Variable rate fertilization in a highly productive vineyard of cv. Trebbiano romagnolo (Vitis vinifera L.) may reduce nitrogen application and vigor variability without yield loss

The site-specific management of the vineyard cultural practices may reduce the spatial variability of vine vigor, contributing to achieve the desired yield and grape composition. In this framework, variable rate fertilization may effectively contribute to reduce the different availability of mineral nutrients between different areas of the vineyard, and so achieving the vine’s aforementioned performances. The present study was aimed to apply a variable rate fertilization in a highly productive vineyard of the fertile Po Valley (Italy) and to evaluate the effects on the spatial variability of vigor. The trial was conducted over two consecutive seasons (2018-2019) in a not irrigated, 3 ha vineyard of cv. Trebbiano romagnolo, trained to GDC and grafted onto SO4. During the summer of both years, vine vigor was mapped with a quad equipped with two GreenSeeker sensors in order to calculate the normalized difference vegetation index (NDVI); simultaneously, soil apparent electrical conductivity (ECa) was measured by the TopSoil Mapper sensor. Successively, clusters were harvested and berries were sampled according to a prearranged grid for georeferencing yield components and quality parameters. Then, maps of vigor, yield and soluble solid concentration were created. At the beginning of the vegetative growth of 2019, the variable rate fertilization was performed according to a prescription map based on NDVI data of 2018. The NDVI map of 2018 discriminated two zones with different levels of vigor (high, NDVI ≥ 0.73) and medium, 0.55 ≤ NDVI < 0.73), and showed spots scattered in the vineyard with NDVI lower than 0.55, corresponding to dead vines or vines severely affected by “Flavescence doree” and “Esca disease”. On the other hand, only minor differences were found in soil apparent electrical conductivity, and it was not possible to delineate a map with zones characterized by different levels based on this parameter. Harvest data of 2018 showed that in the zones with higher vigor, vines were more productive that those characterized by medium vigor. As expected, high yield was correlated with low sugar concentration and high titratable acidity. The variable rate fertilization performed in spring of 2019 consisted in the application of 30 kg/ha of N in the high vigor zones, 60 kg/ha of N in the zones to save 15 kg/ha of nitrogen compared to farm fertilization and contributed to reduce the spatial variability of vigor. In fact, the 2019 NDVI map showed that the area of the higher vigor zones increased over the previous year. Harvest data of 2019 showed an overall increase of yield and confirmed the relations between yield and berry composition observed in 2018. Those results show that variable rate fertilization may be adopted also in highly productive vineyards to reduce spatial variability of vigor without negative consequences on yield. Then, it is expected that a large-scale adoption of this technique would generate a remarkable reduction of fertilizer with beneficial consequences on the environment.