The fate of labelled15N urea and ammonium nitrate applied to a winter wheat crop

A field experiment was conducted on a winter wheat crop in Northern France with either15N-urea or ammonium nitrate, labelled either on NH4 or on NO3. The fertilizer was split between two dressings, one applied in early March and the second in mid-April, labelled separately. N uptake by the crop was measured at 8 successive times after each dressing. The N uptake efficiency of nitrate was higher than that of ammonium or urea over the whole growth cycle for both dressings. The RUC (Real Utilization Coefficient) reached a maximum at anthesis or even before anthesis, and decreased during the grain-filling period, indicative of N turnover in shoots. Thus the annual N use efficiency appeared highly dependent upon the date of measurement. At harvest, the contribution of soil N (residual N+mineralized N) to the crop was comparable to that from fertilizer, but the two pools were utilized at different periods.

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