Excellent paper so far.
I would like to provide a couple of suggestions:
Soil C in Space and Time
You might consider providing a range of global soil C estimates to really emphasize the difficulties (and variability) of measuring and quantifying SOC.
Soil C depletion and Management
The importance of soil management to achieve soil security (and it's central role in other global challenges outlined by Alex) could be reinforced here.
A vote up will earn me a one-time waived author fee :D
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