Effects of variable rate nitrogen application on cv. Barbera performance: yield and grape composition

This paper follows a first contribution on the long-term effects of variable rate nitrogen application on vegetative growth and leaf nutritional status of a small Barbera vineyard, where three vigor levels classified as low, medium, and high (L, M, H) were identified pretrial from normalized difference vegetation index (NDVI) values determined by remotely sensed imagery (5 megapixel resolution). Variable rate fertilization supplying no N (0 kg/ ha; C), standard rate (60 kg/ha; S), and variable rate application (VRA) of 60 or 120 kg/ha to H, M, and L over four consecutive seasons showed that variability among vigor levels was significantly reduced in years three and four, while mean effects related to fertilization strategy were not significant. In this companion paper, L vines were most balanced, suggesting a change in fertilization strategy to no N application in M and H areas, and for maintenance supply in L plots. However, significant vigor level × year interactions indicated that within-vigor variability for total yield and berry weight was considerably reduced, while concomitant seasonal accumulations of sugar and color in 2014 and 2015 were much less variable within dates across vigor levels, suggesting the efficacy of the VRA approach. While the main effects of fertilization supply were mostly non-significant, seasonal accumulation of total soluble solids and anthocyanins in 2014 and 2015 showed that plots receiving 60 kg N/ha had lower within-date values than C (no N), suggesting increasing sensitivity over years. However, the L vines had an almost negligible response to increased N supply, and most observed responses were primarily due to reduced vine capacity.