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Fate of Tagged Fertilizer Nitrogen Applied to Winter Wheat¹

ABSTRACT

A field experiment using ammonium sulfate tagged with 7.65 A% ¹⁵N was conducted to assess how rates and times fertilizer is applied influence the fate of N applied to winter wheat (Triticum aestivum L.). Bottomless metal boxes were pressed 142 cm into the soil to confine the tagged fertilizer. Treatments, replicated four times, consisted of two rates (50 and 100 kg N/ha) and two application times (fall and spring). Fertilizer N used by the crop and that remaining in the upper 180-cm of the soil after harvest were measured. Amounts of ammonium-N and nitrate-N from the fertilizer in the 0 to 10-cm layer also were determined.

The N balance indicated 9.7 – 10.3 kg N/ha were unaccounted for at the 50-kg N rate and 19.7 · 23.4 kg, at the 100-kg rate. Losses did not differ significantly between application times. With 100 kg/ha the crop removed significantly more fertilizer N, and significantly less remained in the soil with spring than with fall applications. The crop used similar amounts of total N in each case, but with the spring applications more fertilizer N but less soil N was taken up. Fertilizer applications caused no priming effect on mineralization of indigenous soil N. Most of the fertilizer remaining in the soil was in the 0 to 10-cm layer, with no evidence of N moving deeper than 50 cm, so losses were from gaseous loss rather than leaching.

Most of the fertilizer N in the 0 to 10-cm layer of soil after harvest was immobilized, with only 9.6 – 11.5% remaining in inorganic forms. Immobilization was the principal reason for differences in spring and fall applications and for limited leaching. The percentages of NH₄⁺-N and NO₃^--N, which originated from the fertilizer were 4 – 8 times the percentages of total N from the fertilizer, so more fertilizer N was in inorganic forms than was the case with indigenous soil N. Since there was a direct relationship between amounts of NO₃^--N in the surface soil from fertilizer and amounts of N unaccounted for, it was concluded that gaseous losses resulted from denitrification processes.

¹ Contribution 79-171-J, Department of Agronomy, Kansas Agric. Exp. Stn., Manhattan, KS 66506.

² Professor of Agronomy, former Professor of Agronomy, Professor of Chemistry, and Associate Professor of Agronomy, respectively. Dr. Murphy is now Western Midwest Director, Potash-Phosphate Institute.

Received for publication December 29, 1978. Accepted for publication May 14, 1979.

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