Fate of fertiliser 15N applied in early and late spring to winter wheat

A field experiment was carried out to compare fertiliser-N recovery in crop and soil from N-15-labelled urea applied in early spring to winter wheat (Triticum aestivum L.) at rates of 0, 150 and 200 kg N ha(-1). In two additional treatments, 150 kg N ha(-1) was applied in split applications (100 kg N ha(-1) applied in early spring plus 50 kg N ha(-1) at booting as either solid urea granules or dissolved in water). Grain yields and protein contents were not significantly influenced by fertiliser N applications. In comparison with 150 kg N ha(-1) applied in early spring, split applications of N (whether in solid or liquid form) increased the recovery of N-14 and N-15 in grain at the expense of that in straw. Total recovery of N-15 amounted to 86-88% for the early spring-applied treatments and 99-106% for the split N applications, indicating losses of 0-14%. These were thought to be mainly gaseous losses of N2O and N-2. Recovery of fertiliser N-15 in the soil ranged from 41 to 64%, and the bulk of this (63-68%) was recovered in the 0-10 cm soil layer, predominantly in organic form. The amount of fertiliser N-15 present in soil mineral form at harvest amounted to only 1.6 to 3.2% of that applied. Thus, a negligible amount of fertiliser N applied in spring is present in residual mineral form after harvest. Consequently fertiliser N applications at these rates are unlikely to contribute to nitrate leaching losses over the following winter.