Due to excessive propaganda and intentional mis-education and other methods of indoctrination, the large majority of Americans believe that they inherently owe federal income taxes as a result of the 16th Amendment to the federal Constitution; however, this is not at all true.
Prior to the 14th Amendment, the term "Citizen of the United States" was an informal term used to denote citizenship in one of the states composing the American Union. However, once passed, it provided for a formal legal definition of the term.
Thus, from the 14th Amendment and onward, the term specifically refers to a species of federal citizenship (as opposed to state citizenship). To this accord, the 16th Amendment applies to federal citizens and other federal persons; though ignorance would suggest otherwise.