Differences in N use efficiency, N translocation and N immobilization capacity of their residues of oilseed rape varieties due to N fertilization

Winter oilseed rape (OSR) is known to poorly utilize nitrogen (N), thus increasing the risk of N losses. In five environments in Germany, eight OSR varieties were grown in combination with five fertilizer N treatments enabling to fit N response curves for each variety separately. At Hohenschulen, additional plant sampling after flowering and at harvest allowed to calculate N translocation from the vegetative plant parts into the seeds and to estimate the potential of the residues to immobilize N after harvest. Nitrogen fertilization increased seed and N yield. Varieties differed significantly in their yields and consequently in their NUE, however, without any significant interaction with the N supply. Total N accumulation at harvest, (N) harvest index and N translocation after flowering into the seeds as well as the N immobilization potential (Ipot) of the residues followed a similar pattern. Without N, the low amount of residues reduced Ipot despite the wide C/N ratio, while a high N fertilization only slightly increased the amount of residues, but clearly decreased their C/N ratio resulting in a lower Ipot. Our results support the approach that breeding for increased seed yields seems to be the most promising way to also improve NUE.