27.9: The Nitrogen Cycle
Nitrogen atoms, present in all proteins and DNA, are recycled between abiotic and biotic components of the ecosystem. However, the primary form of nitrogen on Earth is nitrogen gas, which cannot be used by most animals and plants. Thus, nitrogen gas must first be converted into a usable form by nitrogen-fixing bacteria before it can be cycled through other living organisms. The use of nitrogen-containing fertilizers and animal waste products in human agriculture has greatly influenced the natural nitrogen cycle.
Biological Nitrogen Cycle
About 78% of the air we breathe is nitrogen gas. However, in this form, N2, few organisms are able to use it. Nitrogen makes up essential molecules in all organisms, like proteins and DNA. Unable to use the atmospheric form of nitrogen, most organisms use the byproducts of nitrogen-fixing and nitrifying prokaryotes. Nitrogen fixation converts nitrogen gas (N2) into ammonia (NH3), whereas nitrification converts NH3 into nitrites (NO2-) and nitrates (NO3-). Plants can directly use the ammonia and nitrates, and plant-eating organisms obtain nitrogen by ingesting plants. When these organisms die, bacteria in the soil are able to convert the organic nitrogen into ammonia in a process called ammonification. Through denitrification, aerobic bacteria can then convert ammonia into nitrogen gas that is released back into the atmosphere, completing the cycle.
The primary reservoir for long-term storage of nitrogen is nitrogen gas in the atmosphere. However, there are other, smaller nitrogen sinks in the ecosystem. Nitrogen can be tied up for relatively long periods of time in swamps, marine sediments, and sedimentary rock. However, since nitrogen compounds are very soluble in water, weathering of sedimentary rock can release nitrogen back into the ecosystem.
Human Impacts on the Nitrogen Cycle
Since nitrogen is often a limiting factor for plant growth in natural environments, farmers add nitrogen to the soil as fertilizer to increase agricultural yield. Agricultural runoff into aquatic ecosystems can result in eutrophication and unnaturally rapid growth of toxic algae species. Raising large numbers of livestock can also increase the amount of nitric waste in the soil and in local water sources.