**3. Results and discussion**

#### **3.1. Arsenic and lead in soil**

The average content of As in the Earth's crust is 1.8 mg/kg, and, that of the soils has been found to be at 6.83 mg/kg. The most mobile forms of As are absorbed in a pH range of 7–9. The ions of As are known to be easily fixed in soils by Fe and Al hydroxides, the clay fraction, P and Ca compounds and organic matter [48]. The arsenic concentrations in this study ranged from 4.7 to 463.2 mg/kg. Changes in arsenic concentration were found mainly between the distances of 0–100 m and 1000–10,000 m (kW-H4,25 = 21.71 p = 0.0002, **Figure 3a**); with the highest concentrations being from the samples located in the first 100 m from the source.

Pb is a toxic metal naturally present in the Earth's crust (15 mg/kg). Mining, metallurgy, manufacturing and recycling activities, and, in some countries, the use of lead paints and gasolines, are among the main sources of environmental pollution [49]. The average level of Pb in soils has been measured at 27 mg/kg [48]. In our study, lead concentrations (mg/kg) ranged from 171.7 to 2319.0; the zones located between distances of 0 and 10,000 m showed the greatest difference (kW-H4,25 = 13.61 p = 0.0086, **Figure 3b**). Arsenic and lead concentrations of the sites located from 0 to 100 m from the source are above the levels established for the remediation of contaminated soils dictated by Mexican regulations (NOM-147-SEMARNAT/SSA1–2004), which establishes the limits at 22 mg/kg of As and 400 mg/kg of Pb.

In Villa de la Paz, Martínez-Toledo et al. [50] found values (mg/kg) of As from 1461.9 to 28274.0 and of Pb from 466.1 to 3486.4 in a site close to the source of contamination. In another study, Márquez-Reyes [51] determined the average total concentrations (mg/kg) of 13,443.4 of As and 1301.7 of Pb. González-Mille [52] reported average concentrations (mg/kg), for the site, of 222.1 of As and 204.3 of Pb. Chipres [53] found the presence of heavy metals of environmental

**Figure 3.** Comparison of As (a) and Pb (b) in distance gradient. The bars indicate the median, and the error bars the interquartile range. Different letters indicate the significant differences (p < 0.01) by the distance to the source. The dotted line represents the Mexican normative.

interest related to the geochemical state of the Altiplano showing ranges (mg/kg) from 4 to 324 for As. Due to the above, the values found in this study are similar and even lower than those found in previous studies for the area. In a study similar to ours, in this regard, Puga et al. [54] evaluated soil contamination by Pb, Cd, Zn and As at different distances (300, 600, 900, 1200 and 1500 m) and soil depth levels (0–40, 40–60, 60–80 cm), in an area of influence of mining waste in San Francisco del Oro, Chihuahua; the results showed that at a greater distance from the tailings, the concentrations of As and Pb decreased in a similar way to the one found in this study.

> Mexican regulations (NOM-021-SEMARNAT-2000). The EC values found in this study were low with respect to other studies [50, 55, 56] done in the area ranging from 2200 to 12,200 μS/m; the differences could be due to the technique of determining EC or the selection of sampling points. A high EC could facilitate the mobility and bioavailability of metals in the soil through two possible means: (1) positively charged ions associated with salts (Na and K) that can replace heavy metals in the absorption sites and (2) negatively charged ions (for example chlorides) which can form stable soluble compounds with heavy metals (Cd, Zn and Hg), and have a tendency to generate acidic soils [59], as could be the case of the first 100 gradient

> **Figure 4.** Comparison of elements and physicochemical parameters in distance gradient. a) pH, b) electric conductivity and c) 0rganic matter. The bars indicate the median and the error bars the interquartile range. Different letters indicate

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the significant differences (p < 0.01) by the distance to the source.

