Fate of nitrogen applied in different fertilizers to the surface of a calcareous soil in Syria

N-15-labelled ammonium sulphate or N-15-labelled urea were each applied in solution at a rate of 30 kg N ha(-1) to the surface of 20 soil cores (52 mm internal diameter x 100 mm deep) located on a field experiment at the ICARDA station, Tel Hadya, Syria. Recovery of N-15-label in the ammonium, nitrate, organic and/or urea-N pools in the soil was measured on days 0, 1, 2, 5 and 13 after application. Total recovery of N-15 was initially 100%, but by day 13 after application it had declined to 51% with urea and 73% with ammonium sulphate. Ammonium nitrate labelled either as ammonium or as nitrate was also applied to the soil surface of 8 other cores at the same time. N-15 recovery in the four soil N pools was measured only on day 12 after application. Total recovery of N-15-label was 75% with labelled ammonium and 57% with labelled nitrate. Volatilization of ammonia from this calcareous soil (pH 8.1) is one probable mechanism of N loss from ammonium and urea fertilizers: with nitrate both leaching beyond the base of the core (i.e. 100 mm) and denitrification were responsible for N losses. These large losses of N immediately after application have implications for fertilizer management practices.