## 4 Discussion
### Effect of Immobilizing Agents on Bioconcentration Factors (BCF)
In this study, regardless of the analyzed metal, the addition of both substances reduced their mean amounts in amelina by 21-37% and by 14-34% in oat in relation to metal levels found in plants cultivated in the control soil. Despite the fact that the addition of both organic and inorganic amendments into soil resulted in a small (12-20%), but statistically significant increase of Cu and Zn amounts in the soil (except for Zn in the soil where amelina was grown), the \(\mathrm{BCF_{T}}\) coefficients for Cu and Zn were significantly lower (on average by 20-40%, regardless of the plant) compared to the values specified for the control conditions. \(\mathrm{BCF_{T}}\) for amelina ranged from 0.2 (TI, TII) to 0.4 (T0) for Cu and from 0.8 (TI, TII) to 1.1 (T0) for Zn. In this study, for oat as a cereal plant \(\mathrm{BCF_{T}}\) was from 0.06 (TI, TII) to 0.08 (T0) for Cu and from 0.3 (TI, TII) to 0.6 (T0) for Zn. [PERSON] et al. [39] reported for another cereal (wheat) a BCF of 0.23 and 0.20 for Cu and Zn, respectively. Taking into consideration the threshold (above 1) given by the cited authors for BCF, in this study the probability of metal accumulation may be indicated only in the case of Zn in amelina cultivated in the control soil. This may be due to the fact that, in general, amelina was characterized by higher contents of Cu and Zn compared to oat and although the content of Cu was greater in the soil, more Zn was found in the cultivated plants. The immobilization effect of the applied additives was also recorded and confirmed statistically in the case of \(\mathrm{Cu_{DTPA}}\) and \(\mathrm{ZnO_{DTPA}}\) as well as \(\mathrm{BCF_{B}}\) for these metals. Generally, the bioavailable amounts of Cu account for 36-53% of \(\mathrm{Cu_{TOT}}\), wherein the highest percentage share was recorded for the control soil. On the other hand, \(\mathrm{ZnO_{DTPA}}\) in total Zn content accounted for 11-17%and similarly the highest percentage share was found in the control soil. Regardless of the cultivated plant, the application of these amendments caused an increase in the total contents of these metals and a reduction of their bioavailable amounts in the analyzed soils. However, the effect of the applied immobilizing agents was weaker, because, regardless of the cultivated plant, the differences between bioavailable amounts of Cu and Zn found for TI and TII were lower by 12-18% (Cu) and by 12-26% (Zn) in comparison to values determined for T0. A similar range of differences was found for BCFB. The presented study confirmed earlier reports [7; 11; 13; 14; 15; 30; 31] concerning the immobilization effects of compost and fly ash on heavy metals in soil and plants, especially in relation to their bioavailable amounts. As was indicated by [PERSON] et al. [9], a compost addition can increase bioavailability of Cu in soil. In this study, a lack of significant differences between compost and fly ash in their immobilizing activity was confirmed, but a stronger effect was observed for fly ash. However, the differences of pH values were not confirmed statistically; fly ash caused an increase in soil pH, which was expressed as a slightly alkaline soil reaction. According to [PERSON] et al. [3], such pH conditions show low mobility of heavy metals, especially Cu and Zn solubility is distinctly diminished. Apart from alkaline pH, the most important factor controlling the mobility of heavy metals is organic matter because of the presence of huge amounts of humic substances, which could associate with heavy metals, immobilize them, and then mitigate their availability for plants. However, it needs to be remembered that the input of fresh organic matter (for example as compost) can result not only in the formation of tight chelates with humic substances, but also in an increase of the bioavailability of metals due to bonding with organic acids [3]. This can explain the weaker effect of applied compost in this study and the lack of the immobilizing effect for Cu reported by [PERSON] et al. [9].