OM varied from 0.8 to 1.2%. A significant reduction of OM was found mainly between the distances of 0 and 10–100 m and the distances of 1000–10,000 m (kW-H4,25 = 21.49, p = 0.0003, **Figure 4c**). OM is very low in the distances from 0 to 100 m and low in the distances of 1000– 10,000 m in accordance with Mexican regulations (NOM-021-SEMARNAT-2000). OM values found were comparable with other studies [50, 55, 56] done in the area ranging from 1.2 to 11.6%. Organic matter is formed by humic acids that provide a broad cation exchange capacity [60]; the interaction between organic matter and clays can improve soil adsorption capacity of inorganic contaminants [61]. It has been shown through various studies that the addition of organic matter to the soil (compost, waste, leaf litter) can help to reduce the toxicity of heavy metals regarding the enzymatic activity of the soil, as well as their bioavailability [48, 62].

meters.

#### **3.2. Physicochemical parameters**

The pH levels in this study ranged from 3.9 to 8.2. A pH decrease was found in the distances of 0 and 10 with respect to the distances of 1000–10,000 m (kW-H4.25 = 19.61, p = 0.0006, **Figure 4a**). According to Mexican regulations (NOM-021-SEMARNAT-2000), pH levels in soils in zones 0 and 10 m can be considered as strongly acidic and moderately acidic, respectively; while for distances from 1000 to 10,000 m, soil can be considered neutral. The pH values found were lower than those in other studies [50, 55, 56] for the area ranging from 7.1 to 8.2%, which may be due to the technique of pH determination or the selection of sampling points. pH is one of the most important parameters that control the change of the chemical forms of the elements in soil [57]. According to Rieuwerts et al. [58], the acid pH influences the absorption of heavy metals by plants and other organisms because the solubility of metals tends to increase at low pH values and decreases when pH is high. Near a neutral pH, the formation of complexes can become a metal immobilization mechanism, meaning that they have a low bioavailability [59]. According to what has been described above, it can be assumed that in the samples near the source of contamination (0 and 10 m), there is a high bioavailability of metals due to the low pH reported.

The EC varied from 82.8 to 1660 (μS/m). An increase of the EC was found mainly between the distances of 0–100 and the distances of 1000–10,000 m (kW-H4,25 = 21.06, p = 0.0003, **Figure 4b**). EC levels can be considered as negligible effects of salinity for all sites in accordance with Evaluation of the Biological Activity of Soil in a Gradient Concentration of Arsenic and Lead… http://dx.doi.org/10.5772/intechopen.80031 43

interest related to the geochemical state of the Altiplano showing ranges (mg/kg) from 4 to 324 for As. Due to the above, the values found in this study are similar and even lower than those found in previous studies for the area. In a study similar to ours, in this regard, Puga et al. [54] evaluated soil contamination by Pb, Cd, Zn and As at different distances (300, 600, 900, 1200 and 1500 m) and soil depth levels (0–40, 40–60, 60–80 cm), in an area of influence of mining waste in San Francisco del Oro, Chihuahua; the results showed that at a greater distance from the tailings, the concentrations of As and Pb decreased in a similar way to the

**Figure 3.** Comparison of As (a) and Pb (b) in distance gradient. The bars indicate the median, and the error bars the interquartile range. Different letters indicate the significant differences (p < 0.01) by the distance to the source. The dotted

The pH levels in this study ranged from 3.9 to 8.2. A pH decrease was found in the distances of 0 and 10 with respect to the distances of 1000–10,000 m (kW-H4.25 = 19.61, p = 0.0006, **Figure 4a**). According to Mexican regulations (NOM-021-SEMARNAT-2000), pH levels in soils in zones 0 and 10 m can be considered as strongly acidic and moderately acidic, respectively; while for distances from 1000 to 10,000 m, soil can be considered neutral. The pH values found were lower than those in other studies [50, 55, 56] for the area ranging from 7.1 to 8.2%, which may be due to the technique of pH determination or the selection of sampling points. pH is one of the most important parameters that control the change of the chemical forms of the elements in soil [57]. According to Rieuwerts et al. [58], the acid pH influences the absorption of heavy metals by plants and other organisms because the solubility of metals tends to increase at low pH values and decreases when pH is high. Near a neutral pH, the formation of complexes can become a metal immobilization mechanism, meaning that they have a low bioavailability [59]. According to what has been described above, it can be assumed that in the samples near the source of contamination (0 and 10 m), there is a high bioavailability of metals due to the

The EC varied from 82.8 to 1660 (μS/m). An increase of the EC was found mainly between the distances of 0–100 and the distances of 1000–10,000 m (kW-H4,25 = 21.06, p = 0.0003, **Figure 4b**). EC levels can be considered as negligible effects of salinity for all sites in accordance with

one found in this study.

low pH reported.

