Nutrient Cycles — 生态系统中的营养循环

📚 Nutrient Cycles | 营养循环

Nutrient cycles describe the movement and exchange of essential elements through the biotic and abiotic components of ecosystems. Unlike energy, which flows in one direction through ecosystems and is ultimately lost as heat, nutrients are constantly recycled. Understanding these cycles is fundamental to A-Level Biology because they explain how ecosystems sustain themselves, why fertilisers are needed in agriculture, and how human activities disrupt natural biogeochemical processes. The three most important nutrient cycles for A-Level exam specifications are the carbon cycle, the nitrogen cycle, and the phosphorus cycle. Each involves a series of transformations mediated by microorganisms, plants, and physical processes, linking the living world to the atmosphere, soil, and water.

营养循环描述了必需元素在生态系统的生物和非生物组成部分之间的移动和交换。与能量在生态系统中单向流动并最终以热量形式散失不同，营养物质是不断循环利用的。理解这些循环对A-Level生物学至关重要，因为它们解释了生态系统如何维持自身、为什么农业需要肥料，以及人类活动如何破坏自然生物地球化学过程。A-Level考试大纲中最重要的三个营养循环是碳循环、氮循环和磷循环。每个循环都涉及由微生物、植物和物理过程介导的一系列转化，将生物世界与大气、土壤和水联系起来。

1. The Carbon Cycle | 碳循环概述

The carbon cycle is the most extensive biogeochemical cycle on Earth, moving carbon between the atmosphere, oceans, living organisms, and geological reservoirs. Carbon enters the biotic component primarily through photosynthesis, where photoautotrophs such as plants and algae fix atmospheric carbon dioxide into organic compounds like glucose. This carbon then passes through food chains as organisms consume one another. Respiration by all living organisms returns carbon dioxide to the atmosphere, completing the short-term cycle. On geological timescales, carbon is stored in fossil fuels and carbonate rocks, representing long-term reservoirs that are only accessed through combustion and weathering.

碳循环是地球上最广泛的生物地球化学循环，碳在大气、海洋、生物体和地质储库之间移动。碳主要通过光合作用进入生物组成部分，光合自养生物如植物和藻类将大气中的二氧化碳固定为葡萄糖等有机化合物。然后，这些碳通过食物链在生物体之间传递。所有生物体的呼吸作用将二氧化碳释放回大气，完成短期循环。在地质时间尺度上，碳储存在化石燃料和碳酸盐岩石中，代表只能通过燃烧和风化作用才能接触到的长期储库。

2. Carbon Cycle Processes | 碳循环的关键过程

Several key processes drive the carbon cycle. Photosynthesis removes approximately 120 billion tonnes of carbon from the atmosphere annually, converting CO₂ and water into glucose and oxygen using light energy. Respiration reverses this equation, with both autotrophs and heterotrophs breaking down organic molecules to release energy, producing CO₂ as a waste product. Decomposition by saprobionts (bacteria and fungi) releases carbon from dead organic matter back into the atmosphere and soil. Combustion of fossil fuels and biomass releases stored carbon rapidly, contributing to the enhanced greenhouse effect. Additionally, oceans act as carbon sinks through dissolution of atmospheric CO₂ and through the biological pump, where marine organisms incorporate carbon into calcium carbonate shells that eventually form sedimentary rock.

几个关键过程推动着碳循环。光合作用每年从大气中移除约1200亿吨碳，利用光能将CO₂和水转化为葡萄糖和氧气。呼吸作用逆转了这一方程式，自养生物和异养生物都分解有机分子以释放能量，产生CO₂作为废物。腐生生物（细菌和真菌）的分解作用将碳从死亡的有机物中释放回大气和土壤。化石燃料和生物质的燃烧迅速释放储存的碳，加剧了温室效应。此外，海洋通过大气CO₂的溶解和生物泵充当碳汇，海洋生物将碳整合到碳酸钙壳中，最终形成沉积岩。

3. The Nitrogen Cycle | 氮循环

Nitrogen is essential for all living organisms as it is a key component of amino acids, proteins, and nucleic acids. Despite the atmosphere being 78% nitrogen gas, this form is inaccessible to most organisms because the triple bond between nitrogen atoms is extremely stable. The nitrogen cycle converts atmospheric N₂ into biologically available forms through four main processes: nitrogen fixation, ammonification, nitrification, and denitrification. Each step is catalysed by specific groups of microorganisms, making the nitrogen cycle one of the most microbially dependent nutrient cycles on Earth.

