Some of this leached nitrate flows through the hydrologic system until it reaches the oceans where it can be returned to the atmosphere by denitrification . Denitrification is also common in anaerobic soils and is carried out by heterotrophic bacteria . Both of these gases then diffuse into the atmosphere. This process is important to the bacteria because it supplies them with oxygen for respiration . Almost all of the nitrogen found in any terrestrial ecosystem originally came from the atmosphere.
Small proportions enter the soil in rainfall or through the effects of lightning.
The majority, however, is biochemically fixed within the soil by specialized micro-organisms like bacteria , actinomycetes , and cyanobacteria . Scientist estimate that biological fixation globally adds approximately 140 million metric tons of nitogen to ecosystems every year. The activities of humans have severely altered the nitrogen cycle. The additional nitrogen entering the groundwater system eventually flows into streams, rivers, lakes, and estuaries. In these systems, the added nitrogen can lead to eutrophication .