See previous posts on nitrogen: Importance of Nitrogen in Winemaking, Yeast Assimilable Nitrogen - Part I
Once the YAN level is determined for a particular juice/must, the winemaker can decide how much and when additional nitrogen will be required (this is not an exact science, but the chart here gives a pretty good estimation of the total amount of YAN required through fermentation; basic rule is the higher the brix, the higher required YAN).
The most common nitrogen supplement used in the wine industry is
diammonium phosphate (DAP). DAP provides approximately one-fourth of its weight in assimilable nitrogen (to be extact, 100 ppm of DAP will provide 22.5 ppm of assimilable nitrogen). It is often used in conjunction with
proprietary fermentation nutrient blends (most of which provide small amounts of nitrogen, but more importantly essential micro-nutrients). Winemakers need to be cautious about overusing DAP; yeast will preferentially take up ammonia ions over amino acids, whose synthesis provides a significant source of aroma and flavor precursors.
Timing is a major concern when planning nitrogen supplementation of juice/must. Depending on how much nitrogen is required, I
like to add approximately half at the beginning of fermentation and the
remainder after approximately one-third brix decrease; this provides the yeast with a steadier
nitrogen supply to carry through the end of fermentation and, if necessary, leaves room for a small addition if undesirable odor/flavor appears later in ferment.
It is not wise to add it all prior to inoculation; high YAN levels at the beginning of fermentation promotes rapid cell growth that can lead to overpopulation of yeast. This in turn depletes YAN at an abnormally high rate and leads to a lot of unhappy yeast producing undesirable odors and flavors through the duration of fermentation. On the other hand, nitrogen supplementation late in ferment is often useless; yeast can uptake nitrogen throughout their growth phase, but alcohol inhibits uptake about halfway or two-thirds through fermentation (i.e. with 8-10° brix left when starting with 22° brix). This means late supplementation of nitrogen will basically have little or no effect on fermentation kinetics or undesirable odor/flavor production; that being said, I know plenty of winemakers (I admit I have done this on rare occasions as well) that will add nitrogen down to 5° brix in the hopes of reviving a troublesome ferment.
Also read Yeast Assimilable Nitrogen - Part II (Comment Responses)