Answer:
Basically, the ecosystem consists of all plants and animals, that interact with each other, and are living in a particular territory. Plants and animals represent the biotic (living) component of the ecosystem, while the soil, minerals, gasses, climate etc. represent abiotic (non- living) part.
Carbon is an essential element for all living beings and it is the main component of both organic and inorganic components.
Now, the question is how does that carbon enter the biotic part of ecosystem from the atmosphere (abiotic part).
In the atmosphere, carbon is mostly found in the form of carbon dioxide (CO₂). Plants are able to use it in this form, and with water and sunlight, through the process of photosynthesis, incorporate it in the organic components, such as sugar and complex carbohydrates. Animals that feed on plants can now ingest these compounds, making it part of their own organisms.
In this way, photosynthesis is the main process by which carbon from the atmosphere enter the biotic part of the ecosystem.