Nitrogen use efficiency, partitioning, and storage in cool climate potted Pinot Noir vines

Optimal timing of nitrogen (N) fertilizer to promote N use efficiency (NUE) in cool climate wine regions is largely unknown. One year-old potted Vitis vinifera L. (Vitaceae) cv. Pinot Noir vines were subjected to soil applied 15N-labelled calcium nitrate (5.5 atom%) fertilizer at four application timings (budbreak to fruit set, fruit set to veraison, veraison to harvest, or spread evenly across the growing season) over two growing seasons (2017/18 and 2018/19) at a rate of 12 g N vine−1. Annual vine parts (leaves and clusters) were collected at commercial harvest in both 2018 and 2019 and the remaining vine was destructively harvested during winter dormancy. Vines were separated into organs (leaves, clusters, shoot, trunk, and roots) and organ dry matter was determined before samples were ground into a fine powder to determine total N percentage and 15N atom percentage using flash combustion isotope ratio mass spectrometry (varioPYRO cube coupled to Isoprime100 mass spectrometer). Compared to the average values obtained from the other treatments, the veraison to harvest treatment significantly reduced whole vine dry matter by 39.1% and vine NUE by 42.1%. Yield was also reduced by 12.9% compared to the budbreak to fruit set treatment. NDF, defined here as the proportion of N within the vine (or vine organ) that was derived from fertilizer N, and overall vine total N content was lower in the veraison to harvest treatment due to the reduction of root and shoot NDF content. This suggests that for young, actively growing vines in cool climates, limiting N applications until veraison is inefficient and ineffective. No significant differences in NUE, N partitioning or N storage were observed between the other fertilizer N application timings