**3.2. Physicochemical parameters**

line represents the Mexican normative.

42 Soil Contamination and Alternatives for Sustainable Development

**Figure 4.** Comparison of elements and physicochemical parameters in distance gradient. a) pH, b) electric conductivity and c) 0rganic matter. The bars indicate the median and the error bars the interquartile range. Different letters indicate the significant differences (p < 0.01) by the distance to the source.

Mexican regulations (NOM-021-SEMARNAT-2000). The EC values found in this study were low with respect to other studies [50, 55, 56] done in the area ranging from 2200 to 12,200 μS/m; the differences could be due to the technique of determining EC or the selection of sampling points. A high EC could facilitate the mobility and bioavailability of metals in the soil through two possible means: (1) positively charged ions associated with salts (Na and K) that can replace heavy metals in the absorption sites and (2) negatively charged ions (for example chlorides) which can form stable soluble compounds with heavy metals (Cd, Zn and Hg), and have a tendency to generate acidic soils [59], as could be the case of the first 100 gradient meters.

OM varied from 0.8 to 1.2%. A significant reduction of OM was found mainly between the distances of 0 and 10–100 m and the distances of 1000–10,000 m (kW-H4,25 = 21.49, p = 0.0003, **Figure 4c**). OM is very low in the distances from 0 to 100 m and low in the distances of 1000– 10,000 m in accordance with Mexican regulations (NOM-021-SEMARNAT-2000). OM values found were comparable with other studies [50, 55, 56] done in the area ranging from 1.2 to 11.6%. Organic matter is formed by humic acids that provide a broad cation exchange capacity [60]; the interaction between organic matter and clays can improve soil adsorption capacity of inorganic contaminants [61]. It has been shown through various studies that the addition of organic matter to the soil (compost, waste, leaf litter) can help to reduce the toxicity of heavy metals regarding the enzymatic activity of the soil, as well as their bioavailability [48, 62].

## **3.3. Respiration and enzyme activities**

The activity of arylsulfatase varied from ND to 11.6 (μmol of p-nitrophenol g−1 h−1). A significant decrease in the activity of the enzyme was found in the distances of 0–100 m and the distances of 1000–10,000 m (kW-H4,25 = 21.86, p = 0.0002, **Figure 5a**), which represents an inhibition of 98.3%. The activity of Arylsulfatase was higher than that found by Montes-Rocha [55] for the area (ND at 0.68 μmol of p-nitrophenol g−1 h−1); but the inhibition pattern in the enzyme was similar (98.8%). Borowik et al. [63] determined that arylsulfatase can be inhibited (85.7%) by high concentrations of Zn (2400 mg/kg). Hernández et al. [64] found an inhibition (64.3%) in the activity of arylsulfatase in a highly contaminated soil (mg/kg) by Zn (18900), Pb: 4930, Cd (15.10) and Cu (11.90).

The activity of β-Glucosidase ranged from 0.02 to 63.9 (μmol of p-nitrophenol g−1 h−1). A significant decrease in the activity of the enzyme was found in the distances of 0–100 m with respect to the distances of 1000–10,000 m (kW-H4,25 = 18.60, p = 0.0009, **Figure 5b**), which represents an inhibition of 99.4%. The activity of β-Glucosidase was higher than that found by Montes-Rocha [55] for the area (0.002–0.132 μmol of p-nitrophenol g−1 h−1), and; the inhibition pattern in the enzyme was lower (86.4%). Experimental studies in soil contaminated mainly with Cu (450 mg/kg [65]), Zn (2400 mg/kg [63]) and mixtures of elements (Zn: 18,900, Pb: 4930, Cd: 15.10, Cu: 11.90 mg/kg), showed inhibitions of β-Glucosidase with respect to the control sample from 36.2 to 89.0%.

