*3.1. Yield and Its Components*

Yield of WOSR was significantly determined by the interaction of Y × S × N or by the interaction of Y × F × Nf (Table 3). It is necessary to stress that weather conditions during the spring vegetation were favorable for WOSR growth. In each year of the study, the total amount of precipitation during the period extending from the onset of flowering to seed maturity (FL-SM) was below 300 mm, as reported by Berry and Spink [12] for British conditions, but it was above the long-term average for this region in Poland (197 mm). The highest yield of 4.41 t ha−<sup>1</sup> was obtained at Venetia with 299 mm of rainfall during FL-SM, directly supporting the prognosis of Berry and Spink [12]. The effect of site on yield was revealed in 2009 and 2011, and that of Nf in 2010 and 2011. Based on a simple regression analysis, two distinct patterns of yield response to the increasing Nf rate were observed (Table A1). The quadratic regression model, which prevailed in four sites, indicates a saturation status of Nf supply with respect to the achieved yield. This conclusion is supported by the well-defined optimum N fertilizer rate (Nfop) for the maximum achievable yield (Ymax) for a particular site. The calculated Nfop of 103.1, 142.7, 104.3, and 128.0 kg ha−1, resulted in a Ymax of 3.142, 3.629, 3.589, and 4.012 t ha−1. The pattern of yield response to the Nf rate in the other two sites, representing Wi in 2010, and Ve in 2011, fitted the linear regression model the best. This type of yield pattern clearly indicates that the applied Nf rate was too low to reach the highest yield.

Total biomass of WOSR (TB) and harvest index (HI) were significantly affected by all studied factors. In 2009 and 2010, the effect of weather on TB and HI was the most pronounced. Any increase in TB resulted in a simultaneous, and at the same time, a significant drop in HI. In 2011, TB was on average much lower compared to both previous years, but HI was significantly higher (29.8% vs. 22.6% in 2009, and vs. 23.1% in 2010). In spite of the non-significant impact of HI on yield, a much higher yield was recorded in 2011 (Table 3).

The key yield component, i.e., SD responded significantly, as in the case of yield, to the interaction of Y × S × N, and also to Y × F × N. In 2009, the difference between sites reached 23.4%, whereas the positive impact of NS fertilizer as compared to N alone was much lower, i.e., 5.2%. In 2010, neither factors affected SD. In 2011, a pronounced effect on SD was exerted by the type of Nf fertilizer (+8% for NS versus N alone). The impact of site on SD, and in consequence on yield was year-dependent. A detailed analysis of SD response to the applied Nf rate clearly showed the occurrence of two regression models, i.e., quadratic and linear (Table A1), which covered the same set of sites as described previously for yield. This fact was documented by the strongest value of the correlation coefficient between yield and SD (*r* = 0.84, Table A2).


**Table 3.** Yield and yield components of winter oilseed rape.

\*\*\*, \*\*, \* significant at *p* < 0.001; *p* < 0.01; *p* < 0.05, respectively; n.s.—non significant; a1 within a year, means within a column followed by the same letter indicate a lack of significant difference between the treatments. Y—yield; TB—total biomass; HI—harvest index; PD—pod density; SD—seed density; Se/Po—number of seeds per pod; TSW—thousand seed weight.

The second basic yield component, i.e., TSW responded mainly to site in particular years of study. In 2009, a slightly lower TSW was the attribute of Ko, whose yield was significantly lower when

compared to Go. In 2010, a significantly higher TSW was recorded for Wi, which yielded at the same level as Bu. In 2011, a significantly higher TSW for Do was in accordance with a higher yield for this site. In general, TSW showed a negative, but not significant relationship with SD, which indirectly indicates the presence of a yield compensation mechanism, which is revealed during the SFP [9].

The other yield components, i.e., pod density (PD), and the number of seeds per pod (Se/Po) exerted a much weaker impact on yield when compared to SD (Table A2). In 2009, the difference between sites with respect to the degree of PD and TSW expression were more pronounced for TSW than for PD (29.6% vs. 9.6%). In 2010, the opposite pattern was observed (13.3% vs. 23%). In 2011, the yield compensation mechanisms were the highest, reaching 28% for TSW, and 38% for PD.

#### *3.2. Characteristics of Nitrogen Accumulation Patterns at Harvest*

The concentration of N in seeds (Nc) in all the study years responded mainly to site, and in 2010 and 2011, to the rate of applied Nf (Table 4). In 2009, a significantly higher Nc was determined in seeds for Ko than for Go. A significant, but a slightly lower difference between sites was observed in 2010. In 2011, the average seed Nc was the highest and the difference between fields was also very high. In 2010 and 2011, the increasing Nf rate resulted in a progressive Nc increase. In spite of the significant impact of site and the Nf rate, the interactional impact of all experimental factors on the Nc in seeds was negligible. The impact of this seed characteristic on yield was positive, but not decisive (Table A2).

