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Cycles of Matter: The Nitrogen Cycle and the Carbon Cycle

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  • 0:05 All Matter Cycles
  • 0:54 Cycling of Carbon
  • 3:56 Cycling of Nitrogen
  • 6:17 Lesson Summary
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Lesson Transcript
Instructor: Peter Jaeger

Pete currently teaches middle school Science, college level introductory Science, and has a master's degree in Environmental Education.

Matter is constantly cycled between living and nonliving parts of the environment. Processes like photosynthesis and nitrogen fixation allow the carbon and nitrogen cycles to regenerate needed substances by recycling Earth's atoms.

All Matter Cycles

One of the ways nature recycles is through the carbon cycle
Carbon Cycle

You know about recycling, right? We all probably sort our trash to save things like aluminum cans, plastic bottles and newspaper. We all probably also know why we do it: to conserve resources. What would happen if we didn't conserve resources? Well, the things that are used to make up those products might become more scarce, leading companies to search for more raw, natural resources from the environment, which drives up prices. In some cases, new ways would have to be engineered to make those products if new sources can't be found.

Recycling is just a good idea, and nature is a master recycler. Even when humans don't, nature will get its way and cycle atoms and molecules back again. Carbon and nitrogen are great examples of how nature does this.

Cycling of Carbon

One of the ways that nature recycles matter is through the carbon cycle. The carbon cycle is carbon cycling through the global environment. Carbon is a chemical element and a key component of many systems in the biosphere, from acting as part of the earth's thermostat to being one of the key elements in photosynthesis, which is when plants make sugars for energy.

In order for the ecosystem to function properly, all parts need an adequate supply of carbon. This is usually not a problem since nature is efficient at carbon cycling. Since the process is a cycle, we need to pick some place to begin.

One of the biggest reservoirs of carbon is the atmosphere, which is about 0.038% carbon dioxide. There are two ways for carbon to be removed from the atmosphere. The first is through photosynthesis, where plants take in CO2, water and sunlight to create sugars for energy, and oxygen gas emerges as a by-product. Once inside plants, carbon moves through food chains, where organisms become nutrients including herbivores, carnivores and ultimately, decomposers. Through living organisms, carbon is either re-released back into the atmosphere through respiration (where organisms use oxygen to generate energy from nutrients and produce carbon dioxide as waste), released by combustion (the process of burning something) or broken down into the soil as part of the organism's body. Once buried in the soil, carbon can be converted into fossil fuels over long periods of time and then also reenter the atmosphere by combustion.

If carbon does not enter land plants by photosynthesis, it can be taken into the ocean
Carbon Can Enter Ocean

If carbon from the atmosphere does not enter a terrestrial (or land) plant by photosynthesis, it can dissolve in the ocean. Here it can be taken up by marine plants through photosynthesis - just like in land plants - or it can be incorporated into sediments. Marine organisms can also take up dissolved carbon molecules and use that along with calcium in the seawater to make calcium carbonate, which is a major component of the shells and skeletons of marine organisms.

When these organisms die, their shells and bones settle to the bottom of the ocean, where they can be covered up and remain for long periods of time. Under great pressure from the water and sediment, these shells break down and form limestone rock.

Limestone is the largest storage reservoir of carbon on the earth. Once formed into limestone, carbon usually stays locked in the rock. However, it can also dissolve very slowly to be released as carbon dioxide back into the atmosphere, or, if the limestone is exposed to weathering and dissolved by acid rain, be released as carbon dioxide. This completes the cycle, returning all carbon back to the atmosphere where it began. So, following this line of thinking, the carbon molecules that are in our body have been cycling on the earth since it was formed and will continue to do so as we exhale each breath, returning CO2 back to the atmosphere.

Cycling of Nitrogen

Unlike carbon, nitrogen cannot be directly used as a nutrient by plants or animals
Nitrogen Not Nutrient

Like carbon, nitrogen also has always been present on the earth, and in the nitrogen cycle, nitrogen cycles through the global environment. Nitrogen is also a chemical element, and it is the most abundant element in the atmosphere, making up 78% of the atmosphere. Nitrogen is very unreactive with other elements, and it cannot be used directly as a nutrient by plants and animals the way oxygen or carbon can. However, nitrogen can be converted into forms usable by plants and animals, either by lightning or bacteria. Once converted to usable forms, nitrogen is able to cycle the rest of the way through the ecosystem.

As mentioned, the atmosphere is the largest reservoir of nitrogen. N2 occurs when two atoms of nitrogen are bonded together very strongly. Lightning has enough energy to split these atoms, which then bond with oxygen in the atmosphere to make nitrates that fertilize the soil and are taken into plants as nutrients.

In a process known as nitrogen fixation, special bacteria can convert nitrogen gas in the atmosphere directly into ammonia, which is an important nutrient for plants. In this manner, nitrogen enters the ecosystem from the atmosphere. This conversion is performed either by cyanobacteria in the soil or by a bacteria that lives in the roots of certain plants known as legumes, such as soybeans or alfalfa.

The bacteria rhizobium fixes nitrogen so that it can be absorbed by the plant roots
Bacteria in Roots

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