Well corn plants will take up carbon dioxide from the air, but the carbon is only temporarily removed fromt the air and as such is really still pretty much part of the carbon cycle. As soon as the corn is eaten by man or animal or burned as fuel in one way or another, or even decomposes in some process the carbon gets back into the air, as carbon dioxide or i suppose as methane in some cases. Carbon tied up as carbonate rock is taken out of the cycle for many hundreds of years or more. Still trees and plants are large stores of carbon taken out of the air. When i think about it carbon as coal has been out of the carbon cycle for a long time as well, until we burn it for heat etc. Transforming carbon dioxide gas to coal is probably too costly/difficult, i think carbonate rock is easier to make but still at a great cost. Really conservation of fossil fuel by burning less is the best solution.
This topic fascinates me since carbon is such an essential part of life and we are part of the carbon cycle. One fellow made criticism of my agreement that carbon dioxide was a pollutant since it is so essential in the life of plants animals etc. Having said all that I am not convinced that global warming can be mitigated by man in any substatial way and we should spend our efforts time and money on more acute problems like health and food and water shortages, and acute pollution problems, like smog, sulfur oxides etc.