1. Fertilizers and Population
Influencing the Nitrogen Cycle

2. Nitrogen and Plants Plants need nitrogen
Proteins
Nucleic Acids
Plants can not use non-reactive nitrogen from the atmosphere N2
Plants can use reactive nitrogen from the soil
NO3– (nitrate)
NO2– (nitrite)
Lots of atmospheric N2
Reactive soil N is limited

3. Nitrogen and Farms
In a natural ecosystem (ie. a redwood forest), N is recycled when plants/animals die
In a managed agricultural system (ie. a wheat farm), N is taken away when the crop is harvested
Soil N must be replenished
Crop rotation
Compost/manure
Artificial fertilizers

4. Fertilizer as Agricultural Input
Fertilization allows for increased yields (kg food/acre)
lb/acre
$100-$300/acre
Not all fertilizer is used by the plants

5. Fertilizer as an Agricultural Input
Both fertilizer use and agricultural yield/production have increased over the past 50 years

6. Nitrogen Cycle

7. Nitrogen Cycle
How does N go from atmosphere (non-reactive) to soil (reactive)?
Lightning
Biological fixation by symbiotic bacteria
Industrial fixation (fertilizers)
How does N go from biosphere – plants and animals – to the soil?
Decomposition
Wastes (urine and feces)

8. Nitrogen Cycle

9. Nitrogen Cycle

10. Artificial Fertilizers
Process developed by Haber and Bosch in early 20th century
Uses N from atmosphere and H from methane to make ammonia (NH3)
Requires lots of energy to generate heat and pressure

11. Haber-Bosch Process Nitrogen From air 400-450º C 200 atmospheres
Iron catalyst
Liquid Ammonia (NH3)
Hydrogen
from methane (natural gas) or petroleum
What will run out first – nitrogen, hydrogen or the energy for heat/pressure?

12. Artificial v. Natural Artificial/Industrial
Produced rapidly and in large quantities
N is in form readily available to plants
N levels are high and can be adjusted
Fossil fuel intensive (both matter and energy)
Natural
Uses plants (compost) + animal wastes (manure)
Process is slow
N must be broken down to be available to plants
N levels are lower than artificial fertilizers
Does not require energy inputs/fossil fuels

13. The Questions
Is human manipulation of the global nitrogen cycle good or bad?
Pros: increased crop yields, increased food, increased profits
Cons: impacts on soil, water and air
Are increased crop yields worth the environmental costs? What evidence would inform this decision?
Is the use of industrial fertilizers sustainable? If so, for how long? What evidence would inform this?
Are there alternatives to industrial fertilizers? Are these alternatives more sustainable? What evidence would inform this?