Add child to root XElement in XDocument

I am trying to add a node to root element in XML file with XDocument.Load . The problem is that it reapeats the header when I add a new node. Here is the function to create XML file:

private void createDoc()
        {
            XDocument doc = new XDocument(new XDeclaration("1.0", "UTF-8", "yes"),
                new XElement("Items", new XComment("Here will be added new nodes")));
            using (IsolatedStorageFile isoStore = IsolatedStorageFile.GetUserStoreForApplication())
            {
              if (isoStore.FileExists("positions2.xml"))
                {
                    Debug.WriteLine("File Exists!!!");
                    isoStore.DeleteFile("positions.xml");
                }              
                else
                {
                    using (IsolatedStorageFileStream isoStream =
                        new IsolatedStorageFileStream("positions2.xml", FileMode.Create, isoStore))
                    {
                        doc.Save(isoStream);
                    }                   
                }
            }
        }

From here everything looks fine and the output is ok:

<?xml version="1.0" encoding="utf-8" standalone="yes"?>
    <Items>
      <!--Here will be added new nodes-->
    </Items>

To add a child to the root node I use this function:

private void AppendToXMLFile(string reg, string butname, int oldposition, int newposition)
        {
            using (IsolatedStorageFile isoStore = IsolatedStorageFile.GetUserStoreForApplication())
            {
                using (IsolatedStorageFileStream isoStream = new IsolatedStorageFileStream("positions2.xml", FileMode.Open, isoStore))
                {
                    XDocument doc = XDocument.Load(isoStream);
                    var newElement = new XElement("channel",
                       new XElement("region", reg),
                       new XElement("name", butname),
                       new XElement("oldposition", oldposition),
                       new XElement("newpostions", newposition));
                    doc.Element("Items").Add(newElement);  //add node to root node
                    doc.Save(isoStream, SaveOptions.OmitDuplicateNamespaces);
                }
            }
        }

And here is the output after the AppendToXMLFile function is called :

<?xml version="1.0" encoding="utf-8" standalone="yes"?>
<Items>
  <!--Here will be added new nodes-->
</Items><?xml version="1.0" encoding="utf-8" standalone="yes"?>
<Items>
  <!--Comment to prevent <Items />-->
  <channel>
    <region>test</region>
    <name>test1</name>
    <oldposition>6</oldposition>
    <newpostions>0</newpostions>
  </channel>
</Items>