Fate of nitrogen-15 applied to irrigated Acala and Pima cotton

Better knowledge on the fate of fertilizer N will aid in developing effective N management strategies balancing plant requirements for optimal lint yield with environmental concerns. Field studies were initiated to determine the fate of N-15 fertilizer applied to Acala (Gossypium hirsutum L.) and American Pima (G. barbadense L.) cotton. Four N regimes (56, 112, 168, and 224 kg N ha(-1) corrected for residual soil nitrate N levels) were established for Acala cotton grown on a Panoche clay loam [fine-loamy, mixed (calcareous) thermic Typic Torriorthents] and on a Wasco sandy loam (coarse-loamy, mixed, nonacid, thermic Typic Torriorthents) in 1998, 1999, and 2000. Pima cotton was evaluated on the Panoche clay loam for the same N regimes in 1999 and 2000. To trace the fate of fertilizer N, N-15-labeled urea was applied to microplots in selected treatments and years. Acala fertilizer use efficiency by N-15 dilution (FUE-N-15) averaged 49% on Panoche clay loam and 43% on Wasco sandy loam. Pima FUE-N-15 on the Panoche clay loam averaged 48% and was not affected by N treatment. Recovery of fertilizer N in the soil was not different between the Panoche clay loam and the Wasco sandy loam in the Acala experiments and combined across both Acala and Pima trials averaged 42%. Averaged across all experiments, more than 75% of the N-15 recovered in the soil was found in the top 0.9-m layer. The total recovery of N-15 fertilizer in plant and soil averaged 89% (76-98%) across all treatments, suggesting that production practices employed in this study resulted in only small losses of fertilizer N during the season of application.