1. The Nitrogen Cycle
Ch 7 Part 3

2. Nitrogen Cycle
Nitrogen cycle: the routes that nitrogen atoms take through the environment
Nitrogen gas (N2) is the most abundant gas in the atmosphere (78%)
Nitrogen gas is inert & cannot be used directly by most organisms.
Nitrogen is cycled through animals, soil, & gases.
Organisms use nitrogen (in various forms) to make vital organic compounds.
Nitrogen is an important component of biological molecules: DNA, RNA, & proteins.
Nitrogen is a plant nutrient & acts as a limiting factor for plant growth.
Too much nitrogen in aquatic systems can cause eutrophication.

3. Abbreviations to know:
Several steps:
Nitrogen fixation
nitrogen to ammonia
Nitrification
ammonia to nitrates
Assimilation
nitrates to proteins, DNA, RNA, etc.
Ammonification
decomposition
Denitrification
ammonia & nitrates to nitrogen
Abbreviations to know:
nitrogen gas (N2)
ammonia (NH3)
ammonium ions (NH4+)
nitrite ions (NO2-)
nitrate ions (NO3-)

4. Nitrogen Fixation
Even though N2 is the most abundant gas in the atmosphere (78%) it must be fixed or converted into a usable form before it can be absorbed and used by multicellular organisms.
Ways of converting N2:
1. lightning
causes reaction with O2 to form NO3-
2. Nitrogen fixation: N2 is combined (fixed) with H2 to become ammonia, NH3
whose water-soluble ions of ammonium, NH4+, can be taken up by plants
Done by nitrogen-fixing bacteria

5. Nitrogen Fixation Specialized bacteria convert N2 gas to ammonia (NH3)
N2 + 3H2  2NH3
Once converted into ammonia, water-soluble ions in the form of ammonium (NH4+) can be taken up by plants
Done mostly by:
Cyanobacteria in soil & water
Rhizobium bacteria in nodules on roots of wide variety of plants
(ex. legumes, soybeans,
& alfalfa)
Requires large amounts of energy

6. Nitrification
Nitrification: bacteria convert ammonium ions, NH4+ (the water soluble form of ammonia, NH3, which is usually produced by decay) through a 2-step process into NO2- (nitrite) & NO3- (nitrate)
Done by 2 groups of specialized aerobic nitrifying bacteria (for use by plants) to:
1. Nitrite ions (NO2-)
toxic to plants
Nitrosomonas
Nitrosococcus
2. Nitrate ions (NO3-)
easily taken up by plants as a nutrient through roots
Nitrobacter

7. NH3

8. Assimilation Assimilation
Plant roots absorb inorganic ammonia (NH3), ammonium ions (NH4+), & nitrate ions (NO3-).
Formed by nitrification (NH3  NH4+  NO2-  NO3-) & nitrogen fixation (N2 + 3H2  2NH3).
Ions are used to make
N-containing organic
molecules such as:
DNA & RNA
Amino Acids
Proteins
Animals get their N by eating plants or plant-eating animals
Assimilation
NH3

9. Ammonification
Bacteria in soil & water decompose N-rich organic compounds, wastes, cast-off particles & dead organisms into:
1. simpler N-containing inorganic compounds such as ammonia (NH3)
2. water-soluble salts containing ammonium ions (NH4+)

10. Denitrification
Dentrification: other specialized bacteria convert ammonia back into gaseous nitrogen through a multistep process:
Ammonia (NH3) & ammonium ions (NH4+)  nitrite ions (NO2-) & nitrate ions (NO3-)  nitrogen gas (N2) & nitrous oxide gas (N2O)
Done by denitrifying bacteria deep in soil & aquatic sediments
Anaerobic conditions… So… use nitrates as an alternative to oxygen for the final electron acceptor in their respiration.
Replenish atmosphere & close the nitrogen cycle

11. Humans Affect the Nitrogen Cycle
Haber-Bosch process: synthetic production of fertilizers by combining nitrogen & hydrogen to synthesize ammonia
Dramatically changed the nitrogen cycle since devised in 1950s
huge increases in crop yields (due to fertilization)…cheaply
Humans are fixing more nitrogen than nature does
Reduced biodiversity of plants adapted to low-nitrogen soils
Calcium & potassium in soil washed out by fertilizers (depletion)
Acidified water & soils
Disposing of N-rich municipal sewage
Raising cattle in feedlots adjacent to waterways (wastes in runoff)
Changed estuaries & coastal ecosystems & fisheries
N in run-off causes bloom in algae & aquatic plants (eutrophication)
When photosythesizers die in the water their decomposition by bacteria removes O2 from the water (hypoxia) & causes death of other organisms.
Burning fossil fuels & grasslands & deforestation removes N from the soil & produces NOx (a precursor for acid rain & ozone depletion)
Decomposition of wastes & commercial nitrogen fertilizers produces N2O (greenhouse gas)

12. Effects of Human Activities on the Nitrogen Cycle
Human activities such as production of fertilizers now fix more nitrogen than all natural sources combined.

13. Human Inputs of N Into the Environment
Fully half of nitrogen entering the environment is of human origin

17. The Nitrogen Cycle

24. N Cycle Animation
Nitrogen Cycle Animation
N Cycle Tutorial