Environmental Science / Unit 2: Ecosystems and Energy Flow
2.5 The Nitrogen and Phosphorus Cycles
Notes
Organisms need nitrogen for proteins and DNA, and phosphorus for DNA, ATP, and bones.
- Air is about 78% nitrogen gas ($N_2$), but most organisms cannot use it directly.
- Nitrogen fixation: bacteria (in soil and legume root nodules) and lightning convert $N_2$ into usable ammonia or nitrates.
- Nitrification: bacteria convert ammonia to nitrite and then nitrate. Assimilation: plants take up nitrates. Ammonification: decomposers turn waste into ammonia. Denitrification: bacteria return $N_2$ to the air.
- Humans add nitrogen with fertilizers and by burning fuel, which can cause water pollution.
- Phosphorus has no gas phase. It cycles slowly through rocks, soil, water, and organisms, and weathering of rock releases it. It often limits plant growth in freshwater.
- Example: planting clover (a legume) adds nitrogen to soil because of bacteria in its roots.
Practice
What is nitrogen fixation?
Fixation makes nitrogen biologically usable.
How does the phosphorus cycle differ from the nitrogen cycle?
Phosphorus is released mostly by weathering of rock and cycles slowly.
Why do farmers often rotate crops with legumes such as beans or clover?
Root nodule bacteria convert $N_2$ into usable nitrogen compounds.
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