Uptake, Distribution, and Remobilization of ¹⁵N‐labeled Urea Applied to Maize Canopies¹

Field‐grown maize ( Zea mays L.) plants (hybrid B73 X Mo17) were sprayed with 15 N‐enriched urea to determine recovery and subsequent distribution within the plant. Plants were sprayed with 15 N enriched urea at 7 days pre‐ (treatment 1) and post‐anthesis (treatment 2) to provide a total of 22.3 kg N ha −1 (350 mg N plant −1 ). Unsprayed plants were used as controls. Plots were located on Flannigan silt loam (fine, montmorillontic mesic Aquic Argiudoll) at stands equivalent to 63 800 plants ha −1 . Timing of spray treatment had no effect on grain yield, stover weight, visible leaf senescence, or N content of the plant parts, relative to controls. About one‐third (29% in treatment 1, 30% in treatment 2) of the foliarly applied urea was absorbed and incorporated by the plant. The amounts and distribution patterns within plant parts of foliarly applied N and of total N (N derived from soil uptake and foliarly applied urea N) indicated that leaves were the organs of uptake, and that the stalk served as a conduit for passage of urea‐derived N from leaves to the ear rather than as a storage organ for foliarly applied N. The greater initial rates of loss and the final extent of depletion of N from leaves was greater for foliarly applied N than for total N. This indicated that the foliarly applied N was incorporated into a different pool of N than had been formed earlier by unenriched soil derived N. This observation provides possible explanation for the failure of foliar N applications to increase grain yields of maize.