氮对所有生物体都是必需的，因为它是氨基酸、蛋白质和核酸的关键组成部分。尽管大气中78%是氮气，但由于氮原子之间的三键极其稳定，这种形式对大多数生物来说无法直接利用。氮循环通过四个主要过程将大气中的N₂转化为生物可利用的形式：固氮作用、氨化作用、硝化作用和反硝化作用。每个步骤都由特定的微生物群催化，使氮循环成为地球上最依赖微生物的营养循环之一。

4. Nitrogen Fixation and Ammonification | 固氮与氨化

Nitrogen fixation is the conversion of atmospheric nitrogen gas into ammonium ions, carried out by nitrogen-fixing bacteria. Free-living soil bacteria such as Azotobacter fix nitrogen independently, while mutualistic bacteria like Rhizobium form symbiotic relationships with leguminous plants, residing in root nodules where they receive carbohydrates in exchange for fixed nitrogen. The enzyme nitrogenase catalyses this reaction under anaerobic conditions, which is why leghemoglobin in root nodules maintains low oxygen levels. Ammonification follows, where saprobionts decompose proteins and nucleic acids from dead organisms and waste products, releasing ammonium ions into the soil. This step makes nitrogen from organic matter available for reuse by plants.

固氮作用是将大气中的氮气转化为铵离子的过程，由固氮细菌完成。自由生活的土壤细菌如固氮菌独立固氮，而共生细菌如根瘤菌与豆科植物形成共生关系，居住在根瘤中，在根瘤里它们以碳水化合物交换固定的氮。固氮酶在厌氧条件下催化此反应，这就是根瘤中豆血红蛋白维持低氧水平的原因。随后是氨化作用，腐生生物分解来自死亡生物体和废物的蛋白质和核酸，将铵离子释放到土壤中。这一步使有机物中的氮可供植物再利用。

5. Nitrification and Denitrification | 硝化与反硝化

Nitrification is a two-step aerobic process carried out by chemosynthetic bacteria. First, Nitrosomonas bacteria oxidise ammonium ions to nitrite ions, releasing energy that they use to synthesise organic molecules from CO₂. Then, Nitrobacter bacteria oxidise nitrite to nitrate ions, which are highly soluble and readily absorbed by plant roots through active transport. Denitrification completes the cycle under anaerobic conditions, where bacteria such as Pseudomonas convert nitrates back into atmospheric nitrogen gas. This process is particularly significant in waterlogged soils where oxygen is scarce, leading to the loss of plant-available nitrogen and explaining why poorly drained agricultural soils require additional fertiliser application.

硝化作用是由化能合成细菌完成的两步好氧过程。首先，亚硝化单胞菌将铵离子氧化为亚硝酸根离子，释放能量用于从CO₂合成有机分子。然后，硝化杆菌将亚硝酸根氧化为硝酸根离子，后者高度可溶，易于植物根系通过主动运输吸收。反硝化作用在厌氧条件下完成循环，假单胞菌等细菌将硝酸盐转化回大气氮气。这一过程在缺氧的涝渍土壤中尤为显著，导致植物可利用氮的流失，这解释了为什么排水不良的农业土壤需要额外施肥。

6. The Phosphorus Cycle | 磷循环

The phosphorus cycle differs from the carbon and nitrogen cycles in a crucial respect: it has no significant atmospheric component. Phosphorus exists primarily in rocks and soil minerals, released slowly through weathering and erosion. Unlike the gaseous cycles of carbon and nitrogen, the phosphorus cycle is sedimentary, meaning phosphorus cycles between rocks, soil, water, and living organisms but does not enter the atmosphere in appreciable quantities. This makes phosphorus a limiting nutrient in many ecosystems, as its availability depends on geological processes operating over millions of years. Phosphate ions in soil are absorbed by plant roots through active transport and incorporated into ATP, phospholipids, and nucleic acids, which are then passed through food chains.

磷循环在一个关键方面与碳循环和氮循环不同：它没有显著的大气成分。磷主要存在于岩石和土壤矿物中，通过风化和侵蚀缓慢释放。与碳和氮的气态循环不同，磷循环是沉积型的，意味着磷在岩石、土壤、水和生物体之间循环，但不会以可观的数量进入大气。这使得磷在许多生态系统中成为限制性营养物质，因为其可用性取决于数百万年尺度的地质过程。土壤中的磷酸根离子通过主动运输被植物根系吸收，并整合到ATP、磷脂和核酸中，然后通过食物链传递。

7. Fertilisers and Eutrophication | 肥料与富营养化

In agricultural systems, natural nutrient cycles are disrupted because crops are harvested and removed, preventing the return of nutrients to the soil through decomposition. Farmers apply fertilisers containing nitrates and phosphates to replenish these lost nutrients and maintain crop yields. However, excess fertiliser can leach into nearby water bodies through surface runoff, triggering eutrophication. In this process, the sudden influx of nutrients causes algal blooms that block sunlight from reaching submerged plants. When the algae die and are decomposed by aerobic bacteria, dissolved oxygen levels plummet, creating hypoxic conditions that kill fish and other aquatic organisms. This sequence of events is a classic A-Level exam question linking nutrient cycles to environmental management.

