That carbon isn't really sequestered. Much of it becomes CO2 and methane as it decomposes.

I am also struggling to imagine how you would interr the mulch without damaging the forest itself. Most trees have root systems as wide as their leaf canopy- depending on the species, you'd end up killing or stressing most of the trees you dig around if you toss it deep enough to not burn during a drought.

Now, with all the dead or stressed trees, you've got another out of control wildfire waiting to happen.