Recovery of ¹⁵N-labeled Fertilizer by Thompson Seedless Grapevines: Effects of N Fertilizer Type and Irrigation Method

Abstract

A study was conducted to determine the N fertilizer recovery efficiency (RE_N) of Vitis vinifera L. cv. Thompson Seedless grown in the San Joaquin Valley of California. Vines were drip- or furrow-irrigated and fertilized with ¹⁵N-labeled potassium nitrate (KNO₃) or ammonium sulfate (NH₄)₂SO₄. The fertilizers were applied shortly after anthesis with a single application of 25 g N/vine (~28 kg N/ha). An additional set of the drip-irrigated vines was fertilized 10 times beginning 20 April across a 20-week period during the growing season with labeled KNO₃ (amounts equivalent to ~31 kg N/ha). Total biomass, N concentrations, and ¹⁵N of all vine organs, including the root system, were measured. There were no significant differences in RE_N (~40%) between the two fertilizer types (nitrate versus ammonium) for the drip-irrigated vines or for the comparison of a single application of KNO₃ to a split application of that fertilizer applied every two weeks. The RE_N of furrow-irrigated vines was ~12%. The partitioning of ¹⁵N-labeled fertilizer taken up by the vine was significantly affected by fertilizer treatment. Approximately 47% of the ammonium fertilizer for the drip-irrigated vines was found in the clusters, while only 30% of the nitrate fertilizer for the split application of the fertilized, drip-irrigated vines was found in the clusters. The greatest amount of fertilizer N found in the roots was for the nitrate fertilized vines under drip-irrigation (both single and split applications). The results indicated that irrigation type greatly affected RE_N of the N fertilizers used in the study, and that irrigation and fertilizer type affected the distribution of N once it was taken up by the vine.