I'm working on a paper about derivation, denominalization, zero-affixation and so on and have the following question: If a verb derives from a noun and it used in a past tense, for example boycott - to boycott - boycotted does boycotted still belong to the zero-affixation?
“Boycotted” has no derivational affixes, so it belongs to zero derivation aka conversion. The affix -ed is an inflectional affix, so you cannot generally say “boycotted” is an example of zero affixation, it does have an affix.
It all depends on what you are talking about. If it goes only about derivation mechanisms, then “boycotted” is an example of a zero-affixation derivation, since it is not anything like *boycottized or *boycottified, no derivational affixes are present.
It looks like this point is exactly what you have to come up with in your paper, you have to just find a way of wording to set apart the derivational and inflectional affixes, to make it clear when you write about the former and when about the latter or both together.