The activity of the dehydrogenase varied from ND to 0.44 (μmol of INTF g−1 h−1). A significant decrease in the activity of the enzyme was found in the distances of 0–100 m with respect to the distances of 1000–10,000 m (kW-H4,25 = 2.19, p = 0.0002, **Figure 5c**), which represented an inhibition of 97.4%. The activity of dehydrogenase was higher than that found by Montes-Rocha [55] for the area (0.001–0.003 μmol of p-nitrophenol g−1 h−1), and the inhibition pattern in the enzyme was considerably lower (18.4%). Kucharski et al. [66] found an inhibition (34.3%) in dehydrogenase in laboratory studies due to exposure to Ni.

The activity of the phosphatase ranged from 0.04 to 0.82 (μmol of p-nitrophenol g−1 h−1). A significant decrease in the activity of the enzyme was found in the distances of 0–100 m with respect to the distances of 1000–10,000 m (kW-H4,25 = 18.46, p = 0.001, **Figure 5d**), which represented an inhibition of 79.6%. The activity of the phosphatase was similar with respect to that found by Montes-Rocha [55] for the area (0.017 to 0.103 μmol of p-nitrophenol g−1 h−1), and the inhibition pattern in the enzyme was considerably lower (1.7%). Forty percent of inhibition of phosphatase is being found by exposure to Cd [67].

The activity of urease varied from 0.109 to 39.53 (μmol of NH4 -N g−1 2 h−1). The concentration gradient in the activity of the enzyme was found (kW-H4,25 = 21.92, p = 0.0002, **Figure 5e**) in the following order: 0–100 < 100 < 1000 < 10,000 m. The inhibition pattern of the distances of 0–100 and 100 m with respect to 10,000 m was 99.9 and 96.7%, respectively. The activity of urease was lower than that found by Montes-Rocha [55] for the area (ND at 0.63 μmol of NH4 -N g−1 2 h−1), and the inhibition pattern in the enzyme was similar (100%). There are numerous studies of the inhibition of urease by different metals. In this regard, Belyaeva et al. [68] found urease inhibitions of 82.3 and 93.6% by Zn (8100 mg/kg). Yang et al. [69] showed that urea was inhibited by 75.1% with the presence of Cd (100 mg/kg) and Pb (500 mg/kg) in a controlled experiment. Kim et al. [70] found inhibitions from 21.5

to 37.7% caused by Cu (200 mg/kg) in two types of soil. Researchers observed an inhibition of urease (65.3%) caused by Cu (800 mg/kg) in agricultural soils [71]. Borowik et al. [63] determined that urease can be inhibited (74.7%) by high concentrations of Zn (2400 mg/kg). Wyszkowska et al. [72] found that urea activity in agricultural soil can be inhibited (34.6–

**Figure 5.** Comparison of enzymes and soil respiration in distance gradient. a) arylsulfatase, b) β-glucosidase, c) dehydrogenase, d) phosphatase, e) urease and f) respiration. The bars indicate the median, and the error bars the interquartile range. Different letters indicate the significant differences (p < 0.01) by the distance to the source. ND: Not

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in soil varied from 0.62 to 1.5 (μg of CO2

decrease in respiration was found mainly between the distances of 0–100 and the distances

g−1 h−1). A significant

78.3%) by different mixtures and concentrations of heavy metals.

The production of CO2

detectable.

Evaluation of the Biological Activity of Soil in a Gradient Concentration of Arsenic and Lead… http://dx.doi.org/10.5772/intechopen.80031 45

**3.3. Respiration and enzyme activities**

44 Soil Contamination and Alternatives for Sustainable Development

sample from 36.2 to 89.0%.

