Nickel concentration in the soil
Ni concentration in the soil after plant harvest remained at a similar level in both growing seasons (Table 2). The concentration of Ni in soil at the end of both growing seasons was slightly higher than doped probably due to the doping process and soil homogenization; however, differences in relation to the expected values were low. The average concentration of this metal in Ni1 ranged from 49.9 to 58.2 mg kg−1, in Ni2—from 89.5 to 111 mg kg−1, while in Ni3 treatment—from 182 to 186 mg kg−1, depending on the plant species. These concentrations corresponded to, respectively, weak, medium and heavy contamination level according to the limits of soil contamination with heavy metals by Kabata-Pendias et al. (1993). This assessment takes into account soil features such as soil fraction < 0.02 mm and soil pH (Table 3). According to the Polish standards (Regulation of the Minister of the Environment 2016), the total Ni concentration in the soil on agricultural areas should not exceed 100 mg kg−1, which coincides with the upper limit of the average soil contamination acc. to Kabata-Pendias et al. (1993). In the present study, the Ni concentration exceeded the allowable limit only in the treatment Ni3.
Ni phytotoxicity, manifested by the reduction in biomass yields, varied depending on the plant species and the level of soil contamination Ni1–Ni3 (Table 4). Plants responded to the Ni reduction of both the biomass of the aboveground parts and roots.
The biomass of the aboveground parts of each tested species systematically decreased relative to the control, together with increasing Ni in the soil. It was noticed in both growing seasons (Table 4, Fig. 1). However, the differences between the seasons in plant responses to Ni should be considered differently for an annual plant Z. mays than for perennials, P. arundinacea and S. viminalis. The biggest reduction in the aboveground parts was recorded for Z. mays. The biomass of this plant in the first growing season decreased significantly, by 19% in N2, and by 71% in N3. It was similar in the second season, where the reduction was 13 and 83%, respectively.
The biomass of the aboveground parts of the perennial grass P. arundinacea decreased only in Ni3, whereas in the second growing season, this reduction was significantly smaller (28%) in comparison with the first season (56%). The authors believe that this may be related to the age and length of roots. It can be assumed that in the second season, the roots of grass reached the deeper, into the uncontaminated soil layers. Microplots used in our experiment were 1 m deep, while Ni was introduced to a depth of 30 cm only. Korzeniowska and Stanisławska-Glubiak (2015) recorded a similar phenomenon in another grass (Spartina pectinata); there was a smaller yield decline under the influence of Ni in the second year than in the first year of growth.
S. viminalis exhibited a much lower sensitivity to high concentrations of Ni in the soil than P. arundinacea and Z. mays. A significant reduction in the aboveground parts in Ni3 treatment for S. viminalis did not exceed 35% in both growing seasons, while for other two plants was much higher, especially in the first season.
Soil contamination with Ni resulted not only in the reduction in the aboveground parts, but also the reduction in the growth of roots, with the exception of S. viminalis (Table 4, Fig. 2). Particularly large reductions were recorded for Z. mays. In the first growing season, the root biomass of this plant decreased significantly by 21% in N2 and by 77% in N3 treatment, while in the second season, by 64% in N3 in relation to the control. The impact of Ni on the restriction of the growth of the Z. mays roots was confirmed by other authors who, under hydroponic conditions, found that Ni accumulated in the roots restricted their branching (Seregin et al. 2003) and a length (Maksimovic et al. 2007).
In the species P. arundinacea, soil contamination with Ni caused a reduction in the biomass of roots only at the level of Ni3. In both seasons, growth reduction was, respectively, 38 and 28% compared to the control. It should be noted that in this perennial grass, in the first season, the reduction in biomass yields caused by an excessive Ni in the soil, was bigger for the aboveground parts (the decrease by 58%) than for the roots (38%), while in the second season, it was similar for the two parts (28% each).
Tolerance indices, calculated for both aboveground parts and roots of the tested plants, indicate that Z. mays is the most sensitive species to excessive Ni in the soil, while S. viminalis is the most tolerant one (Table 5).
There are no studies in field condition that would confirm our results of high tolerance of S. vinimalis and low tolerance of Z. mays for Ni. The field studies with S. viminalis were conducted by Ali et al. (2003), Algreen et al. (2014), and Kacalkova et al. (2014), but only on soils contaminated with several heavy metals jointly, so it was not possible to assess the impact of the Ni exclusively on the formation of willow biomass.
It is possible to find some information about tolerance of S. viminalis and Z. mays to Ni exclusively, but obtained on the basis of pot or hydroponic experiments. Torres et al. (2016), in the pot experiment, observed a 40% biomass reduction in 50-day shoots of Z. mays at a dose of 10 mg dm−3 Ni. A significant tolerance of S. viminalis to Ni in hydroponic conditions was demonstrated by Drzewiecka et al. (2012). According to these authors, this plant can be grown on soils heavily contaminated with Ni. It should be noted, however, that the bioavailability of metals from nutrient solutions or pots is much higher than that of field soils, and such studies cannot be used to assess the actual suitability of plants for phytoremediation.
However, hydroponic and pot studies allow to compare the tolerance of several species between themselves to Ni. A high sensitivity of Z. mays to Ni found in our studies has been confirmed by Antonkiewicz et al. (2016). These authors found that the hydroponically grown Z. mays responded with a bigger reduction of the aboveground parts to the increase in Ni concentration in the culture medium (80% compared to control) than beans (60%) or lettuce (23%).
Nickel decreased the net photosynthesis rate of all the tested plant species (Table 6). A significant decrease occurred with Z. mays in Ni2 and Ni3 and with P. arundinacea and S. viminalis in N3 treatment.
