Many areas of soil have very little vegetation on top of the soil or no vegetation at all. how might this influence the amount of carbon stored in the soil? why?

It will make a reduction of carbon in soil organic matter. As a result, that area will become non-fertile for agricultural purposes in the future. 

Explanation:

Carbon makes up approximately 60% of the soil organic matter (SOM), with the remaining 40% of SOM containing other essential elements such as calcium, hydrogen, oxygen, and nitrogen. When vegetation become lower or no vegetation present in any area, it reduces the carbon of that area because soil carbon comes from the decomposition of plant part such as dead leaves. 

If vegetation is lower in an area, it will reduce the carbon of that area. As a result, gradually, because of the absence of decomposition, that area will become entirely unsuitable for the plant/crop production in the future, which is recognized as non-fertile soil.

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