While there are ways to set the default namespace there seems to be no equivalent way for getting the default namespace declaration.
Also, there seems to be no way to either set or get the base URI declaration (xml:base).
However, both base URI and default namespace declarations of a loaded file are essential and need to be known by many applications where Sesame is used as a back-end since this information will help to construct the apropriate URIs and fragment identifiers without asking a user for the information that is stored in the file anyways.