These results are consistent with the above-mentioned biomass decrease in the aboveground parts of the tested species. The inhibition of photosynthesis rate was the biggest with Z. mays, while the smallest with S. viminalis. Physiological studies confirm that Ni damages the photosynthetic apparatus at almost every level of its organization (Chen et al. 2009). Shafeeq et al. (2012), in pot experiments, recorded a systematic decrease photosynthetic rate of wheat together with an increasing dose of Ni from 50 to 300 mg kg−1.
Ni concentration in plants
Ni concentration in organs of Z. mays was in the following order: ears < steams < roots (Table 7). Ears contained 1.7–5.4 mg kg−1, while stems 1.1–10.7 mg kg−1 Ni, depending on the level of contamination of the soil and the growing season. These concentrations do not exceed the maximal tolerable dietary level of this metal for beef cattle and dairy cattle, which is 50 mg kg−1 dm. (NRC 1996; NCR 2001). The roots contained a dozen or even several dozen times more Ni than the aerial parts. At the highest level of contamination of Ni3, the roots contained, respectively, 16 and 12 times more Ni than the stems, and 44 and 24 times more Ni than the ears, respectively, for the first and second season.
Ni concentration in the aboveground parts of P. arundinacea was higher than in Z. mays, amounting to 10.1–31.6 mg kg−1, depending on the level of contamination and a season, but it never exceeded the maximal tolerable dietary level. In their roots, the plants accumulated only a few times more Ni than in aboveground parts. The roots contained about 4–5 times more Ni than the aboveground parts, regardless of the level of contamination of Ni1–Ni3 or the growing season.
Ni concentration in the organs of S. viminalis was in the following order: twigs < leaves < roots. Twigs contained 2.1–6.2 mg kg−1, while leaves 3.5–10.4 mg kg−1 Ni, depending on the level of Ni and a season. The roots had a several times higher Ni concentration than the leaves or twigs. At the level of Ni3, it was 7.5 and 2.5 times higher than in leaves, and 7.5 and 6.5 times higher compared to the twigs, depending on the growing season.
In our study, Z. mays showed the highest ability to accumulate Ni in the roots among the plants tested. These results are confirmed by Antonkiewicz et al. (2016), who found that hydroponically growing Z. mays accumulated 30 times more Ni in the roots than in the stems at the concentration of 10 mg dcm−3 Ni in nutrient solution. There is a lack of reports on the response of P. arundinacea to Ni. Only Vymazal et al. (2011) found a higher Ni concentration in the roots than in the aboveground parts of the grass growing in a constructed wetland treated with municipal sewage. Other authors also observed a higher Ni concentration in the roots in comparison with the aboveground parts of other plant species (Ahmad et al. 2007; Al Chami et al. 2015; Antonkiewicz et al. 2016).
Similarly as in our research on S. viminalis, Drzewiecka et al. (2012), under hydroponic conditions, observed the smallest Ni concentration in the twigs, higher in leaves, while the highest in the roots. In the study of Kacalkova et al. (2014), willow growing on soils naturally contaminated with several metals jointly, accumulated more Ni in the leaves than in the twigs. Moreover, willow leaves contained more Ni compared with the leaves of maize, sunflower, or poplar. A significant accumulation of Ni in the leaves of S. viminalis was also emphasized by Mleczek et al. (2009), even though the concentration of this metal in the roots was higher.
Ni accumulation and translocation
The tested plants showed a various ability to accumulate Ni, as measured by bioaccumulation factor (BF) (Table 8). S. viminalis accumulated small amounts of Ni, both in the aboveground parts and in the roots (mean BFtwigs and BFleaves ≤ 0.09, BFroots ≤ 0.19). Among the tested plants, P. arundinacea showed the highest ability to bioaccumulate Ni by the aboveground parts and relatively high by roots (mean BFshoots = 0.17–0.18, BFroots = 0.67–0.81). Z. mays accumulated the least Ni in the aboveground parts, and at the same time, the most Ni in the roots (mean BFsteams and BFears ≤ 0.06, BFroots = 0.7–1.0).
Other authors confirm the low bioaccumulation of Ni by the aboveground parts of maize.
Fargasova (2012) and Antonkiewicz et al. (2016) showed a lower BF value for Z. mays than for Vicia
sativa, Raphanus sativus, Synapsis alba, lettuce and field bean.
In our study, all the plants accumulated more Ni in the roots than in the aboveground parts (BFroots > BFaboveground parts). At the same time, all BFaboveground parts were much lower than 1.0, indicating a total lack of usefulness of the tested plants for phytoextraction (McGrath and Zhao 2003). At the same time, the value of BFroots close to 1.0 for Z. mays suggests its suitability for phytostabilization (Cheraghi et al. 2011).
Ni transport from the roots to the aboveground parts of all three species tested was limited, as evidenced by the values of TFs < 1 (Table 9). However, TFs values for Z. mays were significantly lower than for P. arundinacea and S. viminalis, oscillating from 0.02 to 0.08. It means that plants transferred only 2–8% Ni from the roots to the aboveground parts. Also, in the studies of Fargasova (2012) and Antonkiewicz et al. (2016), the values of TFs for Z. mays were lower than for other species investigated plants. According to Seregin et al. (2003), Z. mays belongs to excluder plants, as its roots constitute a barrier limiting Ni transport to the shoots.
P. arundinacea and S. viminalis showed higher TF values than Z. mays. The mean TFshoots for P. arundinacea were 0.20–0.28, while TFtwigs and TFleaves for S. viminalis were 0.18–0.20 and 0.34–0.52, respectively, depending on the growing season.