2. Carbon cycle

The element carbon enters a complex cycle of transformations: oxidation, reduction, biodegradation, photosynthesis... Figure 1 attempts to schematize this complex set of reactions. On purpose, CO ₂ has been placed at the center of this cycle, making it possible to show either its transformations or its production. We thus find inorganic compounds such as zero-valent carbon, carbonates, bicarbonates, which show their importance in the sources and sinks regulating CO ₂ in the atmosphere (cf. § 3