## 4 Discussion
### Fractioned Plant Concentrations and Extractions  
In the year of study, the described initial differences in extractable soil Cu and Zn nutrients (Figure 1, Tables A6 and A7) did not translate into significant concentration differences in the grain, the cob and the biomass of the rest of the plant (Figure 3c,d). Thus, both extractable nutrients were neither limiting nor toxic. In fact, Zn concentrations (Figure 3c) were included in the acceptable levels of Zn in plant leaf tissues, which are from 10 to 100 mg kg\({}^{-1}\)[50]. The upper Cu threshold in maize tissues was established in 20 mg Cu kg\({}^{-1}\)[51], which is also a concentration higher than ours (Figure 3d). Despite DTPA-Cd soil differences (Figure 1), Cd concentrations were low for all treatments, as they were in maize crops. Some authors consider maize to be a Cd hypercumulator, which means a shoot concentration of 0.01 g Cd in 100 g of plant dry matter [52]. Other authors [51] established in maize leaves a Cd critical interval of 5-20 mg kg\({}^{-1}\). In our case, plant Cd concentrations were lower than 41 \(\mu\)g Cd kg\({}^{-1}\) (Figure 3f); thus, they were far below the EU legislation for animal feed [13] and human food [14]. The opposite trend (DTPA-Fe vs. DTPA-Mn) was not maintained in maize-crop Fe and Mn concentrations. In maize grain, the Fe concentration increased in the SNC treatment, but the Mn concentration also increased in cobs for the SNC (always comparing with MIN, Figure 3c). The competence of both elements for root absorption has been well-known for a long time [53], but it was not observed in our experiment. Iron concentrations in maize plants from 194 (SPC) to 244 (MIN) mg Fe kg\({}^{-1}\) were higher than in the literature records [54]. Moreover, the Mn concentration in plants ranged from 191 (SPC) to 244 (MIN) mg Mn kg\({}^{-1}\)values that would be considered high (from 151-200 mg Mn kg\({}^{-1}\)) or even excessive (>200 Mn kg\({}^{-1}\)) if they were recorded in the ear leaf of maize [55]. Despite maize being considered an Fe-inefficient plant [56], our recorded Fe grain values were very high in all treatments (between 26-34 mg Fe kg\({}^{-1}\)), which doubled the figures (14.65 \(\pm\) 1.85 mg Fe kg\({}^{-1}\)) of other authors [57]. However, we coincided with [57] in Zn grain concentrations (21-34 mg Zn kg\({}^{-1}\) in our experiment, and 27.04 \(\pm\) 2.76 mg Zn kg\({}^{-1}\) in theirs). In grain, the SNC treatment also presented the highest values of Cr (1.54 \(\mu\)g kg\({}^{-1}\)) with respect to MIN (0.51 \(\mu\)g kg\({}^{-1}\)) and SPC (0.32 \(\mu\)g kg\({}^{-1}\)) treatments. According to [58], an average concentration of 0.6 \(\mu\)g Cr kg\({}^{-1}\) was found in corn (yellow) for human consumption in Greece. The obtained Cr concentrations did not imply a significantly higher Cr grain extraction in SNC vs. MIN (Figure 4c). Nevertheless, the highest concentration, similar to the one that can be found in different silages (from 1.0 to 2.2 \(\mu\)g Cr kg\({}^{-1}\); [59]), is not limiting for animal feeding. Conversely, higher chromium intake (in form of trivalent chromium from maize grain) might positively affect biochemical parameters related to the carbohydrate and lipid metabolism in ruminants [60]. In humans, it has been argued that Cr contributes to normal macronutrient metabolism and the maintenance of normal blood glucose concentrations [61]. Fertilization with SSs and the related Cr concentration in grain is therefore an aspect that might be interesting to evaluate in the development of functional foods. However, the physiology of Cr accumulation in maize grain, when the SS highest rate is annually maintained for a long period, requires further research. The average Co content in grain (_c._ 1.37 ug kg\({}^{-1}\), Figure 3e) is lower than the concentration in a by-product of the manufacturing of maize starch (and sometimes ethanol) called maize gluten meal. This product is used for animal feed and usually has the highest concentration of Co (2 mg Co kg\({}^{-1}\)) of maize subproducts [62].