Soil physicochemical characteristics are presented in Table 1.
Each of the analyzed parameters in the initial soil samples was slightly lower for S. pectinata plots when compared with M. x giganteus plots. Heterogenic distribution of elements could be caused by irregular dust fall emitted by the smelter and unequal distribution of agricultural treatments in the past. No statistically significant differences were found for the analyzed soil parameters among S. pectinata plots, with the exception of CORG for plot S-C. Contrary to CORG, Mg content was different for each of the M. x giganteus plots. It was found that Mg concentration at the M-NPK plot was higher by 15% although the highest value was measured at 81% for the M-INC plot in comparison to the control. It can be assumed that among species, soil parameters were highly homogenous.
Heavy metals and mineral macronutrients plant concentrations (spider charts and PCA)
Spider charts (Fig. 3a, b) are an easy way to visualize patterns in feature characteristics of plant organisms. The presented spider charts can be divided in to three sections: heavy metal (Zn, Cd, Pb) accumulation, primary mineral macronutrients (N, K, P), and secondary mineral macronutrients (Mg, Ca) with water content. For both cultivated species, overall decrease in all parameters (except Pb) was visible after the second growing season, in comparison to the first one.
Principal component analysis (Fig. 4a, b) shows the multivariate relationships between the plants’ ability to accumulate HMs, primary mineral macronutrients, secondary mineral macronutrients, and, additionally, plant water content. The analyzed cases (Fig. 4a) were divided by PC1 and PC2 axis into four groups, which corresponded to the species and the growing seasons (M. x giganteus growing season 2014, M. x giganteus growing season 2015, S. pectinata growing season 2014, S. pectinata growing season 2015). The distance between the clusters indicates the differences in the analyzed parameters within the groups. The distance between M. x giganteus and S. pectinata after the first growing season is larger than after the second. The differences in the distances within the tested species after the first and second growing season are similar. PCA showed that the treatments are not differentiated into subgroups within the growing season and the tested plant species. Variables (Fig. 4b) can be divided into three groups of parameters: primary macronutrients with water content (1), HMs (except Pb) with secondary mineral macronutrients (2), and Pb (3). The first and second groups are mainly conditioned by PC1, whereas the third group (Pb) is mainly conditioned by PC2. However, Mg and Cd are conditioned by PC1, demonstrating that those parameters have also a high affinity to PC2. Due to distribution of variables, it was found that the increase of Pb is the main conditioning factor of the second growing season with a simultaneously reduced amount of other elements in the aboveground plant organs, which corresponds to the results visible on the spider charts.
Among M. x giganteus samples supplemented by fertilizers in the growing season 2014, the highest HMs accumulation was observed for plants treated with NPK while the lowest accumulation ability was in M. x giganteus untreated and treated with an inoculum. For S. pectinata in the growing season 2014, the highest heavy metals (mostly Zn) accumulation was observed for plants treated with the inoculum; however, in the case of NPK treated and control plants, the same or lower values were observed compared to the inoculated plants. In M. x giganteus samples collected at the end of the first growing season, the highest primary mineral macronutrients content was observed for plants treated with the inoculum, whereas significantly lower values (except N in control plants, which was statistically the same) were observed for untreated and NPK-treated plants. The lowest results for N and K were found in plants treated with NPK fertilizer. Among S. pectinata plants in the growing season 2014, the highest accumulation of primary macronutrients was observed for plants treated with NPK fertilizer. Moreover, equally lower values of these elements were found for S. pectinata control plants and those treated with microbial inoculum in comparison with an NPK treatment.
For M. x giganteus plants in the growing season 2014, Mg accumulation was the highest for all plants treated with NPK and microbial inoculum, with statistical significant difference to the control. Accumulation of Ca in the growing season 2014 was the same for M. x giganteus treated with an inoculum and the control. Moreover, NPK treatment caused a lower Ca accumulation when compared to control. Accumulation of Mg among S. pectinata plants was the same for each experimental option. Different observations were found for Ca, where S. pectinata treated with NPK and with microbial inoculum had slightly lower Ca plant concentration in comparison to the control. Water content after the first growing season was higher for M. x giganteus when compared to S. pectinata; however, in both cases, treatments had no significant influence on this parameter.
After the second growing season, trends for HMs and primary mineral macronutrients were similar to those observed in the first one. Among secondary macronutrients accumulation, Mg and Ca showed different trends in the growing season of 2015 when compared to the growing season of 2014. M. x giganteus plants cultivated with NPK and inoculum fertilizers showed higher Mg concentration in the aboveground organs when compared to the control. Among the treatments, the highest Mg concentrations were observed for inoculated M. x giganteus plants. In the growing season 2015, the highest Ca concentration was observed for NPK-treated M. x giganteus; however, lower and statistically equal values were reported for plants treated with the inoculum and the control plants. Among S. pectinata plants, tendencies in accumulation of HMs and primary macronutrients during the growing season of 2015 were the same when compared to the growing season of 2014. Element contents were the same independent of the treatment, with the only exception of Pb, which was significantly lower in comparison to the control. Concerning the secondary mineral macronutrients, accumulation in the second growing season showed that treatments did not affect Ca and Mg accumulation. After the growing season 2015, water content showed the same trends as in the growing season 2014.
This section corresponds to an overall pattern trend of the three groups: HMs, primary macronutrients, secondary macronutrients and water content. Statistical analysis of values used for spider chart construction, specific for the described parameters is presented in Table 2. In addition to the presented analysis, three-way ANOVA was performed to detect which factors, i.e., plant, year, fertilization, drove changes in the analyzed parameters (Table 3). Undoubtedly, factors corresponding to the plant species and the year of cultivation caused meaningful differences among all the investigated parameters. Fertilization affected significantly only Mg and Zn concentration in the investigated plant biomass. While considering a combined effect of the factors, i.e., plant × year × fertilization, the significant changes appeared among N, Pb, and Zn concentrations in the plant biomass.
Plant biomass production
After the growing season 2014, higher biomass yield was found for S. pectinata in comparison to M. x giganteus (by about 126%). Moreover, the treatments did not influence the yield. A visible effect was observed for treatments in M. x giganteus as well as in S. pectinata plants after the growing season 2015. It inferred that the inoculum had a different impact on both species. M. x giganteus biomass yield was increased by 14%, while S. pectinata biomass decreased by 26% when compared to the control. NPK treatment did not affect S. pectinata biomass production; however, it increased biomass yield by 18% in NPK-treated M. x giganteus (Fig. 5).
Plants visual observation
Plant photography is presented in Fig. S1. Beside visual differences in the height and stem density of individual plants for both growing seasons, there is a visible difference in the surface of the withered zones observed for M. x giganteus plants as well as for S. pectinata, which is larger at the end of the second growing season (beginning in September). Moreover, the withering effect of both species was not determined by the application of fertilizers. The results presented in the “Heavy metals and mineral macronutrients plant concentrations (spider charts and PCA)” section together with the photographic documentation corresponding to this section indicate hasted senescence in the 2015 growing season, when compared to 2014. Plant senescence occurred earlier due to drought conditions during the 2015 growing season.