I've tried to cut stumps down below the soil line at various times over the years and always am reminded, the hard way, that there are lots of little stones and sand and other chain wreckers apparently embedded in stump wood. Even if you carefully dig everything out and clean the area to be cut, you're always going to ruin your chain pretty quick. The closer to the ground you cut any tree, the more grit your chain is going to encounter, if for no other reason that sand and other stuff splashes up against the trunk when it rains or the wind blows, and some of it is going to stick.