Influence of nitrogen fertilization on <em>Vitis vinifera</em> cv. Merlot with permanent grass cover. (Nitrogen content, nature and concentration levels of some amino acids of musts and wines)

Since 1995, it has heen running an experiment on the introduction of nitrogen fertilization on permanent grass cover. This has been done in order to determine both the role and the influence of nitrogen on the fermentability of the must. Located south of Saint-Emilion, the plot area studied has a regular, sandy, clayey soil 60 centimeters deep. The cultivar Merlot is grafted on the Riparia Gloire from Montpellier rootstock. Five N- fertilizations were compared : T (0 kg N/ha/year), A (30 kg N/ha/year), 8 (60 kg N/ha/year), C ( 90 kg N/ha/year), and D (45 kg N/ha at springtime and 45 kg N/ha at flowering period). For the different N-fertilizations, the numher of clusters per vine was the same for the two experimental years. The mean cluster weight allowed for evaluating the nitrogen effect on the yield. Nitrogen fertilization on the permanent grass cover in the vineyard depends upon climatic conditions in the spring and, more specifically, upon the rainfall pattern. Varietal vigor can he increased or decreased with the level of nitrogen. However, in all cases studied, late contribution of nitrogen was found to he harmful. In fact, most nitrogen added at springtime was used by the grass cover and the competitiveness of the latter was increased. Only test « C » proved to be adequate to satisfy both vineyard and grass cover needs (this is shown in the study of the pruning wood weight variation compared to the standard « T»). The duration of alcoholic fermentation is quite closely correlated to the content of nitrogen composites in the must and more particularly in amino acids. The global amino acid contents of musts and wines were higher in 1996 than in 1995, but in this last case more numerous kinds of amino acids were present. However, for both years studied, proline represented at least 50 percent of the global amino acids content. A statistical analysis on the five N-fertilizations showed a clear differentiation between the two vintages studied ; that is to say, each vintage was identified by specific amino acid profiles. This « vintage » effect was proved by tasting results.