There's also these pages as well. This one describes how the Juubi's chakras is just like a huge mass of Natural Energy.
http://www.mangastream.to/naruto-chapter-611-page-7.html
And this one shows Kurama meditating to create more chakra.
http://www.mangastream.to/naruto-chapter-612-page-9.html
So if the Tailed Beasts are the by-product of separating the Juubi from it's chakras, then it's safe to say that they are completely composed of chakra formed from Natural Energy. This is also probably why they can apparently respawn after being killed.
A good way of looking at it from that line of thinking, but ultimately I would have to disagree. I had a similar train of thought when the Juubi was first introduced that it would have all the powers of the Tailed Beasts in one, which it doesn't. It has its own unique abilities just like the rest of them do aside from Tailed Beast Ball. So I say the Natural Energy composition is unique only to the Juubi otherwise the following would not make sense:
1. If this were the case it would have been mentioned way beforehand, especially by Jiraiya who had to fight off Naruto's Four Tailed Version 2 state (yet nothing of Natural Energy or Senjutsu is mentioned) or Naruto when he encountered the Tailed Beasts or fought Kurama.
2. All the shinobi who were given chakra during the war should have been petrified along with anyone else who absorbed Tailed Beast chakra, has it in their system, or atleast showed signs of severe mutation, meaning all Jinchuriki should have died unless each one had a Sage Mode.
3. Tailed Beast chakra and Senjutsu chakra would be virtually synonymous.
4. Like others mentioned, Naruto would not need to infuse the Natural Energy from his surroundings, all he would need is Kurama's chakra, and boom near infinite Sage Mode. Kurama himself had to gather Natural Energy, if any part of him had that, it would then make it pointless.
The Tailed Beast are much like Asura and Indra, their chakra still remains after death and they go through reincarnation, not so much any Natural Energy involvement, they are just immortal like that. Kurama gathering the energy for Naruto most likely from a watch and learn experience.