1. Cycles

2. Carbon cycle Phosphorus cycle Nitrogen cycle Water cycle Oxygen cycle
Biosphere
Carbon
cycle
Phosphorus
cycle
Nitrogen
cycle
Water
cycle
Oxygen
cycle
Figure 3.7
Natural capital: life on the earth depends on the flow of energy (wavy arrows) from the sun through the biosphere and back into space, the cycling of crucial elements (solid arrows around ovals), and gravity, which keeps atmospheric gases from escaping into space and helps recycle nutrients through air, water, soil, and organisms. This simplified model depicts only a few of the many cycling elements.
Heat in the environment
Heat
Heat
Heat
Fig. 3-7, p. 55

3. CARBON CYCLE

4. Effects of Human Activities on Carbon Cycle
We alter the carbon cycle by adding excess CO2 to the atmosphere through:
Burning fossil fuels.
Clearing vegetation faster than it is replaced.
Figure 3-28

5. Phosphorous Cycle

6. Effects of Human Activities on the Phosphorous Cycle
We remove large amounts of phosphate from the earth to make fertilizer.
We reduce phosphorous in tropical soils by clearing forests.
We add excess phosphates to aquatic systems from runoff of animal wastes and fertilizers.

7. Phosphorus
Bacteria are not as important in the phosphorus cycle as in the nitrogen cycle.
Phosphorus is not usually found in the atmosphere or in a gas state only as dust.
The phosphorus cycle is slow and phosphorus is usually found in rock formations and ocean sediments.
Phosphorus is found in fertilizers because most soil is deficient in it and plants need it.
Phosphorus is usually insoluble in water and is not found in most aquatic environments.

8. Nitrogen Cycle

9. Effects of Human Activities on the Nitrogen Cycle
We alter the nitrogen cycle by:
Adding gases that contribute to acid rain.
Adding nitrous oxide to the atmosphere through farming practices which can warm the atmosphere and deplete ozone.
Contaminating ground water from nitrate ions in inorganic fertilizers.
Releasing nitrogen into the troposphere through deforestation.

10. Effects of Human Activities on the Nitrogen Cycle
Human activities such as production of fertilizers now fix more nitrogen than all natural sources combined.
Figure 3-30

11. Nitrogen Fixation
This is the first step of the nitrogen cycle where specialized bacteria convert gaseous nitrogen to ammonia that can be used by plants. This is done by cyanobacteria or bacteria living in the nodules on the root of various plants.

12. Nitrification Assimilation
Ammonia is converted to nitrite, then to nitrate
Assimilation
Plant roots absorb ammonium ions and nitrate ions for use in making molecules such as DNA, amino acids and proteins.

13. Ammonification Denitrification
After nitrogen has served its purpose in living organisms, decomposing bacteria convert the nitrogen-rich compounds, wastes, and dead bodies into simpler compounds such as ammonia.
Denitrification
Nitrate ions and nitrite ions are converted into nitrous oxide gas and nitrogen gas.
This happens when a soil nutrient is reduced and released into the atmosphere as a gas.

14. The Sulfur Cycle
Figure 3-32

15. Effects of Human Activities on the Sulfur Cycle
We add sulfur dioxide to the atmosphere by:
Burning coal and oil
Refining sulfur containing petroleum.
Convert sulfur-containing metallic ores into free metals such as copper, lead, and zinc releasing sulfur dioxide into the environment.