How is topsoil both a "carbon source" and a "carbon sink"?

Topsoil can be a carbon “source” when tilled.  Tilling breaks up and aerates the topsoil and releases the carbon within into the atmosphere.  Tilling also exposes the soil to direct sunlight, which dries it out and depletes it of all its nutrients, effectively killing it.  “Bare soil is dying soil”.

Topsoil can be a carbon “sink” when given access to the key nutrients it needs (nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, silicon, and carbon among others) and left untilled.  When a field is left untilled it uses the organic matter to fuel the plants to take in more and more carbon.  Something else that helps maximize carbon intake among soil is mob grazing.  Mob grazing is when a large herd or animals grazes an entire field, defecates, and then leaves. The herd tramples the grass, making it impossible for another coming herd to eat for a while, giving the grass time to recover. Finally the manure provides organic matter for the soil to stay healthy.  The use of cover crops also adds a lot of carbon to the soil; cover crops are plants that are grown to be put back into the soil. Cover crops sustain themselves as well as the soil, and are very useful for keeping the soil healthy.  Therefore adding significant amounts of carbon to the soil. - Aidan

Carbon sinks are things that can store any form of carbon outside of the atmosphere, and carbon sources release carbon to the atmosphere. Soil across the globe stores around 2,000 billion tonnes (a tonne is equal to 2,205 pounds) of carbon in many different forms at any one time. About 300 billion tonnes can be found as detritus (dead particulate organic material). This makes soil a huge carbon sink, but it can also be a carbon source. During the disintegration of some of the carbon in the topsoil, detritus is respired by decomposing organisms like fungi and bacteria, allowing carbon back into the atmosphere as CO2 (Carbon Dioxide), making topsoils a carbon source as well. - Henry http://www.ghgonline.org/co2sinksoil.htm

Topsoil can be both a carbon sink and a carbon source. It began as a carbon sink because the natural decomposition from animals and the large amounts of plants pulled carbon back into topsoil. Unfortunately the kind of agriculture most people do doesn’t absorb as much carbon as it should, it has started to become more of a carbon source than a sink. Topsoil is a carbon source when farmers just till the soil and don't put anything back in, like compost or recently dead cover crops. - Ryan

Topsoil is one of the biggest carbon sources as well as one of the biggest carbon sinks. Topsoil absorbs all of the dead things that were made of carbon. When farmers till the land and aerate the soil, they release the carbon back into the atmosphere. - Josh

Topsoil is a carbon sink when the carbon from dead plants and animals moves through the topsoil down into the earth and goes through a process of decomposition. It breaks down into humus that makes stable carbon organic matter in the soil. But topsoil is a carbon source when any carbon that is left over from decomposition comes up through the soil as gas and naturally releases. If more carbon is released as gas than is added to the soil, the soil is a carbon source. - Piper

Top soil can be a carbon sink because it is usually very dark with organic matter. It can be a carbon source when that organic matter decomposes and the carbon is released. - CJ

Topsoil can hold carbon, but when it is tilled, it releases it. - Leo

Topsoil is distinguished from subsoil because it contains carbon. That carbon comes from dead and living plants and organisms. Topsoil is a carbon sink because all of that organic matter can stay in the soil. But it is a carbon source because all of the organisms inside of the soil eat carbon and use it to survive, when they eat the carbon, it releases carbon dioxide and methane as by products of decomposition.  When humans till the soil, we speed up decomposition by adding oxygen to the soil and release more carbon and it goes straight to the atmosphere. - Alan

The soil could be a carbon source for the atmosphere by the means of tilling the topsoil and releasing the carbon from the soil. The topsoil is made up of organic matter, and when you till it or use chemical farming it will cause the carbon to release. One of the ways the soil could be a carbon sink is because it stores carbon in itself. The soil could be well laid with compost, which would be a good way to add carbon to the soil. - Emily

In natural ecosystems, topsoil is definitely a carbon sink because much of the carbon from dead plants doesn’t leave the soil. Much the carbon, instead of leaving because the soil was tilled, stays in the soil and simply sits there and feeds the microorganisms and becomes humus. Also when plants die, they decompose and give off methane and carbon dioxide. However when soil is tilled or broken up, it the soil can shift to become a carbon source for the atmosphere. - Owen

Topsoil is an area of soil at the top of the ground. Now that 5-10 inch (or more) area can be used to store large amounts of carbon. This soil can only be maintained through organic farming and good tilling methods.
- Hudson