I found a section in a book, Plants and Nitrogen by Lewis, where it is pointed out that of four species of conifer two grew best on nitrate, one preferred ammonium and the other liked either ammonium or ammonium plus nitrate in a combo, so apparently one can't generalize for conifers at least. It goes on to say that the majority of plants show a better growth response "when fed a mixed ammonium and nitrate nitrogen source than when fed one of these sources alone".
