How to add auto taxonomy to node fields?
If I understand you, there is a few modules which can handle it:


Word Link (I recommend it)



  The Word Link module allows you to automatically convert specific
  words into links. It works as text filter. This can be useful for
  crossposting your site's pages, or for the contextual advertising of
  your partners (SEO).



Active Taxonomy Links



  Active Taxonomy Links module processes nodes of selected content types
  replacing words in the nodes with links to taxonomy terms.



Glossify (this one supports Drupal 6 also)



  The Glossify module provides filters that scan and parse content and
  replace terms in the text with links to their pages. The d7 version is
  a complete rewrite of the module to be as simple and performant as
  possible.