Nickel
-
MSWC provides the highest Ni content for the 1998 to 2007 period for clone A7677 (20 mg kg
−1DW) and in the varieties, Agria and Monalisa and Jaerla, decreasing and then remaining constant for the 2007 to 2016 period (4–9 mg kg
−1DW). In contrast, for 2010 to 2016, SMC, ChMC and CMC provide similar Ni contents in the varieties Agria, Monalisa and Jaerla and with the clone A7677. The highest Ni concentration exceeds normal values (0.02 to 5 mg kg
−1DW), going into phytotoxic levels (10–100 mg kg
−1DW)
[27]. The reported results agree with those by Mahmood and Malik
[48] but are larger than those by Khan et al., (6.84, 7.93 and 8.71mg kg
−1DW)
[26] in potato crops exposed to irrigation by Ni-contaminated wastewater. Ni is considered an essential micronutrient for the growth and development of plants with several metabolic roles
[49]. The maximum Ni contents (2007, into the phytotoxic range) were obtained for MSWC treatment in the clone A7677 and in the varieties Agria, Monalisa and Jaerla but they did not affect yields. However, in 2007, it is noted that only yields of the varieties Agria and Monalisa (40 t ha
−1) with contents of Ni in tubers of 13 mg kg
−1DW declined, compared with the variety Jaerla (54 t ha
−1) and the clone A7677 (76 t ha
−1) whose yields, on the contrary, increased, despite Ni content of 20 and 8 mg kg
−1DW, respectively. These results indicate that Ni has stimulated the growth and development of foliage, which agrees with the positive responses of plant growth in the presence of Ni
[33][50]. Nevertheless, the possible Ni toxicity should be considered, with a negative impact on photosynthesis, on membranes permeability, and on the decrease in the micronutrient’s absorption
[51].
4. Bioavailability of Micronutrients and Heavy Metals
A dynamic equilibrium between metal fractions determines the mobility and bioavailability (more than the total content of metals). The pH, the redox potential, and the quantity and types of Organic Matter (OM) and clays are the most important edaphic factors in their control
[52]. The availability of Fe, Mn, Zn, and Cu would be scarce or very restricted in the crops because the considered soil is slightly alkaline (pH 7.4) and would explain the low Fe, Zn and Cu concentrations in the control samples. Nevertheless, the application of treatments, especially MSWC, leads to a considerable increase in Fe (56 to 118 mg kg
−1DW), Mn (9 to 27 mg kg
−1DW), Zn (28 to 34 mg kg
−1DW) and Cu (14 to 17 mg kg
−1DW). Therefore, adequate amounts of compost applied consecutively to the soils would act very favorably for Fe, Mn, Zn and Cu to be in the rhizosphere, and thus being usable by plants. In addition, soil pH also controls the processes of sorption/desorption and chemical speciation in the soils of Cr and other heavy metals such as Pb and Ni
[42].
Consecutive addition of treatments to the soil, especially MSWC, increased Pb (3 to 6 mg kg
−1DW), Cr (15 to 20 mg kg
−1DW) and Ni (12 to 20 mg kg
−1DW) for all varieties, especially clone A7677. These results can be justified considering that the addition of organic matter could specifically affect the solubility and bioaccumulation of metals, generally causing variations in pH and ionic composition of the soil. Additional studies have also proposed that it may be also an indirect consequence of the microbiological activity
[42].
Nonetheless, it should be emphasized that plants have an extraordinary capacity to absorb heavy metals depending on the species, shape, concentration and bioavailability of the metals in the soil, as well as on the composition of the OM and the microbiological activity
[53]. The mechanism of accumulation of Pb, Cr and Ni still have not been elucidated for
S. tuberosum, but involvement of membrane transporters involved in the absorption of Ca, Cd, Mn, Fe, Zn and Cu has been proposed
[42].
The reported results show that the compost has increased the concentration of macro, micronutrients and heavy metals in the tubers of all the varieties, due to the greater availability of assimilable forms of these. The reported high Cr and Ni concentrations (in the range of phytotoxicity) do not lead to lower yields in potato production between 2004 and 2007, which is probably due to positive interactions between the absorption of minerals
[33][43][50]. Moreover, results reported indicate that the concentrations of heavy metals (Cr, Pb and Ni) are not lethal for the plants, with no visible phytotoxic effects, especially during the first stage with the application of the MSWC (1998 to 2007).The lack of visual perception of phytotoxicity in the studied crops, with high contents of heavy metals (Cr and Ni) in their different organs, can provoke an increase in agri-environmental vulnerability with a potential risk in the food chain, due to the possibility of being bioavailable to humans and animals through their consumption
[54].
The decrease in yield from 2007–2010, mainly with MSWC treatment, could be due to the cumulative effect not only of heavy metals but also of micronutrients. The use of organic amendments that involve an application of microelements above the requirements could generate an accumulation of both micronutrients and heavy metals that over time can be toxic to plants
[55]. This could be from 2010, where a decrease in plant height, reduction in foliage growth, discoloration of leaves, necrosis, epinasty of young leaves was notorious, little or no flowering and smaller tuber size, which produced a lower yield in potato production (2010 to 2016) with the application of the MSWC compared to the other treatments. This reduction in the yields reached up to 83% in the variety Agria, 81% in the clone A7677, 80% in Jaerla and 78% in the variety Monalisa.