**Remigiusz Łukowiak \* and Witold Grzebisz**

Department of Agricultural Chemistry and Environmental Biogeochemistry, Poznan University of Life Sciences, Wojska Polskiego 28, 60-637 Poznan, Poland; witold.grzebisz@up.poznan.pl

**\*** Correspondence: remigiusz.lukowiak@up.poznan.pl; Tel.: +48-61-846-6287

Received: 31 July 2020; Accepted: 7 September 2020; Published: 10 September 2020

**Abstract:** It has been assumed that the managemen<sup>t</sup> of both soil and fertilizer N in winter oilseed rape (WOSR) is crucial for N accumulation in seeds (Nse) and yield. This hypothesis was evaluated based on field experiments conducted in 2008/09, 2009/10, 2010/11 seasons, each year at two sites, di ffering in soil fertility, including indigenous N (Ni) supply. The experimental factors consisted of two N fertilizers: N and NS, and four Nf rates: 0, 80, 120, 160 kg ha−1. Yield, as governed by site × Nf rate interaction, responded linearly to Nse at harvest. The maximum Nse (Nsemax), as evaluated by N input (Nin = Ni + Nf) to WOSR at spring regrowth, varied from 95 to 153 kg ha−1, and determined 80% of yield variability. The basic reason of site diversity in Nsemax was Ni efficiency, ranging from 46% to 70%, respectively. The second cause of Nse variability was a shortage of N supply from + 9.5 soil to −8.8 kg ha−<sup>1</sup> to the growing seeds during the seed filling period (SFP). This N pool supports the N concentration in seeds, resulting in both seed density and a seed weight increase, finally leading to a yield increase.

**Keywords:** seed density; N uptake; indices of N productivity; mineral N; indigenous Nmin at spring; post-harvest Nmin; N balance; N e fficiency
