How is the biosphere both a "carbon source" and a "carbon sink"?

The biosphere has many ways to be a carbon sink. One of the reasons is that the forests collectively are a HUGE carbon sink. The problem is when we cut down forests, that turns them into a carbon source. All the carbon stored in those ginormous trees is released into the atmosphere. - Emily

The biosphere is a very important part of climate change. The biosphere is all the living things, above and below the earth’s surface. All of the plants in the biosphere take in carbon in order to do photosynthesis and is therefore it is a carbon sink. However, when plants die and animals breathe out and release excrement they are putting carbon into the atmosphere and are therefore a carbon source. - Owen

All living things are made up of carbon and also store carbon and take in carbon. For example, if you were to bury an antelope its body would start to break down, releasing carbon. So, while it is alive it is a carbon sink, but when it dies it can become a carbon source. The same thing would happen if you were to bury a flower. The reason that all types of living things are carbon sinks is because all plants absorb carbon and put out oxygen through photosynthesis.     - Hudson

The biosphere is a carbon sink because all living things are made of carbon. When they die their body becomes carbon in the soil. Any agriculture practices that increase the amount of life in a given area can store more carbon in the biosphere, and eventually increase the carbon stored in the soil. The biosphere becomes a source when living things die and decompose, releasing carbon to the atmosphere. - Alan

Animals are an important source and sink. They become a carbon sink when they eat plants and take carbon that would typically be released into the atmosphere and digest it and turn it into body tissues. They are carbon source when they exhale and release carbon dioxide and when they poop and release methane. - Josh

The biosphere is both a carbon sink and a carbon source. For example a plant takes in carbon dioxide and releases oxygen, animals take in the oxygen plants give them and they release carbon and it becomes a cycle. The thing that sets apart plants, tree, animals, insects and fungi is photosynthesis and respiration. Although all living things need respiration to convert stored energy into use (with carbon dioxide entering the atmosphere), only plants, trees, and some microorganisms can perform photosynthesis to capture carbon from the atmosphere. - Piper

All living plants and animals store carbon in them while they are alive, but when they die the carbon decomposes into the soil and much of it ends up in the atmosphere, so the biosphere is only a temporary carbon sink. - CJ

The biosphere is a carbon “source” when deforestation occurs; deforestation is something that has happened to a huge effect ever since the beginning of civilization.  The wood from the trees in the forests is very useful to humans in a variety of ways.  However, when a tree is cut down or a child picks a harmless flower from the ground, the carbon that would have been absorbed by those plants instead goes into the atmosphere.  
The biosphere is a carbon “sink” when deforestation, destruction of habitat, global warming, pesticide use, animal hunting, insect extermination and many more don’t occur.  This allows the biosphere to carry out its natural tasks without further disruption from humans as it has done for billions of years.  This natural cycle also adds carbon to the soil and removes it from the atmosphere. -Aidan

Carbon dioxide is going into the ocean or the biosphere (all living things) making the biosphere an obvious carbon sink, but it can be respired and let CO2 back into our atmosphere. - Henry

The Biosphere is naturally a carbon sink as part of the life cycle. Carbon is emitted into the atmosphere when things die, but then plants absorb the carbon and grow new plants. The Biosphere becomes a carbon source when more things are dying than are replaced by new plants and animals. For example, when a forest is cut down, there is nothing to absorb the carbon back so it just emits carbon instead of cycling it back into the Biosphere. - Ryan

The biosphere serves as a carbon sink, but as most living things breathe, they also release carbon back into the atmosphere. - Leo