Friday, 13 October 2023 15:25

Science Snippet: The complexities of vineyard nitrogen nutrition

Written by  Dr Mike Trought
Wither Hills Wither Hills

Di-ammonium phosphate is frequently added to grape juice at the start of fermentation to ensure yeast have adequate nitrogen to complete fermentation.

However, Di-ammonium phosphate (DAP) does not contain essential amino acids, precursors for key flavour and aromatic compounds in wine. Knowledge of the vine nitrogen status is essential for long-term management with the aim of producing good yields of high quality grapes (Vrignon-Brenas et al., 2019).

Nitrogen reserves (like carbohydrate) in the trunk and root system are used in the spring for shoot growth. Nitrogen deficiency can result in poor and uneven shoot growth, lower photosynthesis, reduced bud fertility and yields (Guilpart et al., 2014; Verdenal et al., 2023). Organic soil nitrogen will not necessarily be available in spring as low temperatures limits mineralisation, and herbaceous plants, in particular grasses, with a fibrous dense root system, will potentially ‘mop up’ the free nitrates in the soil.

In contrast, excessive spring nitrogen may result in a dense canopy, which, particularly in cool conditions, can result in higher botrytis infections (Mundy & Beresford, 2007) and/or inflorescence necrosis, a disorder associated with an accumulation of ammonium in the inflorescence, causing the developing bunch to abort (Gu et al., 1996).

The fertile soils of the Dillons Point area in Marlborough generally give higher grapevine yields (Bramley et al., 2023) and Sauvignon Blanc wines with greater thiol concentrations. Applying nitrogen post-fruit set, pre véraison also enables the vine to accumulate adequate reserves and can have a direct or indirect effect on key flavour and aromatic compounds. Research on Sauvignon Blanc in Bordeaux has demonstrated that fertilisation can enhance amino acid concentrations in fruit, and in turn volatile thiol concentrations in wine (Helwi et al., 2016). In contrast nitrogen had no direct effect on methoxypyrazine concentration which increased as a result of greater shading of the fruiting zone (Helwi et al., 2015). We have found similar results in Marlborough.

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