How do they measure Carbon trapped in the "land" Carbon sink? They appear to assume that the amount has decreased, but by what scientific measurement? Please tell me there is more to this than "topsoil has decreased".

Come to think of it, I'm also unclear as to how they are measuring how much carbon has been absorbed into the "water" sink. Are they simply measuring the carbonic acid content of sea water?

Does their caclulation of carbon trapped in the "water" sink include carbon that has been precipitated out of the water to form reefs, and other Calcium carbonate deposits? If so how is that measured?

In the comments section, individuals point to topsoil loss as evidence of the loss of carbon from the "land" sink. Isn't it probable that any such carbon lost due to erosion is now contained within the "water" sink as silt and other debris at the bottom of the oceans? Again, how is that measured? or do they assume that it has somehow become suspended in the atmosphere?

I'll wait to see what others post, but this analysis you linked to doesn't seem to make a strong case for "mankind is destroying the carbon cycle". At best, it appears to be based on a lot of questionable conjecture.