Nitrogen Cycle
The nitrogen cycle transforms nitrogen among its many oxidation states, from inert atmospheric dinitrogen to reactive forms that sustain life and, in excess, pollute air and water.
Definition
The global cycling of nitrogen through fixation, nitrification, denitrification, and assimilation, spanning multiple oxidation states.
Scope
This topic covers biological and industrial nitrogen fixation, nitrification and denitrification, the assimilation and mineralization of nitrogen, and the cascade of environmental effects from the reactive nitrogen that human activity has added to the cycle.
Core questions
- How is inert atmospheric nitrogen converted to reactive forms?
- What microbial reactions interconvert nitrogen oxidation states?
- How has industrial fixation altered the global cycle?
- What is the nitrogen cascade and why does it matter?
Key theories
- Reactive-nitrogen cascade
- A single atom of human-created reactive nitrogen can move through ecosystems causing successive effects, including smog, acidification, eutrophication, and greenhouse warming, before eventual denitrification.
Mechanisms
Nitrogen-fixing microbes and the industrial Haber-Bosch process convert dinitrogen to ammonia; nitrification oxidizes ammonium to nitrate; assimilation builds biomass; and denitrification returns nitrogen to the atmosphere as N2 and nitrous oxide. Human fixation now rivals natural sources, loading ecosystems with mobile reactive nitrogen.
Clinical relevance
The nitrogen cycle underlies fertilizer-driven food production but also nitrate pollution of water, coastal dead zones, and nitrous oxide emissions affecting climate and ozone.
History
The Haber-Bosch process in the early 20th century transformed the nitrogen cycle by enabling massive industrial fixation, and later work characterized the resulting nitrogen cascade and its planetary scale.
Key figures
- Fritz Haber
- Carl Bosch
- James N. Galloway
Related topics
Seminal works
- galloway2008
- schlesinger2013
Frequently asked questions
- Why is reactive nitrogen a problem if nitrogen is essential?
- Reactive nitrogen is needed for life, but in excess it spreads through air and water causing pollution, eutrophication, and greenhouse gas emissions before being removed.