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Fate of Labeled Nitrogen Fertilizer Applied to Winter Wheat for Five Years¹

ABSTRACT

To assess the effect of time and rate of fertilizer N applications on fate of the applied N, a 5-yr field experiment was conducted with winter wheat (Triticum aestivum L.). Microplots confined in metal frames to a depth of 1.42 m received annual applications of (NH₄)₂SO₄ containing 7.65 atom %¹⁵N. Treatments, replicated four times, consisted of 50 and 100 kg N/ha applied in fall and spring on a Pachic Argiustoll soil. In four of the years, spring applications gave better fertilizer use efficiency than fall treatments, probably because of greater immobilization of fall-applied N. After 5 yr, 27 to 33% of the applied fertilizer N had been removed by the grain. Soil N use was increased by fertilizer in some years and in those cases was correlated highly with increased growth, suggesting that the cause was larger root systems rather than a priming effect. Amounts of fertilizer N in the 1.8-m soil profiles increased each year and at the end of the experiment averaged 54% of that applied during the 5 yr for 50-kg treatments and 47% for 100-kg treatment. From 71 to 77% of the surface-applied fertilizer N remaining in the profiles was in the 0- to 0.1-m soil layers. Most of the residual fertilizer N was immobilized in organic forms, but late applications and dry conditions in 1980 greatly reduced both nitrification and immobilization. There was good agreement between ¹⁵N and difference measurements of crop removal of fertilizer N for the 5-year period, although difference measurements were more variable and did not agree well with ¹⁵N measurements in individual years.

¹ Contribution no. 84-28-J, Dep. of Agronomy, Kansas Agric. Exp. Stn., Manhattan, KS 66506.

² Professor and Associate Professor, respectively.

Received for publication August 19, 1983. Accepted for publication October 17, 1983.