The partitioning of fertilizer-N between soil and crop: Comparison of ammonium and nitrate applications

Abstract

Field experiments were carried out in 1987 on winter wheat crops grown on three types of soil. 15N-labelled urea, 15NH4NO3 or NH415NO3 (80 kgN ha ~) was applied at tiUering. 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 tSN 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- ~ at 9°C daily mean temperature. The differences in immobilization and uptake had almost disappeared at flowering and harvest. 15N recovery in soil and crop varied between 50 and 100%. Gaseous losses probably occurred by volatilization in the chalky soil S + and denitrification in the hydromorphic loamy soil. These losses affected the H 4 and NO 3 pools differently and determined the partitioning of fertilizer-N between immobilization and absorption.