Through the process of photosynthesis, plants remove carbon dioxide from the atmosphere and use it to build all the carbon-based compounds it needs for structure and function. When most plants die, these carbon-based compounds break down into their constituent components with a re-release of carbon dioxide back into the atmosphere. Saguaro cactus uses some of the carbon dioxide it removes from the atmosphere to make compounds called oxalates which combine with calcium ions taken up from the soil by the plants roots. The resulting calcium oxalate takes a different path following the death of the cactus. Rather than degrade to its constituent components, calcium oxalate slowly transforms into solid calcium carbonate (calcite), thus essentially sequestering atmospheric carbon dioxide into the soil.
“Cacti contain large quantities of Ca Oxalate biominerals, with C derived from atmospheric CO2. Their death releases these biominerals into the environment, which subsequently transforms to calcite via a monohydrocalcite intermediate…Calcium oxalates form in plants from soil-derived Ca and biologically synthesized oxalate. The C in the oxalates forms from atmospheric CO2 (Catm) via a series of complex biochemical pathways…A large [Carnegiea] gigantea contains on the order of 1×105 g of the Ca oxalate weddellite—CaC2O4·2H2O. In areas with high C. gigantea density, there is an estimated 40 g Catm m−2 sequestered in Ca oxalates. Following the death of the plant, the weddellite transforms to calcite on the order to 10–20 years. In areas with high saguaro density, there is an estimated release of up to 2.4 g calcite m−2 year−1 onto the desert soil. Similar transformation mechanisms occur with the Ca oxalates that are abundant in the majority of cacti. Thus, the total atmospheric C returned to the soil of areas with a high number density of cacti is large, suggesting that there may be a significant long-term accumulation of atmospheric C in these soils derived from Ca oxalate biominerals. These findings demonstrate that plant decay in arid environments may have locally significant impacts on the Ca and inorganic C cycles.” (Garvie 2006:114)
“The source of the Ca in the saguaro is from water uptake by the roots of the rhizosphere. The Ca enters the soil solution as a result of abiotic and biotic dissolution of Ca-bearing minerals such as calcite, feldspars, and micas. While calcite is undoubtedly a significant source of Ca for the saguaro, the importance of other Ca-bearing minerals cannot be ignored…Organic acids produced by soil microorganisms and plant roots greatly enhance the dissolution of soil minerals, making essential plant nutrients available for transport by the xylem.” (Garvie 2006:117)