The Earth is experiencing major ecosystem and climatic changes due to anthropogenic activities. Movement of organic carbon to the ocean depths is called biological pumping and it lowers the carbon dioxide levels at the surface, which allows the oceans to take up additional carbon dioxide. The sink that takes up the excess carbon remains a mystery. Unfortunately, too little is known about the flux of carbon in soils to know if they serve as the missing sink. Its importance makes it a limiting nutrient in ecosystems.
Nitrogen fixation adds nitrogen to the cycle.
Nitrogen-fixing bacteria include Rhizobium and Azotobacter . Rhizobium is found in a symbiotic relationship in root nodules of legumes. Denitrification results in reversal of nitrification and occurs in anoxic soils and sediments as well as anaerobic deep ocean waters. This process can be accomplished by physical reactions and the bacterium Pseudomonas denitrificans . Nitrogen fixation balances denitrification in most ecosystems.