Mac Post of ESD, one of our experts on the carbon cycle, says that several lines of evidence indicate that the terrestrial biosphere, most likely the Northern Hemisphere, could be taking up a net amount of nearly 2 billion tons of carbon per year. Some of this uptake is perhaps due to regrowth of forests harvested previously this century in North America and Europe. Another factor may be the enhanced growth of natural vegetation resulting from rising atmospheric carbon dioxide concentrations stimulating photosynthesis. This Northern Hemisphere sink is larger than the estimated 0.5 to 1.5 billion tons of carbon emitted to the atmosphere through conversion of natural ecosystems to agriculture, primarily in tropical regions. Thus, the earth's vegetation and soil could act as a huge natural scrubber for carbon dioxide emissions from industrial sources and land-use changes.
Plants release some of the stored carbon back into the atmosphere through respiration.
When a plant sheds leaves and roots die, this organic material decays, but some of it can be protected physically and chemically as dead organic matter in soils, which can be stable for up to thousands of years. The decomposition of soil carbon by soil microbes releases carbon dioxide to the atmosphere. This decomposition also mineralizes organic matter, which makes nutrients available for plant growth. The total amount of carbon stored in an ecosystem reflects the long-term balance between plant production and respiration and soil decomposition. People can help reduce carbon dioxide releases to the atmosphere and enhance carbon sequestration by protecting and adding to ecosystems that store carbon.