Local conditions such as heavy rainfall or the removal of plants may limit the amount of nitrogen, phosphorus, or calcium available to a particular ecosystem, but the amount of carbon available to the system is seldom a problem. Why?
Organisms do not need very much carbon.
Plants can make their own carbon using water and sunlight.
Plants are much better at absorbing carbon from the soil.
Symbiotic bacteria help plants capture carbon.
Many nutrients come from the soil, but carbon comes from the air.