0.63 μmol of NH4

The activity of arylsulfatase varied from ND to 11.6 (μmol of p-nitrophenol g−1 h−1). A significant decrease in the activity of the enzyme was found in the distances of 0–100 m and the distances of 1000–10,000 m (kW-H4,25 = 21.86, p = 0.0002, **Figure 5a**), which represents an inhibition of 98.3%. The activity of Arylsulfatase was higher than that found by Montes-Rocha [55] for the area (ND at 0.68 μmol of p-nitrophenol g−1 h−1); but the inhibition pattern in the enzyme was similar (98.8%). Borowik et al. [63] determined that arylsulfatase can be inhibited (85.7%) by high concentrations of Zn (2400 mg/kg). Hernández et al. [64] found an inhibition (64.3%) in the activity of arylsulfatase in a highly contaminated soil (mg/kg) by Zn (18900), Pb: 4930, Cd (15.10) and Cu (11.90). The activity of β-Glucosidase ranged from 0.02 to 63.9 (μmol of p-nitrophenol g−1 h−1). A significant decrease in the activity of the enzyme was found in the distances of 0–100 m with respect to the distances of 1000–10,000 m (kW-H4,25 = 18.60, p = 0.0009, **Figure 5b**), which represents an inhibition of 99.4%. The activity of β-Glucosidase was higher than that found by Montes-Rocha [55] for the area (0.002–0.132 μmol of p-nitrophenol g−1 h−1), and; the inhibition pattern in the enzyme was lower (86.4%). Experimental studies in soil contaminated mainly with Cu (450 mg/kg [65]), Zn (2400 mg/kg [63]) and mixtures of elements (Zn: 18,900, Pb: 4930, Cd: 15.10, Cu: 11.90 mg/kg), showed inhibitions of β-Glucosidase with respect to the control

The activity of the dehydrogenase varied from ND to 0.44 (μmol of INTF g−1 h−1). A significant decrease in the activity of the enzyme was found in the distances of 0–100 m with respect to the distances of 1000–10,000 m (kW-H4,25 = 2.19, p = 0.0002, **Figure 5c**), which represented an inhibition of 97.4%. The activity of dehydrogenase was higher than that found by Montes-Rocha [55] for the area (0.001–0.003 μmol of p-nitrophenol g−1 h−1), and the inhibition pattern in the enzyme was considerably lower (18.4%). Kucharski et al. [66] found an inhibition

The activity of the phosphatase ranged from 0.04 to 0.82 (μmol of p-nitrophenol g−1 h−1). A significant decrease in the activity of the enzyme was found in the distances of 0–100 m with respect to the distances of 1000–10,000 m (kW-H4,25 = 18.46, p = 0.001, **Figure 5d**), which represented an inhibition of 79.6%. The activity of the phosphatase was similar with respect to that found by Montes-Rocha [55] for the area (0.017 to 0.103 μmol of p-nitrophenol g−1 h−1), and the inhibition pattern in the enzyme was considerably lower (1.7%). Forty percent of inhibition of

gradient in the activity of the enzyme was found (kW-H4,25 = 21.92, p = 0.0002, **Figure 5e**) in the following order: 0–100 < 100 < 1000 < 10,000 m. The inhibition pattern of the distances of 0–100 and 100 m with respect to 10,000 m was 99.9 and 96.7%, respectively. The activity of urease was lower than that found by Montes-Rocha [55] for the area (ND at

There are numerous studies of the inhibition of urease by different metals. In this regard, Belyaeva et al. [68] found urease inhibitions of 82.3 and 93.6% by Zn (8100 mg/kg). Yang et al. [69] showed that urea was inhibited by 75.1% with the presence of Cd (100 mg/kg) and Pb (500 mg/kg) in a controlled experiment. Kim et al. [70] found inhibitions from 21.5



(34.3%) in dehydrogenase in laboratory studies due to exposure to Ni.

phosphatase is being found by exposure to Cd [67].

The activity of urease varied from 0.109 to 39.53 (μmol of NH4

**Figure 5.** Comparison of enzymes and soil respiration in distance gradient. a) arylsulfatase, b) β-glucosidase, c) dehydrogenase, d) phosphatase, e) urease and f) respiration. The bars indicate the median, and the error bars the interquartile range. Different letters indicate the significant differences (p < 0.01) by the distance to the source. ND: Not detectable.

to 37.7% caused by Cu (200 mg/kg) in two types of soil. Researchers observed an inhibition of urease (65.3%) caused by Cu (800 mg/kg) in agricultural soils [71]. Borowik et al. [63] determined that urease can be inhibited (74.7%) by high concentrations of Zn (2400 mg/kg). Wyszkowska et al. [72] found that urea activity in agricultural soil can be inhibited (34.6– 78.3%) by different mixtures and concentrations of heavy metals.

The production of CO2 in soil varied from 0.62 to 1.5 (μg of CO2 g−1 h−1). A significant decrease in respiration was found mainly between the distances of 0–100 and the distances of 1000–10,000 m (kW-H4,25 = 18.64, p = 0.0009, **Figure 5f**), which represents an inhibition of 47.7%. There are no respiration data in soil samples from the area. Speir et al. [73] found inhibitions in basal soil respiration due to addition of As.

