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

Abstract

In 1992 and 1993, eight rates of ¹⁵N-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 ¹⁵N 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 ¹⁵N 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.