The Terrestrial Carbon Cycle
Terrestrial versus Oceanic Biosphere
The bulk of carbon on land is locked up in soils and in trees, with each source respectively representing the "soil carbon" and "biosphere" reservoirs. Of course, living things in the ocean are also part of the biosphere, but the amount of carbon they contain is small when compared with the mass of carbon in wood. In addition plants on land appear, for some reason, to be about twice as efficient in fixing carbon during photosynthesis than photosynthesizing organisms in the ocean.