DOMLSSerializer producing empty xmlns attributes

I use Xerces to create a new XML file with a DOM Node, but when I use class DOMLSSerializer to save it to disk, I found that every node which I created included an empty attribute xmlns=""

For example:

<Test xmlns="">
  <ExtRef xmlns="" Name="Val" fir="tt" way="new"/>
</Test>

When I import an XML file into my new XML file, if the other file has an xmlns attribute, every node which I created included its xmlns attribute.

For example:

<Node id="1.00" name="ttt">
  <line desc="new" xmlns:ext="http://www.com" />
  <rect desc="times" xmlns:ext="http://www.com" ext:eOption="M"/>
</Node>

How can I remove these empty xmlns attributes that were added?