The strongest impact on yield was exerted by the amount of N accumulated in seeds at harvest (Nse) (Table A2). This WOSR characteristic responded to the interaction of Y × S × N, and Y × F × N. The first interaction was significantly stronger (*p* ≤ 0.001 vs. *p* ≤ 0.01). A significant difference between sites were recorded in 2009 and 2011. In 2009, the recorded difference reached 35.1%, but in 2011, only 20%. However, both Nse and yield were higher in 2011. The effect of the Nf rate was recorded in all years. As in the case of yield, Nse patterns followed the same type of regression model (Table A1). The quadratic regression model, presenting a saturation level of Nse accumulation, was recorded in three sites, i.e., at Go in 2009, at Bu in 2010, and at Do in 2011. The optimum Nfop rate for the Nsemax was 92.3 for Go, 110.2 for Bu, and 157.2 kg ha−<sup>1</sup> for Do. The respective Nsemax were as 96.2, 124.1, and 154.1 kg ha−1. The linear regression model, representing the unsaturation pattern of Nse was recorded in the other three sites.

The amount of N in WOSR residues (Nres) and the nitrogen harvest index (NHI) were highly variable between sites within a particular year. The greatest difference between studied fields was recorded in 2009, and resulted in the lowest yields. An opposite trend was recorded in 2011, when the difference between fields was much lower, reaching only 30%, but WOSR yielded the highest. In 2009 and 2010, a significantly higher Nres was recorded for the plot fertilized with NS fertilizer. An opposite trend was recorded in 2011. In this particular year, the applied NS fertilizer resulted in a significant decrease of Nres, which corresponded to the higher seed yield. The NHI indicates the relative share of Nse in the total WOSR biomass at harvest. A significant difference between sites was recorded in 2009 and 2010, but not in 2011, the year with the highest NHI. It is necessary to stress that NHI did not show any significant relationship with the Nc in seeds and Nse, but was strongly, and negatively correlated with Nres. Finally, NHI did not impact seed yield (Table A3).

Total N uptake (TN) was significantly driven by all the studied factors. The effect of N fertilizers on TN was only significant in 2009, and 2010. The strongest response of TN to the increasing Nf rate was observed in 2009, in which WOSR yielded the lowest, in spite of a very high value on the N control plot. TN affected yield significantly, but at a much lower level as observed for Nse (Table A2).


**Table 4.** Indices of nitrogen managemen<sup>t</sup> by winter oilseed rape.

\*\*\*, \*\*, \* significant at *p* < 0.001; *p* < 0.01; *p* < 0.05, respectively; n.s.—non significant; a1 within a year, means within a column followed by the same letter indicate a lack of significant difference between the treatments. N—N content in seeds; Nse, Nres—the amount of N in seeds, harvest residues, respectively; TN—total amount of N in WOSR at harvest; NHI—nitrogen harvest index; UNA—unit N accumulation, UNP—unit N productivity.

In agronomic practice, two indices are used which are reciprocal to each other, i.e., unit N accumulation (UNA) and unit N productivity (UNP). In spite of this, the relationship between both indices was not linear, and was expressed by the power function the best:

$$\text{UNP} = 993.7 \text{UNA}^{-0.99b} \text{ for } \text{n} = 24, \text{R}^2 = 0.99 \text{ and } p < 0.001. \tag{15}$$

This type of relationship clearly shows that the unit productivity of N accumulated by WOSR at harvest was the highest when its accumulation was low, decreasing exponentially with its increase. Both indices responded to the interaction of all studied factors. The greatest difference for UNA, reaching 46%, between sites was recorded in 2009. This difference was slightly higher, with respect to UNP (51%). The smallest difference was recorded in 2011 (17%, 18%, for UNA and UNP, respectively). The effect of Nf type was, in general, low, being in most cases not significant. The effect of the increasing Nf rate was site specific. The most conspicuous impact of UNA was recorded for TN and HI. The first WOSR characteristic responded positively, but the second negatively to UNA increase (Table A2).

