📚 IB OCR Biology: The Nitrogen Cycle – Exam Essentials | IB OCR 生物：氮循环考点精讲
The nitrogen cycle is a cornerstone topic in both IB and OCR A-level Biology. Understanding how nitrogen moves between the atmosphere, soil, and living organisms is essential for explaining ecosystem productivity and nutrient management. This article breaks down every key process, the crucial bacteria involved, and common exam pitfalls, delivering a bilingual revision guide aligned with your syllabus.
氮循环是 IB 和 OCR 生物学的核心主题。理解氮在大气、土壤和生物体之间的迁移，对于解释生态系统生产力和养分管理至关重要。本文分解每一个关键过程、涉及的关键细菌以及常见的考试陷阱，提供与你的考纲对齐的双语复习指南。
1. The Importance of Nitrogen | 氮的重要性
Nitrogen is a fundamental component of proteins, nucleic acids (DNA and RNA), ATP, and chlorophyll. Despite an abundance of molecular nitrogen (N₂) in the atmosphere – about 78% – most organisms cannot use it directly because the triple bond is extremely stable. Living things need nitrogen in reactive forms such as ammonium (NH₄⁺) or nitrate (NO₃⁻).
氮是蛋白质、核酸（DNA 和 RNA）、ATP 以及叶绿素的基本组成元素。尽管大气中分子氮（N₂）含量丰富，约占 78%，但大多数生物无法直接利用，因为三键非常稳定。生物体需要活性形式的氮，如铵离子（NH₄⁺）或硝酸盐（NO₃⁻）。
In ecosystems, nitrogen is often the limiting nutrient that controls primary productivity. The nitrogen cycle therefore links abiotic and biotic components, making it a classic topic for questions on energy flow, food production, and environmental change.
在生态系统中，氮往往是控制初级生产力的限制性养分。因此，氮循环将非生物与生物组分联系起来，使其成为能量流动、粮食生产和环境变化等问题的经典考点。
2. Overview of the Nitrogen Cycle | 氮循环概述
The nitrogen cycle consists of five major transformations: nitrogen fixation, nitrification, assimilation, ammonification, and denitrification. Atmospheric N₂ is converted into ammonia or ammonium by fixation, then oxidised to nitrite and nitrate during nitrification. Plants assimilate these inorganic ions to build organic molecules. Decomposers later return nitrogen to the soil as ammonium through ammonification. Finally, denitrifying bacteria convert nitrate back to N₂ gas, completing the cycle.
氮循环由五个主要转化过程组成：固氮作用、硝化作用、同化作用、氨化作用和反硝化作用。大气中的 N₂ 通过固氮转变为氨或铵离子，随后在硝化过程中被氧化为亚硝酸盐和硝酸盐。植物同化这些无机离子以构建有机分子。分解者随后通过氨化作用将氮以铵的形式归还土壤。最后，反硝化细菌将硝酸盐还原为 N₂ 气体，完成循环。
A simplified schematic often tested in exams is: N₂ → NH₃/NH₄⁺ → NO₂⁻ → NO₃⁻ → plant protein → animal protein → NH₄⁺ → NO₃⁻ → N₂. Notice the recycling of ammonium back into nitrate, and the eventual loss of nitrogen to the atmosphere.
考试中常考的一个简化示意图是：N₂ → NH₃/NH₄⁺ → NO₂⁻ → NO₃⁻ → 植物蛋白 → 动物蛋白 → NH₄⁺ → NO₃⁻ → N₂。注意铵循环回到硝酸盐，以及氮最终流失到大气中。
3. Nitrogen Fixation | 固氮作用
Nitrogen fixation is the conversion of atmospheric N₂ into ammonia (NH₃) or ammonium ions (NH₄⁺). It can occur through biological or non-biological pathways. Biological fixation is catalysed by the enzyme nitrogenase, which is found only in certain prokaryotes.
固氮作用是将大气中的 N₂ 转化为氨（NH₃）或铵离子（NH₄⁺）的过程。它可以通过生物或非生物途径发生。生物固氮由固氮酶催化，该酶仅存在于某些原核生物中。
Symbiotic fixation involves bacteria of the genus Rhizobium that live inside root nodules of leguminous plants such as peas, beans, and clover. The plant supplies carbohydrates and a protective, low-oxygen environment maintained by leghaemoglobin; the bacteroid provides a direct source of usable nitrogen.
共生固氮 涉及根瘤菌属细菌，它们生活在豌豆、大豆和三叶草等豆科植物的根瘤内。植物提供碳水化合物以及由豆血红蛋白维持的微氧环境保护环境；类菌体则提供可直接利用的氮源。
Free-living fixation is carried out by soil bacteria like Azotobacter (aerobic) and cyanobacteria in aquatic systems. Non-biological fixation includes lightning strikes, which provide enough energy to combine N₂ and O₂ to form nitrogen oxides, and the industrial Haber process used to manufacture fertilisers.
