Fate of Nitrogen-15 Applied to Spring Wheat Grown for Three Consecutive Years in a Semiarid Region

Uptake and recovery of fertilizer N applied to spring wheat was monitored over a 3-yr period in a semiarid region. Tagged fertilizer with enrichment from 8.9 to 28.5 atom % ¹⁵N was applied as KNO₃ at rates equivalent to 180 kg N ha⁻¹ as a single application or split equally over 3 yr. The wheat was grown in 1.25- × 1.25-m open plots and in 0.3-m diam microplots enclosed by hard plastic tubes that were inserted 0.6 m into the ground. The ¹⁵N enrichments in plant and soil were determined by emission spectrometry. In open plots fertilizer N recovery in plant and soil was 976 g kg⁻¹ in the first year and 697 to 721 g kg⁻¹ in subsequent years. In the tubes, equivalent recoveries were 755 to 841 g kg⁻¹ and 377 to 560 g kg⁻¹. The greater loss in the tubes is attributed to denitrification that could have occurred under excess waterlogging caused by the rim of the tubes that protruded about 0.05 m above the soil surface. In a dry year, 338 to 558 g kg⁻¹ of the applied fertilizer N was found as mineral N in the soil and, especially in the open plots, was utilized in the second growing season, after which only negligible amounts remained in the mineral N fraction. At the end of the third year, 69 to 84 g kg⁻¹ of the applied fertilizer N was found in the soil as organic N. At the end of 3 yr, recovery of fertilizer N was lower when determined directly with ¹⁵N than when determined by difference between treatments and control. The additional N could result from the controversial “priming effect” of fertilizer N application on release of soil N, although the evidence is not conclusive. It is concluded that when fertilizer N is applied in a dry season, a large amount of residual mineral N in soil remains at the end of the growing period. This N can be utilized in subsequent years and should be taken into account when determining fertilizer requirements. In a wet years, losses can be significant and residual fertilizer N in the soil is negligible.