在农业系统中，自然营养循环被破坏，因为作物被收获并移走，阻止了营养物质通过分解回归土壤。农民施用含有硝酸盐和磷酸盐的肥料来补充这些流失的营养物质并维持作物产量。然而，过量的肥料会通过地表径流渗入附近的水体，引发富营养化。在这一过程中，营养物质的突然涌入导致藻类大量繁殖，阻挡阳光到达沉水植物。当藻类死亡并被好氧细菌分解时，溶解氧水平急剧下降，造成缺氧条件，导致鱼类和其他水生生物死亡。这一事件序列是一个经典的A-Level考试问题，将营养循环与环境管理联系起来。

8. Mycorrhizae and Nutrient Uptake | 菌根与营养吸收

Mycorrhizae are mutualistic associations between fungi and plant roots that dramatically enhance nutrient uptake efficiency. The fungal hyphae extend far beyond the plant’s root system, effectively increasing the surface area for absorption by several orders of magnitude. In exchange for carbohydrates produced through photosynthesis, the fungi supply the plant with water and mineral ions, particularly phosphates and nitrates, extracted from soil pores too small for root hairs to penetrate. There are two main types: ectomycorrhizae, which form a sheath around roots, and arbuscular mycorrhizae, which penetrate root cortical cells. This symbiosis is so widespread that over 90% of land plants form mycorrhizal associations, making it one of the most ecologically significant mutualisms on Earth.

菌根是真菌与植物根系之间的互利共生关系，能显著提高营养吸收效率。真菌菌丝延伸到远超植物根系的范围，将吸收表面积有效增加了几个数量级。作为利用光合作用产生的碳水化合物的交换，真菌为植物提供水和矿质离子，特别是从根毛无法穿透的微小土壤孔隙中提取的磷酸盐和硝酸盐。菌根主要有两种类型：外生菌根在根部形成鞘，丛枝菌根穿透根部皮层细胞。这种共生关系非常普遍，超过90%的陆地植物形成菌根关系，使其成为地球上最具生态意义的互利共生之一。

9. Key Exam Terms and Tips | 关键考试术语与技巧

When answering nutrient cycle questions in A-Level exams, precise terminology is essential. Use “saprobiontic nutrition” rather than simply “decomposition” to describe extracellular digestion by microorganisms. Distinguish between nitrification and nitrogen fixation clearly: nitrification converts ammonium to nitrate, while fixation converts N₂ gas to ammonium. For eutrophication, structure your answer chronologically: nutrient runoff = algal bloom = light blockage = plant death = aerobic decomposition = oxygen depletion = fish death. Remember that the phosphorus cycle lacks a gaseous phase, which is frequently tested as a comparison point with carbon and nitrogen. Drawing annotated diagrams of each cycle in your revision notes can help you recall the sequence of transformations and the specific organisms involved in each step.

在A-Level考试中回答营养循环问题时，精确使用术语至关重要。使用”腐生营养”而非简单的”分解”来描述微生物的胞外消化。清楚区分硝化作用和固氮作用：硝化作用将铵转化为硝酸盐，而固氮作用将N₂气体转化为铵。对于富营养化问题，按时间顺序组织答案：营养物质径流 = 藻类大量繁殖 = 光照阻挡 = 植物死亡 = 好氧分解 = 氧气耗尽 = 鱼类死亡。记住磷循环缺乏气态阶段，这经常作为与碳循环和氮循环的比较点进行考查。在复习笔记中绘制每个循环的标注图解，有助于你回忆转化的顺序以及每一步涉及的特定生物。

Key Bilingual Terms | 关键双语术语： Nutrient cycle · 营养循环 | Carbon cycle · 碳循环 | Nitrogen fixation · 固氮作用 | Nitrification · 硝化作用 | Denitrification · 反硝化作用 | Ammonification · 氨化作用 | Saprobiont · 腐生生物 | Eutrophication · 富营养化 | Mycorrhizae · 菌根 | Rhizobium · 根瘤菌 | Leghemoglobin · 豆血红蛋白 | Phosphorus cycle · 磷循环 | Limiting nutrient · 限制性营养物质 | Algal bloom · 藻类大量繁殖 | Hypoxia · 缺氧