"Preferential" ammonium uptake by sugarcane does not increase the 15 N recovery of fertilizer sources

Aims As ammonium (NH4+) has been reported as the "preferred" mineral N source for sugarcane (Saccharum spp.), the predominant supply of NH4+ has been hypothesized as a means of increasing the 15N recovery and crop yield. We aimed to evaluate the 15N recovery by sugarcane as affected by 15N-NH4+ and 15N-NO3− addition. Methods Under field conditions, l5N-labeled ammonium nitrate [labeling as 15NH4NO3 (15N-NH4+) and NH415NO3 (15N-NO3−)] and urea were surface-applied at a rate of 100 kg N ha−1. Under controlled conditions, sugarcane was grown in hydroponic medium (containing unlabeled N) until the maximum N demand period. Subsequently, plants received 15N-NH4+ or 15N-NO3− (as ammonium nitrate). Plants were then sampled at 1, 3, 7, and 15 d after fertilization (DAF) to estimate 15N recovery. Results Under field conditions, the 15N recovery for urea was lower than that for ammonium nitrate, partly explained by ammonia volatilization, but there was no difference in N use efficiency between the two fertilizer N sources, as well as between 15N-NH4+ and 15N-NO3− supply. Under controlled conditions, however, 15N-NH4+ resulted in higher 15N recovery than 15N-NO3− until 7 DAF. Nevertheless, as 15N-NH4+ became depleted in the nutrient solution, 15N-NO3− was taken up more intensely by the crop. At 15 DAF, highest 15N recovery was detected in the roots and whole plant under 15N-NO3− supply. Conclusions Although uptake of NH4+ is faster than NO3−, the so-called "preference" for NH4+ by the crop did not necessarily translate into higher 15N recovery. Therefore, we suggest that the term "preference" should be used with caution to avoid misinterpretation.