## 3 Results and Discussion
### Ability to Release Selected Mineral Components into the Aqueous Phase
#### 3.2.4 Content of Soluble Forms of Microelements and Trace Elements
exhibits greater susceptibility to changes in Zn bioavailability under the influence of the applied additives. The increased solubility of Zn in these variants may be linked to improved sorption conditions or changes in the pH of the extracts, which affects the greater mobility of microelements in the root zone. The obtained results indicate that appropriate modification of the substrate composition--also dependent on the type of fibre used--may play a significant role in optimising Zn availability for plants under horticultural cultivation conditions. In the case of soluble forms of copper (Cu) and lead (Pb), the application of the examined modifying additives did not result in significant changes in their concentrations in aqueous extracts, regardless of the type of coconut fibre used (A or B). The average Cu and Pb levels in the aqueous solutions remained at a similar level across all experimental variants compared to the control samples, indicating a limited impact of the applied additives on the mobility of these elements under laboratory conditions (Figures 12 and 13). However, it should be noted that in some variants (particularly BW_1 and AW_2), a tendency towards a slight reduction in Cu and Pb content was observed compared to the respective control samples. This may suggest partial binding of these elements by the biochar structures, which are characterised by high porosity and adsorption capacity for  
Figure 12: Content of soluble copper forms in the prepared substrates (mean \(\pm\) SD). NOTE: Mean values \(\pm\) standard deviation. (a–d) Identical superscripts denote no significant (\(p<0.05\)) differences between the experimental objects according to the post hoc Tukey HSD test. Figure 13: Content of soluble lead forms in the prepared substrates (mean \(\pm\) SD). NOTE: Mean values \(\pm\) standard deviation. (a–d) Identical superscripts denote no significant (\(p<0.05\)) differences between the experimental objects according to the post hoc Tukey HSD test. binding heavy metals. A similar mechanism may have occurred with the addition of the biopolymer, which could limit metal mobility by forming complexes or by influencing the redox potential of the root environment [28]. From the perspective of food crop safety, the tendency to limit the availability of Cu and Pb may be considered beneficial, especially in the context of the accumulation of these elements in the edible parts of plants. Although Cu and Pb concentrations in the analysed substrates were within safe ranges, further studies should consider long-term changes in the availability of these metals under real cultivation conditions, as well as their potential accumulation in plant tissues. Different trends were observed in the case of soluble forms of nickel (Ni) and cadmium (Cd) (Figures 14 and 15). Contrary to the previously discussed elements, the application of the modifying additives--both biochar and the biopolymer coating--significantly increased the content of these potentially toxic elements in the obtained aqueous extracts. This relationship was evident for both types of coconut fibre analysed (A and B), suggesting that the increased bioavailability of Ni and Cd was a result of the properties of the additives rather than the base material. The increase in Ni and Cd content in the leachates following the application of biochar may be due to its chemical composition--particularly if the raw material used for biochar  
Figure 14: Content of soluble nickel forms in the prepared substrates (mean \(\pm\) SD). NOTE: Mean values \(\pm\) standard deviation. (a–d) Identical superscripts denote no significant (\(p<0.05\)) differences between the experimental objects according to the post hoc Tukey HSD test. Figure 15: Content of soluble cadmium forms in the prepared substrates (mean \(\pm\) SD). NOTE: Mean values \(\pm\) standard deviation. (a–c) Identical superscripts denote no significant (\(p<0.05\)) differences between the experimental objects according to the post hoc Tukey HSD test. production contained trace amounts of these metals.