THE PARTITIONING OF FERTILIZER-N BETWEEN SOIL AND CROP - COMPARISON OF AMMONIUM AND NITRATE APPLICATIONS

Field experiments were carried out in 1987 on winter wheat crops grown on three types of soil. N-15-labelled urea, (NH4NO3)-N-15 or (NH4NO3)-N-15 (80 kg N ha-1) was applied at tillering. The soils (chalky soil, hydromorphic loamy soil, sandy clay soil) were chosen to obtain a range of nitrogen dynamics, particularly nitrification. Soil microbial N immobilization and crop N uptake were measured at five dates. Shortly after fertilizer application (0-26 days), the amount of N immobilized in soil were markedly higher with labelled urea or ammonium than that with nitrate in all soils. During the same period, crop N-15 uptake occurred preferentially at the expense of nitrate. Nitrification differed little between soils, the rates were 2.0 to 4.7 kg N ha-1 day-1 at 9-degrees-C daily mean temperature. The differences in immobilization and uptake had almost disappeared at flowering and harvest. N-15 recovery in soil and crop varied between 50 and 100%. Gaseous losses probably occurred by volatilization in the chalky soil and denitrification in the hydromorphic loamy soil. These losses affected the NH4+ and NO3- pools differently and determined the partitioning of fertilizer-N between immobilization and absorption.