#### **3.4. Relationship between toxic elements and biological activities**

The activity arylsulfatase (93.6%), β-glucosidase (92.5%), dehydrogenase (85.2%), phosphatase (69.9%), urease (96.0%) and respiration (81.8%) was mainly explained by the variations in As concentrations (**Table 1**). Few studies have been done on the effects of As in soil biological activity; in this regard, Speir et al. [73] found inhibitions in sulfatase, phosphatase, urease and basal respiration of soil due to addition of As (concentrations of 0–50 (μmol As [V] g−1 soil). Researchers conducted studies in soils of the USA contaminated by mixtures of toxic elements (among them 5.64 mg/kg of As and 250 mg/kg of Pb) in which they found a strong inhibition of β-glucosidase and phosphatase [4]. A slight negative effect of Pb (1.2%) is present in urease. Speir et al. [73] showed that urease was inhibited by 75.1% with the presence of Cd (100 mg/kg) and Pb (500 mg/kg) in agricultural soil in a controlled experiment. A 3.6% decrease in soil respiration can be associated with the change of pH of the soil, as mentioned above acidity increases mobility and bioavailability of the toxic elements and has a direct effect on microorganisms.

The predominant variables that negatively affect the total biological activity were electrical

**Figure 6.** Distance-based redundancy analysis (dbRDA) ordination plot for biological activities (soil respiration and

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Different studies show that enzymatic activities of the soil are inhibited by trace elements, for example, α and β-glucosidases, α and β-galactosidases, urease, phosphatases, arylsulfatase, nitrate reductase, l-glutaminase, l-asparaginase, L-aspartase and cellulase [74]. Toxic elements can modify the biological properties of the soil such as microbial activity, enzymatic activity and respiration [75]. Metals and metalloids are a fundamental part for the functioning of an ecosystem; however, when the concentration of these elements rises, they can cause great alterations at different scales. The biological activity of the soil can be seriously affected due to concentrations of certain heavy metals such as Cu, Ni, Cd, Zn, Cr, As and Pb. Several authors have shown that under certain concentrations and conditions, heavy metals and metalloids can cause harmful effects in the soil ecosystem [62]. These can inhibit enzymatic activity in an irreversible or reversible manner occupying the active place of the enzyme that the substrate would occupy preventing in this way the union with the substrate avoiding the formation of the products. Different authors have documented that the inhibition takes place at the level of

Several authors have mentioned that the enzymatic inhibition mechanisms are as follows: (1) by interacting with the enzyme-substrate complex, (2) by denaturing the enzyme protein and (3) by interacting with active protein groups [3, 76]. In places like Villa de la Paz, these

The inhibition of enzymatic activity cannot be attributed only to heavy metals since there are other variables that can influence this activity such as pH. Acosta-Martínez and Tabatabai [74] found that β-Glucosidase is sensitive to changes in soil pH and soil management practices, as well as Sherene [59] found that pH can affect the biological activity of the soil as well as the availability of nutrients. Another property that must be considered is organic matter because they can form complexes with heavy metals and enzymes and in this way reduce their toxicity. Martínez-Toledo et al. [50] found an increase in several enzymes directly associated with

conductivity As, pH and Pb (88.4%) while OM (0.8%) has a positive effect.

enzymes) with elements and covariables.

the SH group. This mechanism occurs in each of the enzymes [76, 77].

processes can be performed chronically for more than 200 years.

pH and OM content in mining sites of San Luis Potosí.

Regarding the total biological activity (dbRDA, **Figure 6**, **Table 1**), the grouping of the total samples into two large clusters corresponding to the distances 0–100 and 1000–10000 is shown as well as a greater dispersion in the information between the group of samples closest to the source of contamination. The direction of the vectors shows the influence of the variables toward the samples and the angles of the vectors represent the correlation between the variables. The DistLM model explained 89.2% of the total biological activity (**Table 1**).


