FIGURE 1. Rivers and the carbon cycle.

a, Photosynthesis in land plants fixes atmospheric CO₂ (inorganic carbon) as organic carbon, which is either stored as plant biomass or in soil, or is decomposed back to CO₂ through plant and soil respiration. This CO₂ can return to the atmosphere or enter rivers; alternatively, it can react with soil minerals to form inorganic dissolved carbonates that remain stored in soils or are exported to rivers. b, The transformations of organic to inorganic carbon through decomposition and photosynthesis continue in rivers; here, CO₂ will re-exchange with the atmosphere ('degassing'), or be converted to dissolved carbonates. These carbonates do not exchange with the atmosphere and are mainly exported to the coastal ocean. Organic carbon is also exported to the ocean, or stored in flood plains. c, In the coastal ocean, photosynthesis, decomposition and re-exchanging of CO₂ with the atmosphere still continue. Solid organic carbon (such as soil particles and phytoplankton cells) is buried in coastal sediments, where it is stored or decomposes to inorganic carbon and diffuses back into coastal waters. Dissolved inorganic and organic carbon are also exported to the open ocean, and possibly deep-ocean waters, where they are stored for many centuries.