**1. Introduction**

Over the last four decades, oilseed rape (*Brassica napus* L., OSR) has become one of the most important global oil crops. The main reason for the rise in OSR production was an intensive breeding progress, resulting in new double 00 varieties which deliver plant oil of high consumption value [1,2]. Between 2009–2018, the world OSR harvested area increased from 31 million (mln) to 37 mln ha. The world average yield for this period increased to about 2.0 t ha−1, being only slightly lower than that recorded recently in Canada (2.2 t ha−1) [3]. The leading producers of OSR are Canada, China, and the European Union (EU). Canada, which delivers about 25% of the world rapeseed production, increased the sown area of this crop from 6.5 mln ha in 2009 to 9.1 mln ha in 2018 [3,4]. In the EU, the leading producers of winter oilseed rape (WOSR) are Germany, France, and Poland. Seed yields in these countries, in spite of high breeding progress, stagnated in the period extending from 2009 to 2018 [3,5].

The main constraints in WOSR production in the EU are weather conditions during the growing season and soil fertility level [6]. The resistance of WOSR to frost does not depend only on temperatures during winter but also on plants' physiological status just before winter, which significantly a ffects plant density [7,8]. Two basic yield components, i.e., seed density (SD, number of seed per m2) and seed weight (thousand seed weight, TSW, g) are responsible for the final yield of WOSR. The first, dominant component is SD, which is indirectly defined by plant density, number of pods (pod density, PD, pods per m2) [9]. The critical period of yield formation, referring to the development of primary yield components, such as inflorescences and succeeding pods, extends from the budding stage up to the pod full size [10]. One of the most specific characteristics of the yield development of WOSR is a strong compensation mechanism, occurring between yield components during the period extending from the onset of flowering towards the end of seed growth, i.e., maturity [11]. As reported by Berry and Spink [12], the amount of water required by WOSR during this period to exploit its yielding potential is 300 mm. Any unfavorable weather conditions during the spring vegetation lead to disturbance in the development of yield components (PD and SD). Weather disturbance during pod and seed growth negatively affects TSW [13,14].

The key nutrient responsible for yield formation by WOSR is nitrogen (N) [15,16]. This nutrient affects the number of inflorescences, flowers, and finally pods and seeds. A balanced structure of yield components depends on synchronization of a crop N requirement with its supply from both soil and applied fertilizers [17,18]. In practice, N supply to a given crop is, in general, oriented on the amount applied in fertilizers without considering soil resources. As a result of this fertilization strategy, N fertilizer productivity is highly variable, being both in shortage or in excess with respect to WOSR requirements during the critical stages of yield formation, leading in both cases to yield reduction [16,19]. In some EU countries like Germany and the Netherlands, the N fertilization strategy of crop plants is based on the measurement of the content of mineral N (Nmin) before the spring WOSR regrowth [20]. This strategy, as has been documented recently, clearly shows that N use efficiency (NUE) depends not only on the Nmin content in the root zone, but also on the content of 16 other nutrients, like P, K, and Mg, being responsible for both N uptake, and its utilization by plants [21–24]. Any shortage of this set of nutrients at the onset of flowering and during the seed filling period (SFP) leads to yield depression [11,25].

In spite of the extensive study on N supply to WOSR, and its impact on the development of yield components, the yield prognosis based on Nmin content is good, but not satisfactory. The *black box* in the effective managemen<sup>t</sup> of N during the growing season of WOSR is a lack of knowledge on N release from its soil resources during the spring vegetation [26]. The main reason for the necessity of the Nf rate optimization is a huge variability in soil potential for release of Nmin, both in the period preceding the spring vegetation and during the full season of WOSR growth [23]. The key question remains, however, to what extent does the development of basic yield components depend on the indigenous resources of N during WOSR spring vegetation? Is the soil N supply at the onset of flowering to WOSR plants sufficient to meet the requirements of the growing pods and seeds? The scientific problem focuses on the impact of three N sources, i.e., (i) N mineral soil resources, indigenous N (Ni) at the beginning of spring WOSR vegetation; (ii) the optimum Nf rate; and (iii) the quantity of the soil Nmin released during the spring vegetation on the degree of WOSR yield components expression, i.e., on the sink capacity development as a prerequisite of high seed yield achievement.

The objective of the study was to define the impact of site-specific variability in in-season N managemen<sup>t</sup> during the WOSR growing season based on the amount of N accumulated in seeds, and its relationship with the final yield.