**Table 1.** Results of the distance-based multivariate model (DistLM) for enzyme activity.

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of 1000–10,000 m (kW-H4,25 = 18.64, p = 0.0009, **Figure 5f**), which represents an inhibition of 47.7%. There are no respiration data in soil samples from the area. Speir et al. [73] found

The activity arylsulfatase (93.6%), β-glucosidase (92.5%), dehydrogenase (85.2%), phosphatase (69.9%), urease (96.0%) and respiration (81.8%) was mainly explained by the variations in As concentrations (**Table 1**). Few studies have been done on the effects of As in soil biological activity; in this regard, Speir et al. [73] found inhibitions in sulfatase, phosphatase, urease and basal respiration of soil due to addition of As (concentrations of 0–50 (μmol As [V] g−1 soil). Researchers conducted studies in soils of the USA contaminated by mixtures of toxic elements (among them 5.64 mg/kg of As and 250 mg/kg of Pb) in which they found a strong inhibition of β-glucosidase and phosphatase [4]. A slight negative effect of Pb (1.2%) is present in urease. Speir et al. [73] showed that urease was inhibited by 75.1% with the presence of Cd (100 mg/kg) and Pb (500 mg/kg) in agricultural soil in a controlled experiment. A 3.6% decrease in soil respiration can be associated with the change of pH of the soil, as mentioned above acidity increases mobility and bioavailability of the toxic elements and has a direct

Regarding the total biological activity (dbRDA, **Figure 6**, **Table 1**), the grouping of the total samples into two large clusters corresponding to the distances 0–100 and 1000–10000 is shown as well as a greater dispersion in the information between the group of samples closest to the source of contamination. The direction of the vectors shows the influence of the variables toward the samples and the angles of the vectors represent the correlation between the variables. The DistLM model explained 89.2% of the total biological activity (**Table 1**).

inhibitions in basal soil respiration due to addition of As.

46 Soil Contamination and Alternatives for Sustainable Development

effect on microorganisms.

**3.4. Relationship between toxic elements and biological activities**

**Table 1.** Results of the distance-based multivariate model (DistLM) for enzyme activity.

**Figure 6.** Distance-based redundancy analysis (dbRDA) ordination plot for biological activities (soil respiration and enzymes) with elements and covariables.

The predominant variables that negatively affect the total biological activity were electrical conductivity As, pH and Pb (88.4%) while OM (0.8%) has a positive effect.

Different studies show that enzymatic activities of the soil are inhibited by trace elements, for example, α and β-glucosidases, α and β-galactosidases, urease, phosphatases, arylsulfatase, nitrate reductase, l-glutaminase, l-asparaginase, L-aspartase and cellulase [74]. Toxic elements can modify the biological properties of the soil such as microbial activity, enzymatic activity and respiration [75]. Metals and metalloids are a fundamental part for the functioning of an ecosystem; however, when the concentration of these elements rises, they can cause great alterations at different scales. The biological activity of the soil can be seriously affected due to concentrations of certain heavy metals such as Cu, Ni, Cd, Zn, Cr, As and Pb. Several authors have shown that under certain concentrations and conditions, heavy metals and metalloids can cause harmful effects in the soil ecosystem [62]. These can inhibit enzymatic activity in an irreversible or reversible manner occupying the active place of the enzyme that the substrate would occupy preventing in this way the union with the substrate avoiding the formation of the products. Different authors have documented that the inhibition takes place at the level of the SH group. This mechanism occurs in each of the enzymes [76, 77].

Several authors have mentioned that the enzymatic inhibition mechanisms are as follows: (1) by interacting with the enzyme-substrate complex, (2) by denaturing the enzyme protein and (3) by interacting with active protein groups [3, 76]. In places like Villa de la Paz, these processes can be performed chronically for more than 200 years.

The inhibition of enzymatic activity cannot be attributed only to heavy metals since there are other variables that can influence this activity such as pH. Acosta-Martínez and Tabatabai [74] found that β-Glucosidase is sensitive to changes in soil pH and soil management practices, as well as Sherene [59] found that pH can affect the biological activity of the soil as well as the availability of nutrients. Another property that must be considered is organic matter because they can form complexes with heavy metals and enzymes and in this way reduce their toxicity. Martínez-Toledo et al. [50] found an increase in several enzymes directly associated with pH and OM content in mining sites of San Luis Potosí.
