Key Points from Lecture on Nitrogen and Phosphorus Cycles

Introduction

Importance of essential elements: Nitrogen and Phosphorus.
Nitrogen and phosphorus are crucial but often in forms not accessible to organisms.

Nitrogen in the Atmosphere:
- 78% of the atmosphere is nitrogen gas (N2).
- Nitrogen molecules are difficult to break due to a triple bond.
Nitrogen Fixation:
- Plants cannot directly use atmospheric nitrogen.
- Nitrogen-fixing bacteria convert nitrogen gas into ammonia, using the enzyme nitrogenase.
- Forms a symbiotic relationship with legumes (e.g., soybeans, clover).
Nitrification:
- Nitrifying bacteria convert ammonia into nitrates and nitrites.
- These forms are more accessible to plants.
Denitrification:
- Denitrifying bacteria convert nitrates back into nitrogen gas, completing the cycle.
Human Impact:
- Synthetic fertilizers enhance nitrogen availability.
- Historical note: Fritz Haber's process for ammonia synthesis.

Phosphorus in the Lithosphere:
- Does not involve atmospheric processes.
- Found in rocks, particularly sedimentary rocks.
Weathering and Erosion:
- Releases phosphates into soil and water.
- Plants assimilate dissolved phosphates; animals obtain phosphorus by consuming plants.
Aquatic Ecosystems:
- Phosphorus is vital for aquatic life.
- Deep ocean cycles can trap phosphorus for extended periods.
Human Impact:
- Overuse of phosphorus fertilizers affects ecosystems.

Despite their abundance, nitrogen and phosphorus are challenging to access due to their natural states.
Human activity has significantly altered their natural cycles, often with negative environmental impacts.
Preview of next topic: Human impacts on the biosphere and the environment.

Written by Jesslyn Shields, Blake de Pastino, and Hank Green.
Technical direction and editing by Nick Jenkins, graphics by Peter Winkler, sound by Michael Aranda.
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