Fungi are the dominant microbes in soil. They are an essential part of many ecosystems because they help recycle nutrients from the atmosphere and ground alike (fungi are often associated with decomposition). Fungi from the genus Alternaria take up nitrate from moist soils and convert it to ammonium ion, increasing the pH of the soil in the process. As a result, the carbon dioxide "exhaled" from their respiration gets converted to solid calcium carbonate in the soil before it has a chance to escape into the air as carbon dioxide gas.

"Formation of CaCO₃ induced by fungal physiological activities is a potential way to sequestrate atmospheric CO₂ in ecosystem. Alternaria sp. is a saprophytic fungus isolated from a forest soil. We examined the precipitation of CaCO₃ induced by the fungus in response to different levels of Ca(NO₃)₂ or CaCl₂ in agar media, and the biogenesis of CaCO₃ was verified by low d13C value. The formed CaCO₃ was identified as calcite by X-ray diffraction analysis. Square, rectangular and rhombic CaCO₃ crystals and amorphous calcium carbonate were observed around mycelia at higher levels of Ca(NO₃)₂. Acidification occurred in media at low concentrations (0 and 0.0002 M) of Ca(NO₃)₂, and no CaCO₃ formed in these media. The quantities of CaCO₃ formed in media increased with increasing concentrations of Ca(NO₃)₂ and were significantly correlated to fungal biomass, pH value and nitrite concentrations. No CaCO₃ was formed in media with CaCl₂ at all levels. These results collectively indicated that the formation of CaCO₃ can be induced by the fungal assimilation of nitrate. The study also revealed that biogenic crystal of CaCO₃ tended to grow on a silicon nucleus and the amorphous calcium carbonate (ACC) was the transient stage of CaCO₃ crystal." (Hou et al. 2010)