Well then make sure there is no element in the DOM tree that is in a
namespace. If the serializer does e.g.
  xmlns=""
when serializing your DOM tree then that means there is an element in
the DOM tree which is in a namespace.
So somewhere up the tree you have an element in a namespace, if you
don't want that then don't create it at all or remove it before serializing.

For instance with the code

    XmlDocument xmlDocument = new XmlDocument();
    xmlDocument.AppendChild(xmlDocument.CreateElement("root"));

xmlDocument.DocumentElement.AppendChild(xmlDocument.CreateElement("child"));
    xmlDocument.Save(Console.Out);

the result is

<root>
  <child />
</root>

so you can see there is no xmlns="" declaration. However if the code is e.g.

    XmlDocument xmlDocument = new XmlDocument();
    xmlDocument.AppendChild(xmlDocument.CreateElement("root",
"http://example.com/2006/ns1"));

xmlDocument.DocumentElement.AppendChild(xmlDocument.CreateElement("child"));
    xmlDocument.Save(Console.Out);

then the result is

<root xmlns="http://example.com/2006/ns1">
  <child xmlns="" />
</root>

You can see that the creation of the element with tag name "child" is
the same in both examples, only the serialization differs as the parent
element is in a namespace in the second example. That means you have to
look at the ancestors of the element where you get xmlns="" and make
sure that the ancestor is in no namespace.