📚 IGCSE WJEC Biology: The Nitrogen Cycle | IGCSE WJEC 生物：氮循环 考点精讲
Living organisms require nitrogen to synthesise proteins, nucleic acids, and other essential biomolecules. Although the atmosphere is 78% nitrogen gas (N₂), this form is inert and unusable by most organisms. The nitrogen cycle describes the series of processes that convert nitrogen between its various chemical forms, making it accessible to living things.
生物体需要氮来合成蛋白质、核酸和其他重要的生物分子。尽管大气中 78% 都是氮气 (N₂)，但这种形式的氮是惰性的，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，使氮能够被生物利用。
1. Why the Nitrogen Cycle Matters | 氮循环的重要性
Proteins and DNA contain nitrogen. Without a continuous supply of usable nitrogen, plants cannot grow, and animals that consume plants would suffer deficiencies. The cycle involves four main types of microorganisms: decomposers, nitrifying bacteria, nitrogen-fixing bacteria, and denitrifying bacteria. Understanding their roles is essential for WJEC exams.
蛋白质和 DNA 都含有氮。如果没有持续的可利用氮供应，植物无法生长，以植物为食的动物就会出现氮缺乏。该循环涉及四大类微生物：分解者、硝化细菌、固氮细菌和反硝化细菌。理解它们的作用对 WJEC 考试至关重要。
2. Nitrogen Fixation – Making Nitrogen Usable | 固氮作用 – 让氮气能被利用
Nitrogen fixation converts atmospheric N₂ into ammonia (NH₃) or related compounds. This can happen through lightning (abiotic) or by free-living and symbiotic nitrogen-fixing bacteria. Lightning provides the energy to combine N₂ and O₂ to form nitrogen oxides, which dissolve in rain to form nitric acid and enter soils as nitrates. However, biological fixation is the dominant pathway.
固氮作用将大气中的 N₂ 转化为氨 (NH₃) 或相关化合物。这可以通过闪电（非生物固氮）或由自由生活的固氮菌和共生固氮菌完成。闪电提供能量使 N₂ 与 O₂ 结合，形成氮氧化物，氮氧化物溶于雨水形成硝酸，以硝酸盐形式进入土壤。然而，生物固氮是主要途径。
Biological nitrogen fixation is carried out by prokaryotes that possess the enzyme nitrogenase. Free-living bacteria such as Azotobacter live in soil and fix nitrogen independently. Cyanobacteria in aquatic systems also fix nitrogen.
生物固氮由拥有固氮酶的原核生物完成。自由生活的细菌如固氮菌（Azotobacter）生活在土壤中，独立固定氮。水体中的蓝细菌也能固氮。
3. Symbiotic Nitrogen Fixation in Root Nodules | 根瘤中的共生固氮
The most significant nitrogen fixers in agriculture are bacteria of the genus Rhizobium, which form mutualistic relationships with legume plants (peas, beans, clover). Rhizobium invades root hairs, causing the plant to develop nodules. Inside these nodules, the bacteria obtain carbohydrates from the plant, while converting N₂ into ammonia and then amino acids, which are directly supplied to the plant.
农业中最重要的固氮者是根瘤菌属（Rhizobium）细菌，它们与豆科植物（豌豆、菜豆、三叶草）形成互利共生关系。根瘤菌侵入根毛，促使植物形成根瘤。在根瘤内部，细菌从植物获得碳水化合物，同时将 N₂ 转化为氨，进而转化成氨基酸，并直接供给植物。
WJEC often asks about the advantage of crop rotation including legumes. After harvest, the nitrogen-rich residues decompose, raising soil nitrate levels for the next crop. The enzyme nitrogenase is oxygen-sensitive; leghemoglobin in nodules maintains a low oxygen environment, allowing fixation to occur.
WJEC 常考问及包含豆科植物的轮作优势。收获后，富含氮的残体分解，提高土壤硝态氮水平，供下茬作物利用。固氮酶对氧敏感；根瘤中的豆血红蛋白维持低氧环境，使固氮得以进行。
4. Nitrification – Converting Ammonia to Nitrates | 硝化作用 – 将氨转化为硝酸盐
Ammonia (NH₃) in soil, whether from fixation or decomposition, cannot be directly absorbed by most plants; they prefer nitrates (NO₃⁻). Nitrification is a two-step aerobic process performed by nitrifying bacteria. First, Nitrosomonas oxidises ammonia to nitrite (NO₂⁻):
土壤中的氨 (NH₃)，无论来自固氮还是分解，都不能被大多数植物直接吸收；植物偏好硝酸盐 (NO₃⁻)。硝化作用是由硝化细菌进行的两步需氧过程。首先，亚硝化单胞菌（Nitrosomonas）将氨氧化为亚硝酸盐 (NO₂⁻)：
NH₃ → NO₂⁻
Then, Nitrobacter oxidises nitrite to nitrate (NO₃⁻):
然后，硝化杆菌（Nitrobacter）将亚硝酸盐氧化为硝酸盐 (NO₃⁻)：
NO₂⁻ → NO₃⁻
These bacteria require well-aerated soil. Waterlogged or compacted soils reduce nitrification and promote denitrification.
这些细菌需要通气良好的土壤。积水或板结的土壤会降低硝化作用并促进反硝化作用。
5. Assimilation – Nitrogen Enters Living Organisms | 同化作用 – 氮进入生物体
Plants absorb nitrates (and ammonium ions to a lesser extent) through their root hairs by active transport. Once inside, nitrate is reduced back to ammonium and incorporated into amino acids. These amino acids form proteins and other nitrogen compounds. Animals cannot synthesise all amino acids; they must consume plants or other animals to obtain essential amino acids and nitrogen.
