虽然它不完全是氮循环的专属过程，但与同化作用紧密相关，常出现在情境类考题中。

11. Key Bacteria and Their Roles | 关键细菌及其作用总结

Bacterium / Group	Process	Conditions	Significance
Rhizobium	Nitrogen fixation (symbiotic)	Anaerobic (in nodules)	Converts N₂ → NH₄⁺ for plants
Azotobacter	Nitrogen fixation (free-living)	Aerobic	Adds NH₄⁺ to soil
Nitrosomonas	Nitrification (step 1)	Aerobic	NH₄⁺ → NO₂⁻
Nitrobacter	Nitrification (step 2)	Aerobic	NO₂⁻ → NO₃⁻
Pseudomonas (denitrifying bacteria)	Denitrification	Anaerobic	NO₃⁻ → N₂ (lost to atmosphere)
Saprobiotic bacteria & fungi	Ammonification	Aerobic / Anaerobic	Organic N → NH₄⁺

Being able to place the correct bacterium into the correct process and condition is a simple way to secure marks on the nitrogen cycle.

准确地将细菌匹配到对应的过程和条件，是稳稳拿住氮循环题目分数的一条捷径。

12. Exam Tips for the Nitrogen Cycle | 氮循环考试技巧

Many students lose marks by confusing the order of nitrification or by forgetting to state the role of decomposers in ammonification. Always explicitly mention the bacteria by name and indicate whether oxygen is required.

许多学生因搞混硝化作用的顺序，或忘记说出分解者在氨化作用中的作用而失分。务必明确写出细菌名称，并指出是否需要氧气。

When describing eutrophication, use precise language: “increased BOD” not just “less oxygen”; “death of aerobic organisms” rather than “fish die”. For nitrogen fixation in root nodules, include the role of leghaemoglobin and the exchange of carbohydrates for ammonium.

在描述富营养化时，要使用精确的措辞：“生化需氧量升高”而不是简单说“氧气减少”；“好氧生物死亡”而不仅是“鱼死了”。对于根瘤固氮，要提及豆血红蛋白的作用以及碳水化合物与铵的交换关系。

Practice drawing and labelling the nitrogen cycle from memory, marking each transformation with the relevant microorganism and oxygen requirement. This visual recall will help you tackle data analysis and application questions confidently.

练习默画并标注氮循环示意图，为每一步转化标记出相关微生物和需氧要求。这种视觉记忆可帮助你有信心地应对数据分析和应用类题目。

Published by TutorHao | Biology Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）

The Nitrogen Cycle | 氮循环 考点精讲