The applied principal component analysis (PCA) clearly revealed the distinct impact of yield components and crop N indices on the WOSR final yield. PCs with eigenvalues greater than 1.0 were used as a primary criterion to determine the number of PCs. The first four PCs accounted for 92.27% of the total variance in the data (Table A3). However, only the variables with scores on PCs over 0.70 (R<sup>2</sup> > 0.50) were taken into consideration. PC1 and PC2 contributed to 49.43% and 22.12% of the total variance, respectively. Four of the nine variables had high loadings on PC1, fulfilling the chosen criteria. The highest, and at the same time positive loadings, were recorded in descending order (*r*) for Y = Nse = TN > SD. The required criteria for PC2 were reached for PD, and for PC3 for Se/Po (negative). PC4 had a positive loading for UNA. The studied variable weight was evaluated by the Eigen vector, which varies between −1 to +1. The Eigen vectors for the examined variables were broadly scattered on the two first PCA axes (Figure 1a). The closest to an absolute of 1 were TN, Nse, and Y. The distance of Nse to Y, as seen from Figure 1b, was the closest among the studied N indices and yield components. The Nc in seeds exerted the same loading on both PC1 and PC2 axes (Table A3).

### *3.3. Indices of Fertilizer N Management*

The partial factor productivity of fertilizer N (PFPN) is the basic index, describing the productivity of applied Nf [29]. In the studied case, PFPN was significantly driven by Y × S × N, and Y × F × N interactions (Table 5). The PFPN indices in 2011 were the highest, and the difference between sites was small (5.3%). The lowest PFPN indices, but at the same time, the highest difference between sites, were recorded in 2009 and 2010 (13%). The PFPN decreased in accordance with the progressively increasing Nf rate, irrespective of other factors.

The next index, i.e., agronomic efficiency of Nf (AEN) was driven by the same set of factors as PFPN. A significant, and at the same time, a very strong difference between fields was recorded in 2010 and 2011, reaching 137% and 81%, respectively. The effect of the type of N fertilizer was significant in all of the studied years, clearly indicating a significantly higher N net productivity in the presence of sulfur. The highest AEN increase in response to the NS fertilizer was recorded in 2011, a year with the highest yields. The lowest drop in AEN in response to the increasing N rates was also recorded in 2011. It was twice as low as recorded in 2010, when it was the highest.


**Table 5.** Indices of nitrogen managemen<sup>t</sup> by WOSR.

\*\*\*, \*\*, \* significant at *p* < 0.001; *p* < 0.01; *p* < 0.05, respectively; n.s.—non significant; a1 within a year, means within a column followed by the same letter indicate a lack of significant difference between the treatments. PFPN—partial factor productivity of fertilizer N; AEN agronomic e fficiency of fertilizer N; PEN—physiological e fficiency of fertilizer N; recovery of fertilizer N.

The physiological N use efficiency index (PEN), describing the utilization efficiency of N taken up by a plant during the growing season, significantly responded to the same set of factors as described for AEN. Each year, the differences between sites were extremely high. In 2009, a net productivity of 1.0 kg of N taken up by WOSR plants was 80% higher in the field located at Go, as compared to Ko. In 2010, the observed difference between sites was twice as high. In 2011, the difference between sites was much smaller, but also high, amounting to 152%. The effect of NS fertilizer was as a rule positive, and a significant response to S was observed in 2009 and 2011. The effect of the increasing Nf rate on PEN was clearly demonstrated in 2009 and 2010, decreasing in accordance with the increased Nf rate. In 2011, index stagnation in plots fertilized with N at the rate of 120 and 160 kg ha−<sup>1</sup> was recorded.

Nitrogen recovery (RN) reveals the contribution of N fertilizer in the total N taken by a crop during the growing season. RN indices responded significantly to all the studied factors, including years. The highest RN values were recorded in 2010, exceeding 100% in the field located at Bu (105%). In this particular year, the difference between sites was low (9.7%). The lowest RN of 39.8% was recorded in 2009 at Ko, being, however, twice as high at Go. The same trend was recorded in 2010, but the difference between sites was 29.2%. The effect of NS fertilizer on RN was as a rule positive, but not significant in 2009. A positive impact of NS on RN was noted in 2009 at Ko; in 2010 at Wi, and in 2011 at Ve. The effect of the increasing Nf rate on RN showed the same pattern, as presented for the other N indices. In 2010, it exceeded 100% on plots fertilized with Nfat the rate of 80 kg ha−1.

**Figure 1.** Score plot of WOSR components and nitrogen indices in PC1 and PC2 axes (**a**) and PC1 and PC3 axes (**b**). Y—yield, SD—seed density, PD—pod density, TSW—thousand seed weight, TN—total nitrogen, Nc—N concentration in seeds, Nse—N accumulated in seeds, UNA—unit N accumulation.

#### *3.4. Indices of Soil N Management*

Nitrogen input (Nin) is composed of two N mineral sources. The first, internal (indigenous) source, is soil mineral N measured in spring (Nmins), which is recorded in the soil at the onset of WOSR spring regrowth. The second one is the amount of Nf applied to the growing crop during spring vegetation [23]. In the studied case, the Nf rate was 80, 120, and 160 kg ha−<sup>1</sup> (Table 6). Due to a fixed amount of applied Nf, it was not possible to evaluate Nin statistically. In spite of this, the difference between sites was the most pronounced in 2011 (90%), followed by 2009 (33%), and the least pronounced in 2010 (12%).

