High School Earth Science: Ecosystems and Human Populations - The Carbon Cycle and the Nitrogen Cycle — Flashcards

The carbon cycle
The carbon cycle describes the process in which carbon atoms continually travel from the atmosphere to the Earth and then back into the atmosphere. Since our planet and its atmosphere form a closed environment, the amount of carbon in this system does not change.

The Nitrogen Cycle    
Nitrogen is vital for life on Earth as an essential component of organic compounds such as amino acids, nucleic acids, and chlorophyll. Nitrogen gas is plentiful in the atmosphere, but plants cannot use nitrogen in the gaseous form. To be useful to plants, nitrogen must be “fixed,” or converted into certain nitrogen compounds. Some nitrogen is fixed by lightning or blue-green algae, but most is fixed by bacteria in soil. Soil bacteria combine nitrogen gas with oxygen or hydrogen and create nitrogen-containing compounds such as ammonia. Nitrogen-fixing bacteria live either freely in the soil or in a symbiotic relationship with leguminous plants (peas, beans, peanuts). The symbiotic bacteria use carbohydrates from the plants to produce ammonia that the plants can use as a source of nitrogen. When the legumous plants die, their nitrogen compounds are returned to the soil, where other plants can use them.    
Animals obtain nitrogen by eating plants. They use the nitrogen to grow animal tissues. After a plant or animal dies or produces wastes, bacteria and fungi in the soil fix the organic nitrogen in the remains or wastes and return it to the soil as ammonia. Nitrifying bacteria oxidize the ammonia to nitrites and nitrates, which can be used by the next generation of plants.    
When there are few usable nitrogen compounds in soil, this can curtail plant growth. Modern agricultural practices increase plant productivity by adding nitrogen fertilizers to soil. This can have unintended consequences:    

- Nitrogen from fertilizers may return to the atmosphere as nitrous oxide or ammonia, both of which may have deleterious effects. Nitrous oxide contributes to the breakdown of the ozone layer, and ammonia contributes to smog and acid rain.    
- Excess fertilizers run off the land and end up in ponds, lakes, and coastal areas of the ocean. The nitrogen causes enormous numbers of bacteria and algae to grow. When these organisms die, their decomposition uses up all the available oxygen. Without oxygen, fish and most other organisms cannot survive. On a large scale, this creates an area called a dead zone.