As far as the total Hg distribution in soils is concerned, the GC zone results to be the area with the highest concentration of Hg, followed by TMB > GO > FN (Fig. 1b). No significant correlation between total and leached Hg is observed, indicating that Hg is heterogeneously distributed in the investigated soils, being likely related to different sources. In contrast to Campos et al. (2018), who reported a correlation between total Hg and soil leached Hg of 0.79, and a correlation between soil leached Hg and humic acid Hg of 0.65, in this work soil leached Hg did not present any correlation between the Hg species analyzed. The top-soils are indeed affected by the presence of anthropogenic materials. In the past, in some portions of the former mining area, post-roasting and anthropic man-made (e.g. bricks, tiles, fragments of concrete) materials were used to fill a small paleo-valley positioned in front of the edifice hosting the Gould and Nesa furnaces (Fig. 1b) (Vaselli et al., 2015).
The thermal speciation data evidenced that most Hg is inorganic although eight top-soils (i.e., ASS3, ASS4, ASS10, ASS14, ASS17b, ASS18, ASS19, and ASS20a) have a percentage of organic-related Hg that prevails over that related to inorganic Hg, as evidenced in the bar plot chart of Fig. 2.
Figure 2 Bar chart plot of organic and inorganic Hg (in %) in the soil samples
In this study, as well as in the Almadenejos metallurgical precinct (Almadén, Spain) by Campos et al. (2018), a positive correlation was found between total Hg and that corresponding to the fractions identified by thermal speciation, i.e. organic Hg and inorganic Hg (Fig. 3a,b and Fig. 4). In Fig. 3a, two distinct trends can be observed. The first one corresponds to a positive correlation between total Hg and organic Hg whereas the second trend is mainly delineated by four samples (ASS8a, ASS8b, ASS17a and ASS21), which are characterized by an increasing concentration of computed inorganic Hg (up to > 80%) whilst that of organic Hg maintains almost unchanged. We can hypothesize that these soil samples are possibly indicating the presence of higher contents of residual mining materials. When these four samples are not considered, the correlation between the two parameters significantly increases as a Pearson coefficient of 0.92 (Fig. 3b) was computed. A similar positive correlation (r = 0.92) is also obtained when total Hg is plotted vs. inorganic-related Hg (Fig. 4). This interdependence is likely indicating that the fractionation of Hg compounds in the soils of the ASS mine is a distinct process unaffected by the relative position of the samples, the amount of organic matter present, or the activity due to enzymatic processes, as also reported by Campos et al. (2018). It is be noticed that the soil samples located in GO (Fig. 1b and Table S1) are more enriched in organic-related Hg with respect to those collected close to the mining facilities, suggesting that the proximity to the machineries to produce liquid Hg affected the soil matrix.
In addition, a second, weaker correlation (r = 0.5) between leachable Hg (in µg L− 1) and organic-related Hg (in mg kg− 1) is reported in Fig. 5. According to Campos et al. (2018), the most labile species of Hg are those containing organic Hg. However, it is to be pointed out that the high concentration of leached Hg (up to 20 µg L− 1) can also be released by solid phases and not necessarily only related to organic Hg.
Figure 4 Binary diagram between inorganic-related Hg vs. Total Hg (in mg kg− 1). The inorganic Hg concentrations are those computed by thermal speciation
The measured DHA contents have an average value of 53.7 µg TPF g− 1day− 1. This value is markedly lower than that measured by Campos et al. (2018) in the Almadenejos soils and approaches that reported by Hinojosa et al. (2004) for soils polluted by heavy metals (average: 70 µg TPF g− 1day− 1).
The concentration of DHA in the soils from the mining and production area appears to be even lower than those measured by Hinojosa et al. (2004) in the reclaimed area of the Aznalcollar mine (SW Spain). According to Pan and Yu (2011), the presence of PTEs in soils can have negative effects on the enzymatic activity, affecting either the enzyme-substrate complexation or the structure of the amino acids. In this case, the total Hg vs. DHA enzyme diagram (Fig. 6) shows a poor correlation (Pearson correlation r = 0.52, p < 0.05) with scatter distribution between the two parameters, suggesting that the presence of Hg, independently by its speciation, is not able to affect the microbial activity in the ASS soils, similarly to what observed by Campos et al. (2018) for the Hg-rich soils from Almadenejos.