The residual N (Nminr) is the amount of Nmin measured just after WOSR harvest. This N characteristic showed a significant impact of all studied factors. As in the case of yield, Nminr was evaluated based on the Y × S × N interaction. The difference between sites was evaluated based on two indices, referring to the Nminr content on the N control plot and to its maximum value, as an indicator of the applied rate of Nf:

1. The Nminr content on the N control plot:

$$\text{a.} \qquad \text{Go (52.0)} < \text{Do (83.5} < \text{Bu (98.3)} \le \text{Ko (102.5)} < \text{Wi (117.2)} < \text{Ve (146.1 kg ha}^{-1}).$$

2. The maximum Nminr content:

$$\text{b.}\qquad \text{Go (85.6)} < \text{Ko (102.5)} < \text{Bu (111.8)} \le \text{Wi (119.5)} \le \text{Do (121.1)} < \text{Ve (146.1 kg ha}^{-1}).$$

3. The net Nminr increase with respect to the N control:

$$\text{c.}\qquad \text{Do (+37.6)} > \text{Go (+33.5)} > \text{Ko (+18.6)} > \text{Bu (+13.5)} > \text{Wi (+2.3)} > \text{Ve (+0.0 kg ha}^{-1})$$

The highest amount of residual Nmin was recorded, irrespective of the applied Nf rate, at Ve. The lack of differences between the control N plot and the Nf highest rate indicates that N was taken up by WOSR plants with the same efficiency. For other fields, the net effect of Nf was highly diversified between years and sites. The difference between sites in 2011 was very strong (47%). In the case of Do, the Nminr content increased progressively with the increased Nf rate. The lowest amount of Nminr was recorded in 2009 and the difference between sites was significant (27%). In the case of Go, the amount of Nminr increased linearly with the applied Nf rate.

Nitrogen balance (Nb) was significantly driven by all factors in the study. A positive Nb value indicates the presence of a sufficiently high pool of Nmin present in spring. A negative Nb value indicates Nmin net release from soil resources during the growing season. The differences between sites in particular years were extremely high. A positive Nb was registered in two sites, i.e., in 2009 for Go, and in 2011 for Ve. In the other four sites, a high release of Nmin from soil resources during the growing season was recorded. In the first two years, Nb was negative, irrespective of the type of applied N fertilizer. A quite opposite trend was revealed in 2011. The effect of increasing Nf rates was year specific. In 2009, negative Nb values were recorded on plots with low N rates (up to 80 kg ha−1). In 2010, the observed trend was opposite. In 2011, Nb decreased with the Nf rate increase.

The next characteristic of N managemen<sup>t</sup> during the growing season is Ngain, i.e., the net amount of N taken up by a crop from soil N resources during the growing season. Its variability was most affected by the interactional effect of Y × S × N. The impact of years was tremendous. In 2009, Ngain was positive, and the difference between fields was 10-fold. In 2010, Ngain was extremely high, and the difference between sites was high, but much smaller when compared to 2009. A 4-fold difference between sites was observed in 2011. The effect of N fertilizer type appeared only in 2009 when the application of NS fertilizer increased the net amount of Nmin by 19%. The effect of increasing N rates on Ngain followed two patterns (Table A4), linear (Ko, Ve, Do), and quadratic (Go, Bu, Wi).


**Table 6.** Indices of nitrogen managemen<sup>t</sup> in the soil/plant system.

\*\*\*, \*\*, \* significant at *p* < 0.001; *p* < 0.01; *p* < 0.05, respectively; n.s.—non significant; n.a.—non analyzed. a1 within a year, means within a column followed by the same letter indicate a lack of significant difference between the treatments. Nin—indigenous N (Ni) + fertilizer N (Nf); Nminr—residual Nmin (post-harvest Nmin); Nb—N balance; Ngain—N mineralized during the growing season and incorporated into WOSR biomass; Nint—total N input into the soil/plant system during the growing season.

Total N input (Nint) responded almost to the same set of factors and their interactions as recorded for Ngain. A significant impact of site was recorded in 2009 and 2010 but not in 2011, during which WOSR yielded the highest. The effect of N fertilizer type was the same as observed for Ngain. The effect of N rates was year specific. The effect of increasing N rates on Nint followed two patterns linear (Ko, Ve, Do), and quadratic (Go, Bu, Wi), i.e., showing a high resemblance to Ngain patterns (Table A4).
