Grapevine nitrogen dynamics as a function of crop thinning

Abstract

Context and purpose. Nitrogen (N) is crucial for plant development but is used inefficiently, with only 30–40% of the fertilizer assimilated by crops, leading to significant environmental losses. In viticulture, N metabolism is influenced by genetics, soil management, vineyard practices, and environmental factors. Understanding these influences is key to optimizing grape composition and production sustainability. Yeast assimilable nitrogen (YAN) in grape must is critical for fermentation and wine bouquet. Levels below 140 mg N/L can cause fermentation issues and sensory deviations, especially in white wines. As wine sensory profiles depend primarily on grape composition, managing N at the vineyard level is crucial. Previous studies demonstrated that canopy size (with different trimming height) negatively correlates with YAN concentration. However, the effects of crop thinning on N partitioning remain unclear.

Material and methods. This study evaluated the impacts of crop thinning on N uptake, mobilization, and allocation in grapevines using isotope 15N-labelling method. Conducted over two years on the white cultivar Vitis vinifera Chasselas, the study employed a gradient of crop loads and N fertilization (20 kg N/ha as 15N-labelled urea applied at veraison stage). Vines were destructively sampled over eight periods and the organs separately analyzed to monitor N dynamics. Musts were analyzed for soluble sugars, acids, NH4+, and amino acids.

Results. Crop thinning did not affect grape N concentration, which remained constant in the grape must at harvest, to the detriment of N content in the roots. Both N uptake and root N mobilization were reduced in response to crop thinning. Fertilization efficiency was higher under high-yield conditions in terms of N uptake and grape N accumulation. Therefore, without affecting plant vigor, urea supply efficiently increased YAN concentration in the must (+55 mg/L) only under higher-yield conditions. Carry-over effects of crop regulation in the following year were highlighted. Grapevine seemed to compensate higher N demand from the grapes with higher N uptake from leaves and roots and higher N reserve mobilization. Urea supply limited N mobilization from the roots, thus preserving N reserves for the following year. Depending on the yield, musts were discriminated about their amino-acid profiles, hinting at potential wine aroma modifications.