THE FATE OF LABELED N-15 UREA AND AMMONIUM-NITRATE APPLIED TO A WINTER-WHEAT CROP .1. NITROGEN TRANSFORMATIONS IN THE SOIL

Labelled urea or ammoninum nitrate was applied to winter wheat growing on a loamy soil in Northern France. Two applications of fertilizer were given: 50 kg N ha-1 at tillering (early March) and 110 kg N ha-1 at the beginning of stem elongation (mid-April). THe kinetics of urea hydrolysis, nitrification of ammonium and the disappearance of inorganic nitrogen were followed at frequent intervals. Inorganic nitrogen soon disappeared, mainly immobilized by soil microflora and absorbed by the crop. Net immobilization of fertilizer N occurred at a very similar rate for urea and ammonium nitrate. Maximum immobilization (16 kg N ha-1) was found at harvest for the first dressing and at anthesis for the second dressing (23 kg N ha-1). During the nitrification period, the labelled ammonium pool was immobilized two to three times faster than the labelled nitrate pool. No significant net 15N remineralization was found during the growth cycle. The actual denitrification and volatilization losses were probably more important than indicated from calculations made by extrapolation of fluxes measured over short intervals. However microbial immobilization was the most important of the processes which compete with plant uptake for nitrogen.