4.1. Limitations and implications
The mean values of single-element indices for conventional and organic agricultural management practice Igeo and CF were presented in Fig. 5, Fig. 6 and Table S7 (Supplementary Material). The Igeo values for conventional agricultural practice were in following descending order: Cr > Cu > Co > Cd > As > Zn > Ni > Pb. Chromium and Cu had higher Igeo values indicating that soils were moderately contaminated (1 < Igeo≤ 2, degree 2). Moreover, for other PTEs soil samples were recorded as uncontaminated (0 < Igeo≤ 1, degree 1). Descending order of Igeo values for organic agricultural management practice were Cr > Co > Cu > Ni > As > Cd > Zn > Pb. Igeo indices for organic agricultural management practice were classified as degree 1 (Zn, Ni, Cd, As and Pb), degree 2 (Co and Cu), and degree 3 (Cr) which indicated heavy contamination for Cr according to the Igeo value. Based on CF values contamination by human activities (CF > 1) for major of investigated PTEs was found with exception of Pb in both agricultural practice. On the other hand, in conventional agricultural management practice As, Cd, Cu and Ni were classified as moderate contamination (CF of 3–6), whereas Cr showed very high contamination (CF > 6). For organic agricultural management practice, only values of CF obtained for Pb indicated low contamination, while CF for Co and Cr showed considerable contamination.
The individual Igeo values for Cr in all analyzed samples were ranged from 0.368 (sample no. 11) to 5.10 (sample no. 20), and 12.5%, 37.5%, 33.33%, 12.5% and 4.17% of all investigated samples were classified as uncontaminated to moderately, moderately, moderately to heavily, heavily and extremely polluted, respectively. When Igeo values of Cd were taken into consideration obtained range was from 0.175 (sample no. 10) to 2.34 (sample no.5) and 83.34%, 8.33% and 8.33% of sampling location were classified as uncontaminated to moderately, moderately and moderately to heavily polluted. According to the Igeo values for other investigated PTEs all studied location were classified as unpolluted to moderately polluted, Fig. 5. The individual values of CF for conventional and organic agricultural management practice ranged from 0.142 (Pb-sample no.10) to 11.41 (Cr-sample no.1) and from 0.282 (Pb-sample no.17) to 25.43 (Cr-sample no.20), respectively (Fig. 6).
According to the CF values for Pb, only location no.12 (2.84) was moderately polluted while all other locations were classified as low contaminated with CF values being lower than 0.463. Based on the CF values, Cr could cause very high contamination for organic agricultural practice, while 30.77% and 69.23% of convention soil locations were considerable and very high contaminated, respectively. On the other hand, calculated CF values of As, for conventional management practice indicated that 7.70%, 76.92% and 15.38% were low, moderate and considerable polluted, while 18.18%, 72.72% and 9.10% of studied fields for organic agricultural practice were moderate, considerable and very high polluted, respectively. Furthermore, CF values of Cu were ranged between 2.34 (sample no. 2) to 14.02 (sample no.14), and 15.38%, 61.54% and 23.08% of locations for conventional production, and 9.10%, 72.72% and 18.18% of organic locations were moderately, considerable and very high contaminated, respectively.
Relating calculated CF values for Cd and Ni, it was indicated that 30.78% of locations for conventional agricultural practice and no one for organic management practice were low polluted. Additionally, only one of the all studied locations based on CF values for Cd was very high contaminated (sample no. 4), while majority of the studied locations were moderately contaminated. However, 36.36% locations for organic production based on CF values for Ni was very high polluted with no one intended for conventional production. Moreover, CF values of Co, ranged between 1.03–6.82, indicated that only 15.38% of locations for conventional production were moderately while the rest were considerable contaminated. In the case of organic locations, the CFs were between 5.33–6.87, with 45.45% and 54.55% of studied fields being considerable and very high polluted, respectively. The average PLI value for organic agricultural management practice was found to be 3.12, while for conventional agricultural management practice was 3.05. Since the average PLI was above unity for all studied samples, deterioration of site quality for investigated region is noticed based on applied background values of the studied PTEs in Serbian arable soils (Ubavić et al., 1993) used in the risk estimation. Moreover, if the integrated risk that takes into account the influence of the number of studied PTEs has calculated, the mean value of the NIPI was 8.14 indicating that the majority of the sites are strongly polluted. Furthermore, the mean value for NIRI in the studied area was 91.2 indicating considerable risk. However, based on the calculated NIPI and NIRI values only conventional location no.10 with NIRI of 20.68 and NIPI of 2.44, indicated low risk, while the other conventional locations were under considerable (61.54%) and high (30.77%) risk, respectively. Moreover, taking into consideration, locations used for organic agricultural practice, based on NIRI values 45.45%, 45.45% and 9.10% were under considerable, high and extreme risk, respectively, while based on calculated values of NIPI all locations were strongly polluted. Namely, as it is mentioned earlier integrated risk is significantly influenced by the number of calculated potential toxic elements and toxic response factors being the highest for As and Cd. Thus, the values of the NIRI in this study were largely influenced by concentrations of As and Cd.
