Fig. 2.

Plants take up carbon dioxide (CO₂) from the atmosphere and convert it to plant organic matter, of which light-absorbing lignin is a major component. When plants die, plant and soil organic matter is decomposed by soil microbes to CO₂ and carbon monoxide (CO) that is returned to the atmosphere, and to smaller pools of organic matter (i.e., particulate and terrestrial dissolved organic matter; POM and tDOM, respectively). POM and tDOM are flushed to streams, rivers, and lakes in rain and snow, and to coastal waters via riverine export. On land and in sunlit surface waters, UV and visible radiation help decompose plant organic matter, POM, and tDOM to CO₂ and CO. PAR, photosynthetic active radiation (400–700 nm).