Effect of post-bloom foliar nitrogen application on vines under two level of soil fertilization in increasing bud fertility of ‘Trebbiano Romagnolo’ (Vitis vinifera L.) vine

•Two sets of grapevines differing for soil mineral N were sprayed with 15N.•Nitrogen soil application increased soil mineral N.•Nitrogen soil application did not enhance N concentration in canopy organs.•June was more effective than July spray in increasing bud N concentration.•A positive correlation between bud N in 2014 and bud fertility in 2015 was found.