Soil,leaf,and grain natural 15 N abundance was measured in corn (Zea mays L.) and soybean (Glycine max L.) plots which had received various rates of fertilizer N for 20 years. Soil 15 N abundance did not significantly reflect the amount of fertilizer N applied and should not be used to estimate fertilizer IM additions. Corn leaf and grain 15 N abundance reflected additions of fertilizer N only at low rates of fertilizer N applied. Soybean leaf 1S N abundance reflected a decrease in symbiotic N fixation with additional increments of fertilizer N applied. Additional Index Words: corn 15 N, isotopic fractionation, soybean 15 IM, symbolically fixed N.

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Ammonia volatilization can reduce the effectiveness of surface applied N fertilizers. Our objectives were to evaluate N source by timing interaction under field conditions conducive to NH3 volatilization and to compare the sensitivity of three approaches to assessing N source by timing interaction. Ammonium nitrate and urea fertilizers were topdressed at rates of 0, 34, 67, and 101 kg N/ha on 19 November or 23 March to ‘Winalta’ winter wheat (Triticum aestivum L.) grown on a nonirrigated Scobey silt loam (Aridic Argiboroll, fine, montmorillonitic). Plots receiving topdressed N contained sub‐plots fertilized at the appropriate rate and time with 15NH4 15NO3 or 15NH2CO15NH2 labeled fertilizers containing 1.31 atom percent 15N. Nitrogen fertilization increased straw yield, grain yield, grain protein, test weight, and N content of straw and grain. Uptake of soil N and fertilizer N in straw and grain were determined. Grain yield was fitted to a concurrent Mitscherlich regression model while total N uptake and fertilizer N uptake were fitted to multiple linear regression models to evaluate the relative effectiveness of each N source—timing combination. Comparison of regression coefficients for either grain yield or total N uptake revealed no significant differences between N source—timing combinations. Fertilizer N uptake regressed on rate of applied N was a better parameter for evaluating N source by timing interaction and showed that fall topdressed urea was only 69% as effective as fall topdressed ammonium nitrate. Spring topdressed urea, spring topdressed ammonium nitrate, and fall topdressed ammonium nitrate were equally effective in supplying N to wheat plants. Volatilization of NH3 from surface applied fertilizer probably accounts for the lower relative effectiveness of urea topdressed on 19 November.