Towards identifying the fate of ¹⁵N applied in spring as ammonium nitrate fertiliser to a kikuyu pasture

Abstract

Nitrogen was applied as ammonium nitrate in solution to kikuyu pasture at 225 kg N ha^‐1, split into three equal dressings and applied in October, December and February. The first, at 75 kg N ha^‐1 was labelled in the ammonium ion and the latter two dressings were not labelled. Herbage, soil and roots were sampled monthly to determine the proportions of the labelled N in the herbage, root and soil over the season. Total ¹⁵N recovered over the season, as a percentage of that applied as ¹⁵NH₄NO₃, was 99.0%. A total of 52.1% of the labelled N was recovered in the herbage over the season, the highest recovery being in November. Recovery rates declined from November through to March. However, the accumulated recovery of ¹⁵N was curvilinear over the season. The recovery rate of ¹⁵N in the roots was lower than in the herbage, and the highest percentage recovered was in December. The ¹⁵N recovered, as a percentage of that applied, in the soil depth classes showed similar trends. The highest percentage of ^I5N was recovered in the 0–300 mm depth class and the lowest in the 600–1000 mm depth class. Leaching of ¹⁵N, although minimal, did occur over the season.