Analyzing Soil Metal Toxicity: Spiked or Field-Contaminated Soils?

Abstract

Most ecotoxicological studies on soil metal toxicity are performed using spiked soils (i.e., uncontaminated or artificial soils to which increasing amounts of metals in the form of soluble salts are added in a laboratory setting). Such an approach has been widely criticized due to the difficulty in extrapolating the results directly into real field situations (e.g., Spurgeon and Hopkin 1995; Smolders et al. 2004). It is well known that metal toxicity is greater in spiked soils than in field-contaminated soils where pollution may have occurred decades ago (Table 1). For instance, total soil metal concentrations yielding 10% inhibition in freshly spiked soils were up to 100-fold smaller (median 3.4-fold) than those in corresponding aged soils or field-contaminated soils (Smolders et al. 2009). Such disparity is attributed to the fact that metal toxicity depends on metal residence time in soils, among other factors. This process is referred to as “aging” and takes place over a period of time. For this reason, many investigators argue that metal-spiked soils are of limited use for environmental assessment and soil quality decision-making, and emphasize the importance of using field-contaminated soils for toxicity assays. Nevertheless, few such studies have been performed. For instance, we have found only 6 studies in which copper phytotoxicity thresholds have been determined using field-contaminated soils (Hamels et al. 2014; Kolbas et al. 2014, 2018; Verdejo et al. 2015; Mondaca et al. 2017; Lillo-Robles et al. 2020). Likewise, we are aware of only one study on the arsenic toxicity threshold for Eisenia fetida in field-contaminated soils (Bustos et al. 2015). Although the importance of using field-contaminated soils—rather than spiked soils—is evident, interpretation of the results from field-contaminated soils presents several difficulties. First, field-contaminated soils often contain several metal pollutants, which makes it difficult to distinguish between the effects of specific metals on plant and soil organism responses. Second, the chemical form in which the metal is deposited into the soil also affects the level of soil metal toxicity. Third, field-contaminated soils might not offer the range of metal contamination necessary to conduct ecotoxicological studies (e.g., Lillo-Robles et al. 2020). In the case of plants, fluctuating nutrient availability in soil may also affect responses, in addition to soil metal toxicity. Using field-contaminated soils, we have, however, demonstrated that detailed characterizations of soil properties and metal concentrations in plant and earthworm tissues yield appropriate estimates of metal toxicity thresholds (Bustos et al. 2015; Mondaca et al. 2017). Another alternative approach to the use of spiked soils is the so-called fading technique, whereby a range of metal concentrations is obtained by mixing field-contaminated soil with uncontaminated soil in various proportions. The range of metal contamination obtained this way provides more accurate results compared with metal-spiked soils. In addition, the fading technique has proved useful in obtaining dose–response relationships to determine threshold metal concentrations in soil and in soil porewater (Hamels et al. 2014; Kolbas et al. 2014, 2018). Another potential approach to advance our understanding of metal toxicity thresholds is to use metal-containing compounds that mimic the source of contamination. In the case of contamination resulting from mining activities, such compounds may include metal oxides, sulfides, and carbonates. These solid phases represent various chemical forms with different weathering rates and solubility. The use of oxides, sulfides, and carbonates—rather than the sulfate, nitrate, and chloride salts typically utilized for metal spiking—provides a more realistic source of contamination. Although an argument can be made that a leaching procedure after spiking with metal salts may be used to remove excessive metal salts (Smolders et al. 2009), such a procedure is complex and labor intensive. It must be pointed out, however, that the proposed approach of using oxides, sulfides, and carbonates does not solve the problem of aging, which implies that the threshold values of total added metal will not represent real field conditions unless experiments are carried out for long periods of time (e.g., decades and more) to allow for dissolution (weathering) and redistribution of the metal contaminant among soil solid phases. Nevertheless, our recent study (A. Neaman et al., unpublished manuscript) shows that addition of oxides, sulfides, and carbonates produces meaningful threshold values of exchangeable/soluble metal concentrations that are comparable to the data obtained in field-contaminated soils. Thus, this approach seems to be useful in understanding metal behavior in real field conditions. In sum, we encourage the analysis of dose–effect relationships using native field-collected soils with varying degrees of metal contamination due to long-term industrial emissions rather than applying the standard approach, which relies on spiked soils. The present study was written with the support of the Fondo de Fomento al Desarrollo Cientifico y Technológico (project ID17AL0056). For further information, please contact the corresponding author ([email protected]).

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