**1. Introduction**

In current agriculture, the amount of food production depends on the use of N fertilizers, both mineral and organic [1]. In modern agriculture, which is dominated by varieties of a huge yield potential, resulting from a high rate of biomass growth, the requirement for N is high [2]. This requirement can be fulfilled by taking into account the required rate of a crop plant growth and the uptake of the adequate amount of N, and synchronization of its supply with a plant requirement. N is taken up by plants in two chemical forms, i.e., as nitrogen nitrate (N-NO3, NN), and ammonium (N-NH4) [3,4]. This N form, as opposed to ammonium, does not undergo fixation by soil colloids therefore being easily available to a crop plant within a broad range of soil pH [3]. Nitrate N, in spite of a higher metabolic cost of assimilation, results in the production of carbohydrates, consequently leading to a higher rate of plant growth with respect to ammonium [5,6]. In the light of present knowledge, NN, in fact its soil resources, can be considered as the soil nitrogenous growth factor (NGF). This hypothesis is also supported by thousands of scientific papers in the area of agriculture

that point to the strong yield increase in grown crops in response to the application of fertilizer N. The classic example is winter oilseed rape (WOSR) [7–11].

A plant requires about 16 (19) nutrients other than N to cover its life cycle. All these nutrients are responsible for N efficiency in metabolic and physiological processes in a crop plant, affecting both the rate of its growth, and final yield [5,12]. The primary source of these nutrients is the soil solution, being enriched by compounds incorporated into the soil through the application of organic or mineral fertilizers [13,14]. The rate of uptake of these nutrients by the growing plant, including ammonium, is significantly distinct from nitrate ions, being as a rule much slower [15]. Most of this set of nutrients, but especially phosphorus (P), potassium (K), magnesium (Mg), and calcium (Ca), undergo numerous agrochemical processes, temporally changing their concentration in the soil solution and consequently reducing/increasing their available pools to crop plants [16,17]. The shortage of a particular nutrient leads, as a rule, to an inefficient use of N, irrespective of its source (soil, manure, mineral fertilizers) [8,14,18]. It is well-recognized by farmers that the efficient use of N requires its balancing through an adequate supply of other nutrients. Therefore, the production potential of a given soil, including both the current content of available nutrients, and soil pH is, in general, recognized as soil fertility, and can be defined as the soil fertility factor (FF).

One of the most important characteristics of any crop plant is the concentration and consequently the amount of any given nutrient accumulated by the crop plant during the growing season, which affects the rate of biomass growth and expression of yield components [2,19]. The total vegetative season of a seed crop growth, based on the mode of development of its particular organs, can be divided into three major periods [20]. The period of crop foundation (PCF), extending from seed emergence to the onset of stem elongation (BBCH 30, STME, phenological stages of crop plants growth as proposed by Mayer [21] and Böttcher et al. [22], respectively), covers the WOSR growth stages related to root system growth and the rosette build-up. The second major period, termed as the period of yield foundation (PYF), ending at the end of inflorescence emergence (INFE), comprises stages responsible for the build-up of the primary generative yield components. The third major period, termed as the period of yield realization (PYR), extends from the onset of flowering (BBCH 60, flowering (FL)) up to physiological crop maturity (BBCH 89, the end of pod development-maturation (PDV-M phase). The borderline phases define the cardinal stages of WOSR growth. It can therefore be assumed that the plant nutritional status depends on both the status of the NGF and soil fertility factors (FFs) just at a borderline between the major phases, which can be named the cardinal stages. Thus, the status of NGF in a respective cardinal stage should be related to the content of NN, and the FF current availability status of other nutrients present in the soil occupied by WOSR roots. Accumulation of N by WOSR yields a high level (3.75 t ha−1) continuous up to BBCH 79, i.e., to the end of pod growth (BBCH 79) [23]. The soil zone occupied by roots for WOSR is usually related to a soil depth down to 0.9 m [24,25]. So far, the analysis of mineral N has been limited to the onset of crop growth in spring [26,27]. For N budgeting, taking into account NN pressure on environment, its content should be also analyzed after harvest [28,29]. The content of available nutrients, defining the current status of FFs should also be measured in the same soil layer as is practiced for mineral N. To date, the in-season status and variability of both groups of production factors is still a classic *black box*.

The key sources of N in intensive crop production are mineral N fertilizers (Nf) [1,3]. Consumption of Nf has progressively increased during the last hundred years, and it is expected to have increased up to 182 × 10<sup>6</sup> t by 2050, i.e., 75% more as compared to 2010 [30]. Some of the projected Nf consumption can be potentially substituted by other N sources, mainly organic by-products (wastes) of human activity [31,32]. As a fast-developing source of renewable energy, biogas plants seem to be a grea<sup>t</sup> source of N, which could be used in crop production. The by-product of the anaerobic digestion is digestate, which is rich in mineral and organic N compounds. However, its concentration in raw biogas slurry is highly variable both in total content (0.1–0.5% fresh weight) and in ammonium (30 to 70% of total N) [33,34]. Digestate, depending on the substrate, also contains other nutrients (macro, and micronutrients), as well as enzymes and hormones. All these compounds significantly

affect the level of soil fertility, including the content of soil available N, and in consequence a ffect plant growth and yield [35,36]. However, a key question remains respecting the e fficiency of biogas N in crop production in comparison with the classical N source, i.e., to Nf. The latest study on maize response to digestate showed its higher e fficiency, i.e., a net yield increase as compared to ammonium nitrate [37].

The objective of the study was to discriminate the three nitrogen fertilization systems, resulting from the application of two distinct nitrogen forms applied as mineral and organic, and their mixture on WOSR yield, based on the relation between the nitrogenous growth factor (NGF) and soil fertility factors (FFs) in the cardinal stages of winter oilseed rape growth.
