EFFECT OF ORCHARD SOIL-MANAGEMENT ON THE UPTAKE OF NITROGEN BY ESTABLISHED APPLE-TREES

15N was supplied as 15KNO3 to mature Cox/M.26 apple trees grown in conventional wide herbicide strips, with overall herbicide or with complete grass cover. Uptake was measured, following placement, at 10 cm depth within the tree rows or at 10 or 25 cm depth in the alleys between the rows. Uptake of 15N in the strip treatment was always much greater from the strip than from the grassed alley, while with the overall herbicide trees, uptake was highest from the tree row in June and from the alley in Aug. With overall grass, uptake was highest from the alley. Management treatments did not affect the total N content of the leaves, so results were similar whether expressed on an atom percent or .mu.g g-1 DW [dry weight] basis. In the herbicide strip treatment, application of fertilizer N at rates between 0 and 126 kg ha-1 had little effect on the amount or the pattern of 15N uptake. Recovery of added 15N tracer by the trees during the year of treatment ranged 2-25% depending on treatment, position and depth of injection. Absorption of 15N apparently was greater in mid-summer than in spring or autumn. Pattern of N uptake was affected by interaction between soil water status, periodicity of root growth and total N demand.