*3.2. Metal Concentrations in Soil, Wheat, DGT, CE, CSOL, CaCl2 Extractable Metals*

Total metal (Cd, Cu, Mn, Ni and Zn) concentrations measured in the control soil were lower than in the roadside soil (Table 2). The highest Zn and Cd concentrations (45 mg/kg and 0.2 mg/kg) were recorded adjacent to old roads and the highest Mn concentration (599 mg/kg) was recorded next to a new road. The highest Cu concentration (12 mg/kg) and highest Ni concentration (21 mg/kg) were recorded from medium-aged roads [20].

**Table 2.** Metal concentrations (Cd, Cu, Mn, Ni and Zn) in soil, wheat, DGT, CE, CSOL, CaCl2 extractable metals for soil and soil after the growth of wheat.



**Table 2.** *Cont.*

DGT = Diffusive gradients in thin film, CE = effective concentration, CSOL = pore water metal concentrations, CaCl2 extractable = available metals, R = ratio of CDGT and CSOL, C = soil not exposed to vehicular emissions (control), N = new aged roads (2–5 years), M = medium aged roads (5–10 years) and O = old aged roads (≥15 years). Values are means (*n* = 3), ± standard error, a, b, c, denotes the significance between the sites at *p* < 0.05.

Metal concentrations in wheat shoots varied across the sites. Significantly higher wheat shoot concentrations were measured for Mn (*p* = 0.041), Ni (*p* = 0.024) and Zn (*p* = 0.035) for roadside soils compared to the control sites (Table 2). Cadmium showed a significant accumulation in wheat shoots grown in medium-aged roadside soil compared to control site grown wheat. Copper accumulation did not vary significantly in wheat shoots at any site. Some roadside soil samples showed a significant difference in CaCl2 extracted metals, soil solution metals (CSOL) and effective concentrations (CE) among roadside and control soils (Table 2).

Both the DGT technique and CaCl2 extractable metals were not statistically correlated to metals in wheat shoots for plants grown in roadside soil (Table 3), except for Cu, where a significant negative correlation r = −0.562 was found for CaCl2 extractable Cu. In general, the factors best correlated with shoot metal concentrations were total metal concentrations in soil (Ni: r = 0.750, *p* = 0.005; Zn: r = 0.833, *p* = 0.001), CSOL concentrations (Cu: r = −0.528, *p* = 0.009; Mn: r = −0.544, *p* = 0.005; Ni: r = 0.465, *p* = 0.025; Zn: r = 0.485, *p* = 0.016) and soil pH (Ni: r = 0.632, *p* = 0.001; Zn: r = 0.589, *p* = 0.004).


**Table 3.** Pearson correlation coefficients (r) between metal concentrations in wheat tissues, soil physiochemical properties and extractable metals measured by different methods in soil. Statistically significant values (*p* < 0.05) are in bold font.


**Table 3.** *Cont.*

\*\*\* Correlation is significant at the 0.001 level, \*\* Correlation is significant at the 0.01 level, \* Correlation is significant at the 0.05 level, Wheat = metal concentrations in wheat shoots, Soil = total metal concentration in soil, R = CDGT/CSOL.

Soil pH also correlated with several other soil metal measurements, in particular with R, CSOL and Tc (Table 3). The dissociation time for Cd (r = 0.630, *p* = 0.028), Ni (r = 0.661, *p* = 0.027) and Zn (r = 0.807, *p* = 0.002) in soil was significantly correlated with soil pH (Table 3), while Tc showed a strong correlation with soil TOC for Mn (r = 0.701).
