Fate of N-15-labelled fertilizer in a long-term field trial at Ropsley, UK

In 1992 and 1993, eight rates of N-15-labelled fertilizer (0-245 kg ha(-1)) were applied to winter wheat growing on the Ropsley long term field trial where eight different N amounts had been applied annually since 1978. The fate of the labelled N in the crop and topsoil (0-23 cm) was determined at harvest in the year of N-15 application and in the first and second residual years. By harvest in the second residual year, 60-77 % of the original labelled application had been recovered in the crop and topsoil with 23-40 % lost. These losses virtually all occurred within the first two growing seasons; there was no significant loss during the second residual year. Significant changes in the N-15 balance were observed at N applications in excess of the range 140-175 kg ha(-1) which suggested a marked decrease in the efficiency of N use and an increase in residual labile N in the soil at harvest. At low N applications (< 175 kg ha(-1)), a positive added nitrogen interaction (ANI) was observed: 40-50 % of this was a residual ANI due to the short or long term effect of applying N fertilizer, and the remainder was probably an apparent ANI due to pool substitution in the immobilization process. At large N applications (> 175 kg ha(-1)) a negative ANI was observed: large N applications resulted in a net suppression of soil N uptake due to substitution by fertilizer for a limited plant N demand.