The concentration of yeast assimilable nitrogen in Merlot grape juice is increased by N fertilization and reduced irrigation

Vineyard management practices that can be used to elevate yeast-assimilable nitrogen (YAN) above the 140 mg N L-1 required for efficient fermentation are of critical interest. The effects of N fertilization and reduced irrigation frequency on grape juice YAN, fruit composition and yield were examined in a 5-yr study on Merlot (Vitis vinifera L.) vines. Fertilization with N increased the concentration of YAN in grape juice by improving grapevine N status as indicated by petiole N concentrations. Reduced irrigation frequency appeared to have no effect on grape juice YAN status but short-term reductions in the quantity of applied water during the early stages of berry development in 2 of the study years did increase YAN. Juice pH was sometimes increased by reduced irrigation and N application treatments, but levels remained acceptable for wine production. Other measures of fruit composition were less sensitive to irrigation and N fertilization treatments. Interannual variability played an important role in determining grape juice YAN, fruit composition and yield. Future work should focus on refining management practices, e.g., the timing of N application, to minimize the effects of annual variability on grape juice YAN concentrations.