How does carbon cycle through an ecosystem?
The carbon cycle is much like the water and nitrogen cycles.
The carbon cycle starts when a plant dies and gets buried turning into fossil feul.This fossil feul is burnt by humans converting most of the carbon into carbon dioxide. Carbon dioxide is added to the atmosphere by respiration of animals and the plants.Plants again use this carbon dioxide to perform photosynthesis thus quickly adding it to their bodies from where the cycle continues.
Atmospheric carbon, lithospheric carbon, etc.
A nice illustration of carbon cycle (VisionLearning.com)
A cartoon of the global carbon cycle. Pools (in black) are gigatons (1Gt = 1x109 Tons) of carbon, and fluxes (in purple) are Gt carbon per year. Illustration courtesy NASA Earth Science Enterprise.
image © NASA
As you can see, We have over 400 ppm
The key terms in Carbon cycle are photosynthesis and respiration.