N content in one-year-old needles was 1.2% in the control plot in the last third of September 2014 (Figure 2). It rose to 2.1% after application of N fertilizers to the soil at dose 100 kg ha⁻¹ in all variants of B supply.

N levels in needles in the treatments with nitrogen at 200–400 kg ha⁻¹ were the same as in the variants with boron deficit and its surplus, rising to 2.8%.

As N dosage was increased in B-deficient plots, N levels in needles rose (to 3.2%) until the N dose was 800 kg ha⁻¹, and then, as N doses were elevated further, the level declined to 2.5%. In the B-excessive variant, N content in needles changed little between fertilization at 400 kg ha⁻¹ and the highest dose. In the variant with optimal B supply (3 kg ha⁻¹), N content in needles increased more significantly with elevation of fertilization doses to 400 kg ha⁻¹ (to 3.6%) than in the plots with B deficit and surplus, but starting at 600 kg N ha⁻¹ and higher, it declined to the same level as in the other two variants of B supply. Thus, the optimal dose of B, as opposed to its deficit and surplus, augmented N levels to the maximum in needles where N fertilization doses were 200–500 kg ha⁻¹. Its levels became equal (up to 2.5%) in the needles of trees of three variants of B supply at the nitrogen dose 1000 kg ha⁻¹.

B content in one-year-old needles of pine trees from the control plot in September 2014 was low and amounted to 17 µg g⁻¹. In B-deficient settings, the element’s level in needles changed insignificantly when different N doses were applied. In B-fertilized plots, foliar B content in pine trees was higher than in B-deficient plots in all N dosage variants. In the variants with optimal and excessive B supply, its foliar content was more significantly augmented by application of the second dose of N compared to the first dose, i.e., from 103 and 174 µg g⁻¹ to 149 and 205 µg g⁻¹, respectively. As N doses were further raised, foliar B levels declined, starting with 300 kg N ha⁻¹ in B-optimal plots and starting with 400 kg N ha⁻¹ in B-excessive plots, yet remaining higher in the variants with B boron surplus compared to boron optimum. The decline in foliar B content continued until the nitrogen dose of 600 kg ha⁻¹ in B-optimal plots and until 800 kg N ha⁻¹ in B-excessive plots. When even higher N doses were applied, B content in needles changed little in the two B supply variants. In treatments with the highest N dose, B content in needles was 50 and 86 µg mg⁻¹ in B-optimal and B-excessive plots, respectively. Thus, application of the second N dose coupled with optimal and excessive B supply promoted foliar B content more significantly than the first N dose. Elevation of N doses to higher levels, from 300 to 600 kg ha⁻¹ (for B optimum) or from 400 to 800 kg ha⁻¹ (for B surplus), entailed a decline in the B content in needles.

The treatments of N fertilizers alone and combined with B significantly increased the content of Arg in one-year-old needles of pine trees (Figure 3, Table 1). Differences in tree needle Arg concentration between the 11 treatments of N levels at the three treatments of B levels (B0, B3, and B6), analyzed with a two-sample t-test with equal variances, was significant (p = 0.05) in September in all variants except N0 (Table 2). In March, no significant differences were observed in variants N0, N4, and N10. Followed for pairwise comparison, when the analysis of variance showed significance (p = 0.05), it was possible to reveal that the accumulation of Arg in needles significantly differed in three B levels in variants N3–N5 in September and N3 and N5 in March. Regression analysis results between Arg concentration and treatments of N levels are shown in Figure 3 and Table 1. In the last third of September 2014, foliar Arg levels in plots with soil B deficiency (B0) were similar to the control in the treatments with the first three N doses (N1–N3) and then increased to 145.0 µmol g⁻¹ in the treatments with N at 600 kg ha⁻¹, and they showed little change at higher N doses.

The amount of Arg in the needles of trees in the control variant (B0N0) during the study period did not exceed 8.6 µmol g⁻¹. Optimal B availability to pine, unlike its shortage and surplus, helped significantly augment foliar Arg content (to 102.3 µmol g⁻¹) when coupled with the placement of the second and third N doses (N2 and N3). In the variants with B deficit or surplus coupled with the same N doses, on the other hand, the level of amino acid in needles was lower—17.2 and 46.0 µmol g⁻¹, respectively. In treatments with N doses above 500 kg ha⁻¹, differences in amino acid content between the B deficit and B optimum variants became less significant (Table 2), while the Arg level in the variant with B surplus was lower than in the other two variants.

In the last third of March 2015, as well as in September 2014, Arginine content in one-year-old pine needles in B-deficient plots was similar to that of the control when the first two N doses were applied; as N doses were raised to 500 kg ha⁻¹, the amino acid content rose to 107.5 µmol g⁻¹, and when the N dose was increased further, the change in Arg was insignificant. In plots with optimal B supply in March as well as in September, the first N dose had no significant effect on Arg content in needles; the second and further N doses, up to 500 kg ha⁻¹ inclusively, significantly promoted the Arg level to 175.9 µmol g⁻¹. Foliar Arg level in March, as compared to September, was promoted even more significantly by the application of N starting with the second dose in the B-optimal variant and with the third dose where B was in surplus. The level of the amino acid in needles in the variant with B deficiency, on the contrary, was lower in March compared to September in treatments with N in doses above 500 kg ha⁻¹ and was also lower in the N treatments of 300 kg ha⁻¹ and higher compared to trees from B-fertilized plots.