## 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  
The produced substrates were also evaluated for the content of soluble (available) forms of microelements and potentially toxic elements. In particular, the concentrations of soluble forms of iron (Fe), manganese (Mn), zinc (Zn), and copper (Cu), as well as nickel (Ni), lead (Pb), and cadmium (Cd), were analysed. In the case of iron (Fe), which plays an important role in metabolic processes and photosynthesis, significant differences were observed between the two types of coconut fibre [26]. Coconut fibre A was characterised by an average soluble Fe content approximately twice as high as that of fibre B. These differences may result both from the different origins of the raw material and from the methods of its processing. Additionally, the applied modifiers in the form of biochar and/or coatings by biopolymer influenced the iron content in the aqueous solutions extracted from the substrates. In most cases, an increase in soluble Fe concentration was recorded compared to the control samples, suggesting that these additives may improve the availability of microelements in the plant root zone (Figure 9). This effect may be related to biochar's ability to adsorb and slowly release nutrients, as well as the influence of the biopolymer on the physicochemical properties of the substrate, including its sorption capacity. Similar trends were also observed regarding the content of soluble manganese (Mn) forms. It was demonstrated that the content of available Mn in the substrates varied significantly depending on the additive used and the type of coconut fibre (Figure 10). The greatest increase in soluble manganese content was recorded in variants with the addition  
Figure 9: Content of soluble iron forms in the prepared substrates (mean \(\pm\) SD). NOTE: Mean values \(\pm\) standard deviation. (a–g) Identical superscripts denote no significant (\(p<0.05\)) differences between the experimental objects according to the post hoc Tukey HSD test. of biopolymer isolate, where the average Mn content in aqueous extracts was up to five times higher compared to the control samples (AW_0 and BW_0) (Figure 10). This effect was consistent for both fibre A and B, suggesting that the additive used, rather than the base raw material, had the greatest influence on the mobility and availability of this microelement. This phenomenon may result from interactions between the biopolymer and the organic fraction of the substrate, leading to increased solubility of Mn or reduced sorption [27]. Additionally, changes in the physicochemical parameters of the substrate--such as pH or cation exchange capacity--may have promoted the enhanced release of manganese into the soil solution. In the case of soluble zinc (Zn) content, the observed changes were more varied and dependent on the type of coconut fibre used as well as the type of modifying additives (Figure 11). For fibre A, the average Zn content in aqueous solutions did not undergo significant changes regardless of the variant applied (AW_1-AW_3), indicating a limited impact of additives such as biochar or biopolymer on the mobility of this element. In the case of fibre B, a significant increase in Zn content was observed only in variants BW_1 and BW_2, i.e., following the application of biochar and coating biopolymer, respectively. This may suggest that fibre B, due to its different physicochemical structure,  
Figure 11: Content of soluble zinc 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. Figure 10: Content of soluble manganese 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.