OVERWINTER LOSSES OF NITROGEN-15 LABELLED UREA FERTILIZER

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Abstract
Field studies using 15N microplots were conducted to quantify the uptake and disappearance of fall- and spring-applied urea N on low organic matter, irrigated soils. Urea was mixed with the surface soil to maximize the potential for overwinter nitrification and subsequent losses of the fertilizer N. In three irrigated soils, losses of fall-applied urea averaged 24–31% compared with 11–21% of that applied at seeding. Barley took up 33–42% of spring-applied urea N but only 16–36% of fall-applied urea N. The lower uptake of fall-applied N apparently resulted from higher N losses rather than from the immobilization of fall-applied urea. Fall application resulted in lower soil reserves of residual fertilizer N after the growing season, as compared to spring application, in two of the three studies. Sixty percent of the fertilizer N recovered from the soil remained in the surface 15 cm. The application of 50 mm of water in the fall or 100 mm in the early spring, to intensify any effects of moisture, had a minimal effect on N losses or the distribution of N in the soil. This suggests that an individual rainfall event would not greatly affect the uptake or losses of fall-applied fertilizer on well-drained soil. The observed fertilizer losses, however, support practices such as concentrating fall-applied fertilizers in bands or the use of nitrification inhibitors. Key words: Denitrification, nitrogen, fertilizer, N balance, N losses, urea