自由生活固氮 由土壤细菌如固氮菌（好氧）和水域中的蓝藻进行。非生物固氮包括闪电（提供足够能量使 N₂ 和 O₂ 结合成氮氧化物）以及用于制造化肥的哈伯工业合成氨法。
N₂ + 8 e⁻ + 8 H⁺ + 16 ATP → 2 NH₃ + H₂ + 16 ADP + 16 Pᵢ
4. Nitrification | 硝化作用
Nitrification is the aerobic oxidation of ammonium to nitrate, carried out in two steps by specialised chemoautotrophic bacteria. It occurs in well-aerated soils and supplies plants with the highly mobile nitrate ion.
硝化作用是铵盐在有氧条件下被氧化为硝酸盐的过程，由专门的化能自养细菌分两步完成。它发生在通气良好的土壤中，为植物提供高度移动的硝酸根离子。
First, Nitrosomonas and related genera oxidise ammonium to nitrite (NO₂⁻).
首先，亚硝化单胞菌等属将铵氧化为亚硝酸盐（NO₂⁻）。
NH₄⁺ + 1.5 O₂ → NO₂⁻ + H₂O + 2 H⁺
Next, Nitrobacter oxidises nitrite to nitrate (NO₃⁻).
接着，硝化杆菌属将亚硝酸盐氧化为硝酸盐（NO₃⁻）。
NO₂⁻ + 0.5 O₂ → NO₃⁻
Both groups gain energy from these exergonic reactions and use CO₂ as their carbon source. Because nitrification consumes oxygen, it can reduce soil oxygen levels when ammonium is abundant.
这两类细菌从这些放能反应中获得能量，并以 CO₂ 作为碳源。由于硝化作用消耗氧气，当铵丰富时会降低土壤的氧气水平。
5. Assimilation | 同化作用
Assimilation is the uptake and incorporation of inorganic nitrogen compounds into organic molecules by plants and other producers. Root hairs absorb nitrate and ammonium from the soil solution. Once inside the plant cell, nitrate is reduced back to ammonium via nitrate reductase and nitrite reductase before being combined with carbon skeletons to form amino acids, nucleotides, and chlorophyll.
同化作用是植物和其他生产者吸收并将无机氮化合物结合进有机分子的过程。根毛从土壤溶液中吸收硝酸盐和铵离子。进入植物细胞后，硝酸盐经硝酸还原酶和亚硝酸还原酶还原为铵，然后再与碳骨架结合形成氨基酸、核苷酸和叶绿素。
Animals obtain their nitrogen by consuming plants or other animals – they cannot use inorganic nitrogen directly. This step links all trophic levels and explains why the nitrogen cycle is essential for food webs.
动物通过摄食植物或其他动物获取氮——它们无法直接利用无机氮。这一步骤连接了所有营养级，并解释了为何氮循环对食物网至关重要。
6. Ammonification | 氨化作用
Ammonification is the process by which decomposers – mainly saprobiotic bacteria and fungi – break down organic nitrogen in dead organisms, faeces, and urine, releasing ammonium ions (NH₄⁺) back into the soil.
氨化作用是分解者（主要是腐生细菌和真菌）分解死生物体、粪便和尿液中的有机氮，将铵离子（NH₄⁺）释放回土壤的过程。
Extracellular enzymes are secreted onto the organic matter, hydrolysing proteins to amino acids and nucleic acids to nitrogenous bases. Deaminase enzymes then remove amino groups, producing ammonia, which is protonated to ammonium at soil pH. This returned ammonium becomes available for nitrification or direct re-assimilation.
分解者向有机物分泌胞外酶，将蛋白质水解成氨基酸，将核酸水解成含氮碱基。脱氨酶随后移除氨基，产生氨，在土壤 pH 下质子化为铵离子。这些返回的铵可用于硝化作用或直接被再同化。
7. Denitrification | 反硝化作用
Denitrification is the anaerobic reduction of nitrate to nitrogen gas (N₂) by facultative anaerobic bacteria such as Pseudomonas and Thiobacillus. It occurs in waterlogged, compacted, or oxygen-depleted soils and returns nitrogen to the atmosphere, completing the cycle.
反硝化作用是兼性厌氧细菌如假单胞菌和硫杆菌在厌氧条件下将硝酸盐还原为氮气（N₂）的过程。它发生在积水、板结或缺氧的土壤中，使氮返回大气，完成循环。
2 NO₃⁻ + 10 e⁻ + 12 H⁺ → N₂ + 6 H₂O
Farmers try to minimise denitrification because it removes plant-available nitrogen. Good soil drainage and aeration help keep nitrate in the soil. However, from a global perspective, denitrification prevents the endless accumulation of fixed nitrogen in ecosystems.
农民尽量减少反硝化作用，因为它去除了植物可利用的氮。良好的土壤排水和通气有助于将硝酸盐保留在土壤中。然而，从全球视角看，反硝化作用防止了生态系统中固定氮的无止境积累。
8. Human Impacts on the Nitrogen Cycle | 人类对氮循环的影响
Human activity has roughly doubled the amount of reactive nitrogen in the biosphere, primarily through the Haber-Bosch process for fertiliser production and the cultivation of leguminous crops. While this has increased food production, it has also disrupted natural nitrogen balances.
