Metals such as Cu, Cd, Cr, Pb, As, and Zn are all potential soil pollutants, and a wide range of these elements enters the environment through many sources, including industrial processes, mining, and irrigation (Liu et al. 2007). Tested soils, containing various ranges of metals, collected from nearby groundwater well collection sites. Leaching activities from these sites may have significance effects on groundwater quality and subsurface environments. Total metal contents of tested soils, excluding arsenic, were in the following order: Zn, Cr > Pb, Cu ≫ Cd. Arsenic content was in the range of 1–627 mg/kg soil and was highly variable depending on the site (Table 1). These metal concentrations were often higher than maximum permissible contents based on two standards in Korea (Jung 2001). As generally known by other investigators, cadmium displayed the lowest content.
Metal contents by weak acid (0.1 N HCl) extraction were in the range of 0.3–6.3 % of total contents (average 1.4 %), depending on the site. A low correlation (r2 = 0.186) between total and acid-extracted contents was observed. High percentage of Cd was found in the acid extractants, showing an average of 7.9 %. However, total concentrations of Cd in these soils were very low (~1.0 mg/kg soil), which means the corresponding absolute Cd concentrations in acid extractants were also very low (average 0.87 mg/kg soil). Percentage of acid extractants may vary according to soil characteristics, such as pH, organic contents, redox potential, as well as metal properties. Sample #F, which showed the highest percentage (6.3 %) of acid extraction, contained high contents of organics and DOC (5.31 % and 15.87 mg/L, respectively), whereas sample #E showing the lowest percentage (0.3 %) of acid extraction contained a low content of organics and DOC (1.09 % and 5.08 mg/L, respectively) compared to other samples. Correlation of acid extractant contents (% of total content) with organics (%), organic-C (%), and DOC showed r2 values of 0.3972, 0.3971, and 0.4866, respectively, whereas there were no observable correlations with soil properties such as pH and CEC, showing r2 values of 0.2084 and 0.0021, respectively. These results may be attributed to extraction of organic complex fraction metals by 0.1 N HCl. The distribution of fractions is an important parameter for determination of metal availability in soils. The organic fraction released in the oxidizable fraction is not considered to be very mobile or available since it is associated with stable high molecular weight humic substances, which slowly release small amounts of metals (Ure and Davidson 2001). Many research studies have also suggested that soil reactions, organic matter contents, and composition of the finest fraction may influence the mobility of metals in the environment (Venditti et al. 2000; Agnieszka et al. 2014). In this study, metal contents of water extractants, which close to exchangeable fraction in nature, were measured prior to the bioassays. Total metal contents of water extractants were very low, sometimes lower than the instrument detection limit, and less than 0.5 mg/L (<0.05 % of total contents) for all tested soils regardless of the soil to water ratio.
Kungolos et al. (2009) reported that the toxicity of single compounds varied up to two orders of magnitude, depending on the bioassay examined. Therefore, the combined results of different bioassays will better reflect effects in contaminated soils. In this study, various patterns appeared depending on the types of samples and bioassays (bacterial bioluminescence, seed germination, root, and shoot growth). In the case of bioluminescence, either inhibition or stimulation was observed with no complete inhibition during exposure periods. Relative bioluminescence activity (%) was in the range of max. 118 % and min 55 % of control (average 82 ± 16.4 %). Bioluminescence activity was considerably correlated with organic-C (%) (r2 = 0.7204) rather than with total and acid-extracted metal contents. For example, total metal contents of the #A and #F samples were 920 and 216 mg/kg soil, which were 91 % (9 % toxicity) and 55 % (45 % toxicity) of relative bioluminescence activity, respectively. This correlation might be attributable to the effects of organics themselves or the bioavailability of organic-metal complexes.
Liu et al. (2005) reported that seed germination is one of the best known indicators of plant development among other endpoints, including root length, shoot height, root biomass, shoot biomass, and total biomass. In contrast, Kapustka et al. (1995) reported germination as the least effective technique for vegetative response endpoints. In this study, seed germination was less sensitive than both root and shoot growth. Inhibition of specific enzymatic reactions by metals permeated into seed reserves is one of the main mechanisms behind metal toxicity on seed germination. In addition, seed germination activities of samples did not show any observable correlation with soil properties or metal contents, as all correlation coefficients were less than 0.0607. Effects on root and shoot growth, especially root growth, were clearly greater than that on seed germination (Table 3). Average root growth (62 ± 22.1 %) was nearly 0.75 to 0.67 times lower than bioluminescence activity, seed germination, and shoot growth (83 ± 16.5, 92 ± 11.4, and 87 ± 21.4 %, respectively). In general, an increase in metal concentration leads to reduction of root and shoot growth in plants. Therefore, root and shoot growth of germinated seeds are likely more affected by metals than seed germination itself. Differences between root and shoot toxicity might be due to the movement of metals from roots to shoots as well as direct contact with the root surface. As metals tend to be retained in root tissues, the effects are generally greater in roots than in shoots (An et al. 2004). Liu et al. (2007) also reported that plant measures are inhibited in the following order: root length > shoot height > biomass > germination frequency. Similar to reports, root growth was clearly more sensitive than shoot growth in all samples (An et al. 2004; Liu et al. 2005). RRL and RSL showed a positive correlation (r2 = 0.6849) regardless of the sample characteristics, indicating a reduction in the shoot growth strongly depended on the reduction of root growth.
Research has demonstrated that the toxicity and bioavailability of metals in soils can be strongly influenced by variations in soil chemical and physical properties (Langdon et al. 2014). The specific soil properties that have been shown to play the greatest roles in controlling toxicity, bioavailability, partitioning, and speciation of metals include pH, clay content, organic carbon, and CEC (Smolders et al. 2004; Oorts et al. 2006; Rooney et al. 2007; Criel et al. 2008; Heemsbergen et al. 2009; Li et al. 2011). In this investigation, no considerable correlation was observed between metal contents (total and acid extractants) and any results of the tested methods, showing all r2 values less than 0.2788. However, specific soil properties were considerably correlated with certain bioassays. Bioluminescence activity was highly correlated with organic-C (%), showing a correlation coefficient of r2 0.7204. In contrast, root growth was highly correlated with CEC, showing a correlation coefficient of r2 = 0.6676, whereas correlations with other soil properties were in the range of 0.0183–0.2788. Shoot growth also showed a little high correlation (r2 = 0.3288) with CEC. Both RSL and RRL showed a better correlation with total metal contents (r2 = 0.2723, 0.2788) compared to other observations, showing r2 values of 0.1025 for bioluminescence and 0.0104 for seed germination. The effects of these soil properties on the behavior and availability of metals have been shown to be metal-specific, with different properties, or combinations of properties, having the greatest influence. Due to the complex characteristics of soil, it is difficult to generalize any relationship with toxicity. Li et al. (2013) reported that soil pH and organic-C are the most important soil properties controlling the effects of Cu and Ni toxicity on tomato and Bokchoy shoot growth. Other studies reported that one metal species in a mixture can influence or even decrease uptake of other metals, which may constitute a novel reduction mechanism (Peralta-Videa et al. 2002). Therefore, it seems not possible to apply the relationships between toxicity and soil properties that have been observed for one case to the behavior and availability of other cases.