Effects of 15N-labelled ammonium nitrate and urea on soil nitrogen during growth of wheat (Triticum aestivum L.) under field conditions

We studied the effects of ¹⁵N-labelled ammonium nitrate and urea on the yield and uptake of labelled and unlabelled N by wheat (Triticum aestivum L., cv. Mexi-Pak-65) in a field experiment. The dry matter and N yields were significantly increased with fertilizer N application compared to those from unfertilized soil. The wheat crop used 33.6–51.5 and 30.5–40.9% of the N from ammonium nitrate and urea, respectively. Splitting the fertilizer N application had a significant effect on the uptake of fertilizer N by the wheat. The fertilizer N uptake showed that ammonium nitrate was a more available source of N for wheat than urea. The effective use of fertilizer N (ratio of fertilizer N in grain to fertilizer N in whole plant) was statistically similar for the two N fertilizers. The application of fertilizer N increased the uptake of unlabelled soil N by wheat, a result attributed to a positive added N interaction, which varied according to the fertilizer N split; six split applications gave the highest added N interaction compared to a single application or two split applications for both fertilizers. Ammonium nitrate gave 90.5, 33.5, and 48.5% more added N interaction than urea with one, two, and six split N applications. A values were not significantly correlated with the added N interaction (r=0.557). The observed added N interaction may have been the result of pool substitution, whereby added labelled fertilizer N replaced unlabelled soil N.