人类活动通过哈伯-博斯法生产化肥和种植豆科作物，大约使生物圈中的活性氮量翻了一番。尽管这增加了粮食产量，但也扰乱了自然的氮平衡。
Excess nitrate leaches from agricultural fields into groundwater and surface waters, causing eutrophication: algal blooms block sunlight, and subsequent decomposition by bacteria depletes dissolved oxygen, creating dead zones. High nitrate concentrations in drinking water can also cause health problems, such as methaemoglobinaemia (‘blue baby syndrome’).
过量的硝酸盐从农田淋溶到地下水和地表水中，引起富营养化：藻类爆发遮挡阳光，随后细菌分解耗尽溶解氧，形成死区。饮用水中高浓度的硝酸盐还会导致健康问题，如高铁血红蛋白血症（”蓝婴综合征”）。
Combustion of fossil fuels releases nitrogen oxides (NOₓ), which contribute to photochemical smog and acid rain. In addition, deforestation and soil erosion disturb the natural re-cycling of nitrogen in terrestrial environments.
化石燃料的燃烧释放出氮氧化物（NOₓ），导致光化学烟雾和酸雨。此外，森林砍伐和水土流失扰乱了陆地环境中氮的自然再循环。
9. Key Bacteria and Their Roles | 关键细菌及其作用
Rhizobium – Symbiotic nitrogen-fixing bacterium inside legume root nodules. Infected plant cells form bacteroids that convert N₂ into ammonium, which is exported to the plant.
根瘤菌——豆科植物根瘤内的共生固氮细菌。被侵染的植物细胞形成类菌体，将 N₂ 转化为铵，并输送给植物。
Azotobacter – Free-living aerobic soil bacterium that fixes nitrogen independently, contributing to natural soil fertility even without leguminous hosts.
固氮菌——自由生活的需氧土壤细菌，独立固氮，即使没有豆科宿主也能贡献自然土壤肥力。
Nitrosomonas – Chemoautotrophic nitrifying bacterium that oxidises ammonium to nitrite in the first step of nitrification.
亚硝化单胞菌——化能自养的硝化细菌，在硝化作用第一步中将铵氧化为亚硝酸盐。
Nitrobacter – Completes nitrification by oxidising nitrite to nitrate, providing the preferred nitrogen form for most crops.
硝化杆菌——通过将亚硝酸盐氧化为硝酸盐完成硝化作用，提供大多数作物首选的氮形式。
Pseudomonas – A common denitrifying bacterium; under anaerobic conditions, it uses nitrate as a terminal electron acceptor and releases N₂ gas.
假单胞菌——常见的反硝化细菌；在厌氧条件下，利用硝酸盐作为末端电子受体，释放出 N₂ 气体。
Saprobiotic bacteria and fungi – Carry out ammonification by secreting enzymes that mineralise organic nitrogen into ammonium, linking decomposition back to nitrification and assimilation.
腐生细菌和真菌——通过分泌酶将有机氮矿化为铵离子来完成氨化作用，将分解过程重新连接到硝化作用和同化作用。
10. Exam Tips for the Nitrogen Cycle | 氮循环考试技巧
Many marks are lost by confusing nitrification with denitrification. Remember: nitrification requires oxygen and produces nitrate; denitrification occurs in the absence of oxygen and converts nitrate back to N₂. Always state the specific bacterial names where possible – IB and OCR examiners expect Nitrosomonas, Nitrobacter, Rhizobium, and Pseudomonas to be used accurately.
许多失分源于混淆了硝化作用与反硝化作用。记住：硝化作用需要氧气并产生硝酸盐；反硝化作用在无氧条件下发生，将硝酸盐还原为 N₂。在可能的情况下，始终说出具体的细菌名称——IB 和 OCR 考官要求准确使用亚硝化单胞菌、硝化杆菌、根瘤菌和假单胞菌。
When sketching the nitrogen cycle in an exam, label all input and output arrows with the name of the process and the key chemical forms (N₂, NH₄⁺, NO₂⁻, NO₃⁻). Include both assimilation and ammonification to show that nitrogen moves between organic and inorganic pools. If you are asked about eutrophication, link the use of nitrate fertilisers to increased algal growth, subsequent decomposition, and oxygen depletion.
在考试中绘制氮循环简图时，用过程名称和关键的化学形式（N₂、NH₄⁺、NO₂⁻、NO₃⁻）标注所有输入和输出箭头。务必将同化作用和氨化作用都包括进去，以表明氮在有机库和无机库之间移动。如果被问到富营养化问题，请将硝酸盐肥料的使用与藻类大量繁殖、随后的分解以及氧气耗尽联系起来。
Be ready to explain why nitrogen fixation is energetically expensive (16 ATP per N₂ fixed) and why leghaemoglobin is essential inside nodules. Finally, practise describing how over-application of fertilisers alters the entire nitrogen cycle, from leaching to groundwater contamination and atmospheric pollution.
准备好解释为什么固氮耗能巨大（每固定一个 N₂ 需要 16 个 ATP）以及为何豆血红蛋白在根瘤内是必要的。最后，练习描述过量施肥如何改变整个氮循环，从淋溶到地下水污染和大气污染。
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