It may sound like some kind of monster, and in some ways it is, though normally it just appears as in innocuous fungus at the base of hardwood trees. It’s a fungus called Kretzschmaria deusta found in temperate climates worldwide that can cause fairly severe decay. It has two visual forms. In the spring it grows new fruiting bodies that are gray with white edges that emerge directly at the base of trees and logs that are infected. Later in the year these turn into black crusty lumps that look like charcoal blisters. Both forms can be seen on a Big Leaf Maple in the photos below that I took during the spring in South Seattle.
The fungus is a decayer of both cellulose (the fibers that add flexibility) and lignin (which give rigidity to wood). Thus it can be a fairly damaging fungus.
Sodium nitrate does not dissolve the tree stump but adds nitrogen to speed the growth of fungi in the stump. It is the fungi that break down stump not the sodium nitrate.