On the other hand, based on the comprehensive survey of PTEs concentration ranges, mean, median, geochemical values, maximums allowed concentrations of PTEs set by European regulatory bodies for European agricultural soils (Reimann et al., 2018) and the determined levels of PTEs in agricultural soils from the northern part of Serbia and southern part of Hungary belonging to the Pannonian Basin (Table 1) it might be concluded that evaluation of risk by using the indices such as Igeo, CF, PLI, NIPI and NIRI with the accepted background values from Ubavić et al. (1993) might not give the real picture of the state of the agricultural soils. Namely, application of the estimated indices may overestimate the risk what is confirmed by the finding in this study, particularly considering that mean values determined for investigated soil samples in this study were several times lower for the most of PTEs than the geochemical (non-toxicological) threshold values determined for southern and whole European agricultural soils (Reimann et al., 2018).
Additionally, the geochemical threshold values published for European agricultural soils are several times higher (being approximately for (Cr) 111, (Cu) 40, (Co) 27, (Ni) 25, (Pb) 22, (As) 20, and (Cd and Zn) 3 times higher) than the background values determined for Serbian agricultural soils (Ubavić et al., 1993). However, in European scale, the variation in the natural background concentrations of all investigated elements in agricultural soil samples is much larger than any anthropogenic impact (Reimann et al. 2018). Thus, it can explain the fact that direct comparisons of PTEs concentrations with accepted background values provides one side view relating elements levels control, and might fails to adequately interpret the level of risk of those PTEs can pose (Fei et al. 2022). Anyhow, the indices also should be taken with precaution in the case of agricultural soil PTEs risk estimation. Namely, the proper choice of PTEs background concentrations is crucial, particularly taking into consideration the definition of “ambient background” given in ECHA, 2008 (Reimann et al., 2018) as “the sum of the natural background of an element and diffuse anthropogenic input in the past or present, without the massive influence of point sources” which indicates that the chosen background values depend also of time the soil samples were taken. Moreover, if taken into account, the proposed indication limits of food chain contamination for standard texture soils for selected elements (Cd, Ni and Pb) at pH higher than 6.5 (Vacha et al., 2014), as was the case for the investigated arable soils, it could be concluded that in this study the determined levels of Cd, Ni, and Pb, were 1.29 times for Cd, 15.2 times for Ni and 22.90 times for Pb lower than the proposed indication limits of the ones. Moreover, if the limit values of phytotoxic risk elements (Ni, Cu and Zn) (Vacha et al., 2014), which indicate the plant growth inhibition that results in significant yield reduction, are taken, it was revealed that the mean concentration levels determined in this study were lower 15.2 times for Ni, 6.51 times for Cu and 6.28 times for Zn than the set phytotoxic risk limits. Thus, generally, considering the proposed data for indication limits of food chain contamination and phytotoxic risk it could be concluded that there are no any risk relating food chain contamination and plant growth inhibition with studied elements. These, additionally, support the fact that the application of indices might not give the real picture of the state of the agricultural soils and can overestimate potential risks (Reimann et al., 2018; Škrbić et., 2010; Sakan et al., 2015). Therefore, the use of proper geochemical trash hold values is crucial and should be set by regulatory bodies of each region based on definition given in (Reimann et al., 2005.) to adequately manage the ecological and health risks for sustainable production (Fei et al. 2022).