MICROBIAL PRECIPITATION OF PEDOGENIC CARBONATE

Scanning electron microscopy of calcite in arid and semiarid soils indicates that some of the calcium carbonate is precipitated by microorganisms. Thus, laboratory and field experiments were performed to test the hypothesis that calcite in these soils is a microbially-mediated authigenic mineral. Lab experiments revealed that certain bacteria (less than 1 percent) could precipitate calcite when grown on a calcium-rich medium in petri dishes. Calcite crystal in the bacterial colonies occurred in two days. In addition, several fungal species (about 50%) formed calcite on the same calcium-rich medium, but at a slower rate of one month. Field experiments showed that fungal hyphae precipitated calcite within 5 weeks on microscope slides inserted into soils that were treated with a calcium-rich medium. Even in a forest ecosystem of the southeastern United States, soil fungi precipitated pedogenic carbonate when the hyphae escaped the low pH environment of the soil and enter protected microenvironments between cover slips containing the calcium-rich medium. These results reveal that an unknown amount of soil calcite is mediated by microorganisms, which, in turn, are linked to aboveground ecosystems and global climatic regions. Soil carbonate is the third largest C pool in the active global carbon cycle, containing at least 800 x 10¹⁵ grams of carbon, thus exceeding the amount in the atmosphere (ca. 750 x 10¹⁵ grams) or in land plants (ca. 560 x 10¹⁵ grams). Recognizing that some, if not most, soil carbonate is biogenic will advance our understanding of terrestrial carbon dynamics.