Fate of 15N fertilizer applied to trickle-irrigated grapevines

Information on fate of nitrogen applied to vines is needed to improve fertilizer management. Nitrogen-15 enriched ammonium and nitrate fertilizers were applied in the spring through a trickle irrigation system to six Thompson Seedless vines of a vineyard on the West Side of the San Joaquin Valley of California. At fruit harvest, all above-ground plant parts were removed and analyzed for (15)N. Soil around each vine was also sampled and analyzed for (15)N in the inorganic and organic N fractions. Spatial patterns of fertilizer N for soil inorganic and organic N were analyzed using a median polish technique which indicated large variability with respect to direction, distance, and depth. There was a tendency for the fertilizer N from NH(4) to be located directly beneath emitters than from the NO(3). Nitrogen from the NH(4) application penetrated to only the 150-cm depth, whereas some N from the NO(3) application reached 210 to 240 cm. Most of the organic fertilizer N for both NO(3) and NH(4) applications was in the top 60 cm of soil where the vine roots were likely of greatest density. Overall recovery of fertilizer N was also quite variable, probably due to variability in soil physical properties and uneven surface application of water and fertilizer due to local surface pending. Although not statistically significant, uptake of fertilizer N by above-ground plant components was slightly higher for the NH(4) application (24.2% of applied N) than the NO(3) application (21.5%). Soil organic N had significantly (95% level) higher N from NH(4) (19% of applied N) than from NO(3) (13%). This probably occurred due to longer residence time of N from NH(4) within the top 60 cm, where the bulk of roots and microbial activity existed, than for NO(3). Overall, about 67% to 79% of the fertilizer N applied in spring remained in the soil at harvest, and the vines took up the rest. There was no indication of significant N leaching below 2.4 m or denitrification of fertilizer N for the trickle-irrigated vines during the growing season.