Biogeochemical Cycles
Chapter 3.5-3.6
AP Environmental Science
What is nutrient cycling?
Hydrological Cycle
Carbon Cycle
Nitrogen Cycle
Phosphorous Cycle
Sulfur Cycle
Nutrient Cycling
• Driven directly or indirectly by solar energy and
• Nutrients can accumulate in “resiviors” at
different points along a nutrient cycle.
• Nutrients on Earth and finite and must be
continually recycled
Hydrological Cycle
• Natural renewal of water
• 90% atmospheric water comes from
• 0.024% of Earth’s water is usable
Human Impacts
1. Withdrawal of large amounts of water from
natural systems, sometimes higher than
replacement rates
2. Clearing of vegetation around water sources
1. Decreased transpiration
2. Increased runoff
3. Increased flooding
Carbon Cycle
Based on atmospheric carbon dioxide
0.038% of Earth’s atmosphere is CO2
Photosynthesis and aerobic respiration play a
very important role in this cycle
Human Impacts
• Increasing amounts of carbon dioxide leads to
increased greenhouse gases
▫ Clear cutting of forests reduces CO2 absorption
▫ Warming ocean waters can solubulize less CO2
▫ Fossil fuels are “out” of the cycle and we add them
back into it
Nitrogen Cycle
• Major reservoir is the atmosphere
• N2 cannot be used by most organisms
▫ Nitrification – bacteria convert N2 into NH3
▫ Denitrification – bacteria convert NH3 and NH3+
back in N2
Human Impacts
1. Nitric oxide (NO) is converted to nitric acid
2. Nitrous oxide (N2O) is a greenhouse gas
3. Release of nitrogen faster than normal through
habitat destruction
4. Excess nitrates in the environment from runoff
and sewage
5. Nitrogen removal from crap growing
Phosphorous Cycle
• Does not have an atmospheric component
• Based around phosphate (PO4-3)
• Phosphate can leave the cycle for a long time
once it enters the ocean and settles
Human Impacts
• Phosphate is a major limiting factor in plant
• We release lots of phosphate into the
environment due to fertilizers and sewage runoff
Sulfur Cycle
• Focuses on sulfates (SO4-2) and H2S
• Sources include volcanoes, dust storms, sea
spray, and bacteria byproducts
• Some bacteria can use sulfur to power their
metabolic activities
Human Impacts
• Excess sulfur in the atmosphere can lead to
sulfuric acid rain
• Mining and extraction from the Earth input’s a
large amount of sulfur into the cycle
• Burning sulfur laced coal leads to acid rain
Methods for studying ecology
• Study science directly
• Study ecosystems in the laboratory
• Use models to simulate ecosystems

Biogeochemical Cycles