Impact of foliar nitrogen supplementation on Chardonnay and Sauvignon Blanc wines

The effectiveness of an application of foliar nitrogen at veraison depends on the initial level of vine nitrogen deficiency, an Agroscope study has shown. The threshold levels of assimilable nitrogen deficiency in the grape must are validated for Chardonnay but still need to be confirmed for Sauvignon Blanc.

Cultivation practices in our vineyards are gradually evolving towards reduced fertilisation and the increasing use of cover crops, intensifying competition for nitrogen. In this context, managing nitrogen nutrition in order to achieve a sustainable balance between vine vigour and grape composition is a genuine challenge. The amount of yeast assimilable nitrogen (YAN) present in the grape must at harvest determines the vinification conditions and ultimately, the quality of the wine. Threshold levels of nitrogen deficiency have been defined for Chasselas musts: a YAN level below 140 mg N/L is considered very low, between 140 and 200 mg N/L low, and above 200 mg N/L correct. A six-year trial was conducted at the Agroscope vineyard in Nyon to test the effectiveness of a late foliar application (veraison) and so validate the threshold levels of assimilable nitrogen for Chardonnay and Sauvignon Blanc wines.

The application of nitrogen at veraison is effective except for severe deficiencies

The Chardonnay vines were moderately deficient in nitrogen initially (1.84% leaf DM at veraison) whereas the Sauvignon Blanc vines were more severely deficient (1.63% DM) and showed signs of reduced vigour. Foliar fertilisation increased nitrogen levels in the two varieties by 0.26±0.11% DM on average over a six-year period. Foliar fertilisation increased concentrations of YAN in the must (+69 mg/L for Chardonnay and +67 mg/L for Sauvignon Blanc). After foliar fertilisation, the average concentration of YAN in the Chardonnay control increased from a severely deficient level (125±32 mg N/L) to a correct level (194±52 mg N/L); whereas that of the Sauvignon Blanc control was so low (65±26 mg N/L) that it remained at a severely deficient level despite fertilisation (132±39 mg N/L).

Threshold limits of deficiency for Chardonnay and Sauvignon Blanc

During tasting, the Chardonnay wines from the fertilised variants gave a better general impression compared with the control wines from the same year. The delicacy of their bouquet came to the fore, with notably fewer negative aromas linked to nitrogen stress in the must (hay, wax, wet floorcloth). In the mouth, these same wines had a better balance associated with a noticeably reduced level of bitterness and astringency (Figure 1). The Sauvignon Blanc wines showed the same tendencies as the Chardonnay, but differences [between the treated and control versions] were not significant due to the low level of assimilable nitrogen, which remained below the critical 140 mg N/L. The correlation between the level of YAN in the grape must and the general impression given by the wine was highly significant for the Chardonnay (p = 0.003), but negligible for the Sauvignon Blanc (Figure 2).

Thus, it seems that the threshold levels of YAN deficiency in grape must established for Chasselas are also valid for Chardonnay but still need to be confirmed for Sauvignon Blanc.