碳循环(carbon cycle)
各种形式的碳在自然界中的循环。碳是一切有机化合物（包括地球上生命不可缺少的那些有机化合物）的基本组分。生物所含碳的来源是空气中的或溶於水的二氧化碳。藻类和绿色植物（制造者）都是透过光合作用将二氧化碳转化成碳水化合物。这些碳水化合物被制造者用来维持新陈代谢，多余部分作为脂肪和多醣贮藏起来，供消费动物（从原生动物到人）食用，再转化成其他形式。所有生物将二氧化碳作为其呼吸的副产品直接送回大气。动物排泄物和一切生物体中的碳经过腐烂（或分解），即微生物（主要是细菌和霉菌）的一系列转化过程变成二氧化碳进入大气。一部分有机碳（有机体残骸）则作为化石燃料（煤、天然气和石油）、石灰石和珊瑚贮於地壳中。化石燃料中的碳是史前从碳循环中脱离出来的，现正透过工业过程和农业过程以二氧化碳形式大量释出，大部分很快进入海洋，形成分解的碳酸盐，有些则留在大气中（参阅greenhouse effect）。
English version：
carbon cycle
Circulation of carbon in the form of the simple element and its compounds through nature. The source of carbon in living things is carbon dioxide (CO2), from air or dissolved in water. Algae and green plants (producers) use CO2 in photosynthesis to make carbohydrates, used in the processes of metabolism to make all other compounds in their tissues and those of animals that consume them (herbivores). The carbon may pass through several levels of herbivores and carnivores (consumers). Animals and (at night) plants return the CO2 to the atmosphere as a byproduct of respiration. The carbon in animal wastes and the bodies of organisms is released as CO2, in a series of steps, by decay organisms (decomposers), chiefly bacteria and fungi (see fungus). Some organic carbon (the remains of organisms) has accumulated in the earth's crust in fossil fuels, limestone, and coral. The carbon of fossil fuels, removed from the cycle in prehistoric times, is being returned in vast quantities as CO2 via industrial and agricultural processes, some accumulating in the oceans as dissolved carbonates and some staying in the atmosphere (see greenhouse effect).