Got some reading for you, byron-ed!...The Other Slavery: The Uncovered Story of Indian Enslavement in the Americas by Andres Resendez...I can get you more if you like that - it's an appetizer.
And yet quite certain that this source will not support that a quarter of the slave population in the U.S. slave South was indian. It will likely demonstrate that yes, natives were the first population to be enslaved by Euros in North America, and it will likely demonstrate that
indigenous populations made up a considerable population of slaves in the early colonization years, and continued as such in the Caribbean, middle and South America, but not in the developed U.S. Cotton South.
By the 19th century --
the period we are interested in after all, that leading up to the Civil War -- indians (meaning native Americans) had been phased out as slaves in favor of readily-marketed and much more containable black African slaves.
The condition we know as chattel slavery in the secession South and the Confederacy was most entirely that. There was nowhere near a 25 percent content of indian slaves left in the slave population of the U.S. South by that time -- not that indians fared any better otherwise.
I also trust your source will concur that among those indians who remained U.S. slaves in Antebellum times, many of them were by then mixed-breed. In other words that population was not so distinct that they were specifically sorted out in any document as a percent. Nobody took the time to count out the indian and mixed-breed indian slaves that way. With that in context, I'm confident your source, being academically viable, doesn't actually claim that a quarter of the population of slaves in the U.S. slave South were indian.
Sometimes we find the answers we want from a source (the way we want things to have been) and kind of don't remember the context.