Safe permanent sealants?

I have three very large Amur maples that I have let rot for about four years. I have transplanted two of them and notice that all the roots are at the surface with essentially none under the rotted trunks.  While I have now preserved the upper part of the rotted trunk there is a smaller section extending below the soil line. That will continue to wick water up the trunk.

So I'd like to permanently seal the subsurface trunk.  I was going to permanently seal it and then set it on a plate.  But I can't think of what to use to seal it because everything I think of takes a while to dry.  I can't leave the feeder roots exposed that, or can I?

Can you give us a picture? There may be other solutions.