Figure 6 Scatterplot of DHA (mg TPF g− 1d− 1) vs. Hg (mg kg− 1) in the soils from the ASS mining area. Blue circles: samples from CG, red circles: samples from FN, cyan circle: samples from GO and dark yellow circle: samples from TMB
Considering the metals and metalloids usually found in the AAS ore deposits and the surrounding Hg-mining areas (Rimondi et al., 2014a), the analytical spectrum should be enlarged to evidence whether, the enzymatic activity may be jeopardized by other PTEs (e.g. As and Sb).
4.1. Hg and BF in plants
The bar graphs in Fig. 7 depict the Hg distribution in each portion of the sampled plants, except for Acer pseudoplatanus and Salix spp., for which only one sample (bark trunk and root, respectively) was collected. Robinia pseudoacacia, Sambucus nigra, Castanea sativa and Popolus spp. are characterized by the highest Hg concentrations in the roots, as well as Salix spp. while the bark trunk of Acer is enriched in Hg. Different is the behavior of Cytisus scoparius and Verbascum thapsus as Hg is found in high contents in the foliage. Notably, is the fact that the highest Hg concentrations are related to the leaves of Cytisus scoparius, located in the TMB zone, where the gaseous elemental Hg in the atmosphere were found almost constantly up to 50,000 ng m− 3 or even higher (Vaselli et al., 2013).
This is likely related to the fact that the leaf system is one of the main pathways of Hg uptake due to both dry deposition, as also suggested by Chiarantini et al. (2016) and Campos et al. (2018), as well as gaseous Hg from Hg-rich environments (such as that recorded in the air nearby the mining machineries and furnaces) and diffuse Hg from soil, although no Hg flux measurements are presently available.
The high Hg concentrations detected in the leaves of Verbascum thapsus (related to ASS 17 soil), collected from GO (Fig. 1b), can be explained by the main winds at ASS that blow from NNE/NE (https://www.meteoblue.com/it/tempo/historyclimate/climatemodelled/abbadia-san-salvatore_italia_3183581), thus, favoring the deposition of the Hg-rich atmospheric particulate and atmospheric Hg from TMB to GO (Fig. 1b). Therefore, according to the investigation on the different parts of plants analyzed in this study (Fig. 7), foliage is likely the main mechanism of Hg-uptake that can be invoked for the ASS plants, thus confirming previous investigations, e.g. Naharro et al., (2019). Occasionally, roots seem to play a role in the Hg-uptake. However, further analyses on the leaf apparatus for those plants where the foliage was not collected are necessary.
The distribution of Hg between external and internal roots is reported in Fig. 8. All the studied root samples indicate that Hg concentration increases, as expected, in the external roots, with the exception of Sambucus nigra, showing an external root/internal ratio of about 1.7, i.e. more than one order of magnitude lower that those recorded for those samples characterized by Hg content > 20 mg kg− 1. To the best of our knowledge, few are the studies related to the partitioning of Hg between internal and external roots and this calls for more detailed investigations.
According to Hussain et al. (2022), BF is a partitioning coefficient that mimics the ability of plants to absorb PTEs and, in this study, it was applied to the concentration of Hg in each part of the analyzed plants and that related to the soil leachable Hg. The BF values are highly variable when the different plant sectors are considered (Table S3). All BF values are < 1 (Table S3), but BF > 0.6 values correspond to the bark trunk and bark root of Sambucus nigra, and the leaves of Verbascum thapsus (0.63, 0.93, and 0.65 respectively). The BF values in the leaves of Verbasum thapsus seem to confirm that the leaves are likely the main path of Hg-uptake by plants. On the other hand, the relatively high BF values measured in the bark trunk and bark roots of Sambucus nigra are possibly due to the difficulty in efficiently and completely removing all the soil-related particles during cleaning. This means that the concentration of mercury in the outermost part of roots and trunk is likely affected by the presence of soil material.