Preferential uptake of ammonium nitrogen from soil by ryegrass under simulated spring conditions

SUMMARY The 15 N isotope was used to compare the uptake and recovery of NH 4 + and NO 3 – nitrogen by ryegrass growing in soil under controlled simulated spring conditions when the temperature of the root was appreciably lower than that of the shoot. A preference was shown for NH 4 + N over N0 3 – N, particularly at low soil temperatures. The limited uptake of NO 3 – does not appear to be due to an inhibitory effect of the NH 4 + ion. Ryegrass not only absorbed less N from NO 3 – but also translocated a lower percentage of the total nitrogen taken up, to the shoots, the difference being greatest at low soil temperatures. The results give increasing support for the recommendation that early spring nitrogen should be applied in the NH 4 + form. However, the preferential uptake and translocation of NH 4 + ions would be of agricultural importance only if it led to an increase in dry-matter production.