XPath to get namespace of root node

Given the following xml snippets:

<ns2:Invoice xmlns="http://ift.tt/1tYqNLC" xmlns:ns2="http://namespace2.com"
  xmlns:ns3="http://namespace3.com">
.........
</ns2:Invoice>

and:

<ns3:Invoice xmlns="http://ift.tt/1tYqNLC" xmlns:ns2="http://namespace2.com"
  xmlns:ns3="http://namespace3.com">
.........
</ns3:Invoice>

Can xpath be used to get the namespace associated with the root () element?

Expected results would be as follows: In the first example, the root is prefixed "ns2" so I would expect "http://namespace2.com" In the second example, the root is prefixed "ns3" so I would expect "http://namespace3.com"