植物通过根毛主动运输吸收硝酸盐（少量吸收铵离子）。进入植物体后，硝酸盐被还原成铵并掺入氨基酸。这些氨基酸构成蛋白质和其他含氮化合物。动物无法合成所有氨基酸；它们必须食用植物或其他动物来获取必需氨基酸和氮元素。
6. Ammonification and the Role of Decomposers | 氨化作用与分解者的作用
When plants and animals die, or when animals excrete urea and faeces, the organic nitrogen in proteins, nucleic acids, and urea is returned to the soil. Saprotrophic bacteria and fungi (decomposers) secrete enzymes that break down these complex molecules. Through ammonification, they release ammonium ions (NH₄⁺) into the soil. Ammonium can be absorbed by some plants or used by nitrifying bacteria.
当动植物死亡，或动物排出尿素和粪便时，蛋白质、核酸和尿素中的有机氮返回土壤。腐生细菌和真菌（分解者）分泌酶将这些复杂分子分解。通过氨化作用，它们释放铵离子 (NH₄⁺) 进入土壤。铵可被某些植物吸收或被硝化细菌利用。
Decomposers are crucial; without them, nitrogen would remain locked in dead materials. WJEC questions often require naming bacteria and fungi as saprotrophs and explaining their role in recycling nutrients.
分解者至关重要；没有它们，氮将长期锁定在死亡物质中。WJEC 题目常要求指出细菌和真菌为腐生生物，并解释它们在营养物质再循环中的作用。
7. Denitrification – Returning Nitrogen to the Atmosphere | 反硝化作用 – 氮回归大气
Denitrifying bacteria, such as Pseudomonas, convert nitrates back into nitrogen gas (N₂) under anaerobic conditions (e.g., waterlogged soils). They use nitrate as an alternative electron acceptor in respiration, releasing N₂ to the atmosphere. This process depletes soil of valuable nitrates, reducing fertility.
反硝化细菌，例如假单胞菌（Pseudomonas），在厌氧条件下（如积水土壤）将硝酸盐转化回氮气 (N₂)。它们利用硝酸盐作为呼吸作用中的替代电子受体，向大气释放 N₂。这一过程消耗土壤中宝贵的硝酸盐，降低肥力。
Farmers try to minimise denitrification by ensuring good drainage and avoiding over-watering. Ploughing also improves aeration and reduces waterlogging.
农民通过确保良好排水和避免过度灌溉来尽量减少反硝化作用。犁地也能改善通气，减少积水。
8. Human Impact: Fertilisers and Eutrophication | 人类影响：肥料与富营养化
Intensive agriculture requires adding extra nitrogen to soils, often as inorganic nitrate fertilisers or manure. Excess fertiliser can run off into freshwater, causing eutrophication. The sequence begins with nitrate and phosphate enrichment, leading to algal blooms. Algae block sunlight, underwater plants die, and aerobic decomposers multiply, consuming oxygen. Eventually, the water becomes anoxic, killing fish and other aquatic life.
集约农业需要向土壤中添加额外的氮，通常是施用无机硝酸盐肥料或粪肥。过量的肥料会流入淡水，引起富营养化。其过程始于硝酸盐和磷酸盐的富集，导致藻类暴发。藻类遮蔽阳光，水下植物死亡，好氧分解者大量繁殖，消耗氧气。最终水体缺氧，导致鱼类和其他水生生物死亡。
WJEC students must be able to describe this sequence and link it back to the nitrogen cycle. Organic fertilisers such as manure also contribute to eutrophication and must be managed carefully.
WJEC 考生必须能够描述这一顺序并将其与氮循环联系起来。有机肥如粪肥也会导致富营养化，必须谨慎管理。
9. Summary of Key Microorganisms | 关键微生物总结
Below is a table consolidating the roles of the main microbial players. Memorising their names and processes is a common exam requirement.
下表汇总了主要微生物的角色。记忆它们的名称和过程是常见的考试要求。
| Process / 过程 | Microorganism (English / 中文) | Role / 作用 |
|---|---|---|
| Nitrogen fixation | Rhizobium / 根瘤菌 Azotobacter / 固氮菌 |
Convert N₂ → NH₃ / NH₄⁺ 将 N₂ 转化为 NH₃ / NH₄⁺ |
| Nitrification | Nitrosomonas / 亚硝化单胞菌 Nitrobacter / 硝化杆菌 |
NH₃ → NO₂⁻ (Nitrosomonas) NO₂⁻ → NO₃⁻ (Nitrobacter) |
| Ammonification | Saprotrophic bacteria & fungi / 腐生细菌和真菌 | Break down organic N → NH₄⁺ 将有机氮分解为 NH₄⁺ |
| Denitrification | Pseudomonas / 假单胞菌 | NO₃⁻ → N₂ (anaerobic) 在厌氧条件下将 NO₃⁻ 转化为 N₂ |
Learning these processes in order, using a diagram, helps. Start with fixation → nitrification → assimilation → ammonification → denitrification. Note that in some textbooks decomposition and ammonification are merged.
结合图表按顺序学习这些过程很有帮助。从固氮 → 硝化 → 同化 → 氨化 → 反硝化。注意有些教材将分解和氨化合并。
10. Exam Tips and Common Pitfalls | 考试技巧与常见误区
Students often confuse nitrification and denitrification. Remember: nitrification adds oxygen, creating nitrates; denitrification removes nitrates, returning to N₂. Also, do not state that plants use nitrogen gas directly – they don’t. Only bacteria fix nitrogen.
学生常混淆硝
Published by TutorHao | IGCSE Biology Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导