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

Abstract

AbstractTo 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.