FATE OF N-15 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 ferti lizers were applied in the spring through a trickle irrigation system to six Thompson Seedless vines of a vineyard on the West Side of the S an Joaquin Valley of California. At fruit harvest, all above-ground pl ant parts were removed and analyzed for N-15. Soil around each vine wa s also sampled and analyzed for N-15 in the inorganic and organic N fr actions. Spatial patterns of fertilizer N for soil inorganic and organ ic N were analyzed using a median polish technique which indicated lar ge variability with respect to direction, distance, and depth. There w as a tendency for the fertilizer N from NH4 to be located directly ben eath emitters than from the NO3. Nitrogen from the NH4 application pen etrated to only the 150-cm depth, whereas some N from the NO3 applicat ion reached 210 to 240 cm. Most of the organic fertilizer N for both N O3 and NH4 applications was in the top 60 cm of soil where the vine ro ots 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, upta ke of fertilizer N by above-ground plant components was slightly highe r for the NH4 application (24.2% of applied N) than the NO3 applicatio n (21.5%). Soil organic N had significantly (95% level) higher N from NH4 (19% of applied N) than from NO3 (13%). This probably occurred due to longer residence time of N from NH4 within the top 60 cm, where th e bulk of roots and microbial activity existed, than for NO3. Overall, about 67% to 79% of the fertilizer N applied in spring remained in th e soil at harvest, and the vines took up the rest. There was no indica tion of significant N leaching below 2.4 m or denitrification of ferti lizer N for the trickle-irrigated vines during the growing season.