Carbon, a major constituent of structural and energy-releasing organic molecules, is essential to life. A substantial amount of C found in soils is available as soluble organic molecules (SOM) or particulate organic matter (POM) derived from plant primary production. Available atmospheric C as CO2, used by plants during photosynthesis, is part of a larger biogeochemical cycle, including the cyclic release of CO2 by respiration in living organisms and its fixation through photosynthesis. Carbon also is fixed through biological calcification and is eventually bound in carbonate-containing rocks, especially in aquatic and marine environments. This book focuses mainly on soil carbon and its environmental benefits.