Long-term assessment of variable rate N-fertilization in a Vitis vinifera L. 'Barbera' vineyard

Variable rate technologies allow site-specific management of parcels characterized by different levels of vigor and/or yield. N-fertilization based on actual plant needs is one of the most promising applications of precision farming aiming at improving efficiency, optimizing yield-to-quality relationship as well as limiting environmental impact. Although this strategy appears suitable for developing new viticultural models, few experiences validating this hypothesis are available in literature. Based on a pre-trial remotely sensed vigor map (NDVI-derived, 5 m resolution) a four-year study was performed in a Vitis vinifera L. 'Barbera' vineyard situated in the Colli Piacentini wine district. Vigor level (L = low, M = medium and H = high) and fertilization strategy (standard, variable rate application, and unfertilized control) were the main factors in a randomized block design. For each vigor level the study compared no N-supply (0 kg ha-1), standard supply (60 kg ha-1) and VRA supply delivering 0, 60 and 120 kg ha-1 to H, M and L, respectively. Post-trial effects were assessed by remote and proximal sensing considering a 5 m multispectral image and a very high resolution characterization of the canopy growth, respectively. Vine growth, yield, leaf nutritional status and fruit composition were assessed. Results show that variability among vigor levels was significantly reduced over year 3 and 4, whereas effects related to fertilization strategy were often non-significant. The canopy index calculated by proximal sensing proved that VRA was able to reduce variability among vigor levels (CV=1.42%) vs. standard fertilization at 60 kg ha-1 (8.20%). Leaf area did not vary among strategies while N-fertilization affected yield and vine balance. Despite VRA increased yield as compared to Standard, technological and phenolic maturity was similar in both fertilization strategies. The VRA reduced N waste in H while vines from L blocks showed a low sensitivity to increased N-supply.