In my own veg garden, I have noticed that my soil quality starts degrading around >6% of carbon contents.

It wasn't a problem until recently, but with all the droughts in recent years, too high carbon can make it hydrophobic and lead to water run-off that happens too quick once it has entirely dried out. I.e. it becomes too lose/aereated. This then means that the occasional heat storm no longer replenishes water reserves and you end up a bit wirh the same problem as if you had 1% Carbon.

My soil is still way more resilient at the beginning of the drought (I can easily cover 1mo eithout rain without having to water most crops, except some smaller spices). But in recent times I have stopped mulching for a while.