- biogeochemical cycle converting nitrogen between chemical forms across Earth’s systems
- stages: N2 fixation, nitrification, assimilation, ammonification, denitrification
- N2 fixation: bacteria (Rhizobium, cyanobacteria) and lightning convert atmospheric N2 to ammonia
- nitrification: bacteria oxidize ammonia to nitrite then nitrate in soil
- assimilation: plants absorb nitrate/ammonia, build proteins, DNA, and chlorophyll
- ammonification: decomposers break organic nitrogen back to ammonia
- denitrification: anaerobic bacteria convert nitrate back to N2, closing the loop
- nitrogen is essential for amino acids, nucleotides, and energy carriers (ATP, NADH)
- 78% of the atmosphere is N2 gas, yet most organisms cannot use it directly
- industrial Haber-Bosch process fixes nitrogen for agriculture, doubling global food capacity
- excess reactive nitrogen causes eutrophication in rivers and oceans
- tightly coupled to the carbon cycle and water cycle