sooby,
The conversion of the ammonium to nitrate by the microorganisms is one key point in one of the video, but it goes on to break down the plants that benefit the most from each stage of the conversion(a very general wide ranging type of break down). Things like weeds, shallow rooted grasses, at the early part then field crops etc. and I do remember mature forests were at the end of the conversion process as the main beneficiary . I may be reading more into the video that is actually there, but they did seem to me to draw a pretty distinct line between annuals and perennials.
