Microbial community and heavy metals content in

GEODRS-00173; No of Pages 9 Geoderma Regional 14 (2018) e00173

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Microbial community and heavy metals content in soils along the Curu River in Ceará, Brazil David C. Anjos a,⁎, Fernando F.F. Hernandez a, Gary S. Bañuelos b, Sadikshya R. Dangi b, Rebecca Tirado-Corbalá c, Francisco N. da Silva d, Paulo F.M. Filho a a

Department of Soil Sciences, College of Agricultural Sciences of the Federal University of Ceará, Fortaleza 60440554, Ceará, Brazil Water Management Research Unit, San Joaquin Valley Agricultural Sciences Center, United States Department of Agriculture, Agricultural Research Service, Parlier, 93648, CA, USA Agro-Environmental Science Department, University of Puerto Rico-Mayagüez, Mayagüez, 00681, PR, USA d College of Agricultural Sciences, University of International Integration of Afro-Brazilian Lusophony, Redenção 62790000, Ceará, Brazil b c

a r t i c l e

i n f o

Article history: Received 22 January 2018 Received in revised form 4 May 2018 Accepted 7 May 2018 Available online xxxx Keywords: Contaminants Heavy metals Microbial community composition PLFA Entisols Watershed

a b s t r a c t This study aimed to survey the soils surrounding the Curu River with respect to the content of heavy metals and microbial community, besides evaluating the effects of heavy metals on the microbial community. Soil samples were collected from the layers of 0–5 cm and 5–30 cm at 22 sites, close to areas possibly contaminated by heavy metals. The samples were analyzed for granulometry, pH, electrical conductivity, organic matter (OM) and the contents of arsenic, cadmium, cobalt, chromium, copper, molybdenum, nickel, lead and selenium. The responses of microbial structures to the different soil uses were evaluated using phospholipid fatty acid PLFA analysis. Canonical correlation was used to verify possible relations between groups of variables (soil physicalchemical characteristics, heavy metals and soil microbiological attributes). The soil chemical and physical characteristics (clay, OM, silt, pH and sand) contributed to explain the contents of heavy metals in this soil, but the behavior of the microbiological attributes cannot be explained by the heavy metals in this soil. According to the national standards of the Environmental Company of São Paulo State (CETESB), some sites in the studied area are contaminated by heavy metals. © 2018 Elsevier B.V. All rights reserved.

1. Introduction The Curu River watershed extends for 198 km, located in the North center of Ceará – Brazil, with an area of 11,719,392 km2 exposed to the effects of the irregular rainfalls of the semiarid region and low water balance indices throughout the entire year (Brasil, 2010). Distinct environmental units are found along the Curu River, which are explored according to their natural potentials and particular conditions of use or exploration. These units are eventually used by city supply systems, big landowners, farmers and the small subsistence farmers who cultivate in the humid soil left by a river when its water level recedes. These users ultimately interfere with the environmental processes and conditions, due to intensive and, in many cases, inadequate use of these areas. The drainage systems of the Curu River watershed permeate many sites of concentrated and diluted pollution, such as municipal

Abbreviations: EC, electrical conductivity; OM and the contents of arsenic As, organic matter; Cd, cadmium; Co, cobalt; Cr, chromium; Cu, copper; Mo, molybdenum; Ni, nickel; Pb, lead; Se, selenium; CETESB, Environmental Company of São Paulo State. ⁎ Corresponding author. E-mail address: [email protected] (D.C. Anjos).

headquarters and areas of farming and industry (Gorayeb et al., 2006). These potential pollution sources cause damages to the environment, such as pollution of surface waters and contamination of soils with heavy metals (HM) (Erdem et al., 2004). Among the main anthropogenic sources of soil contamination are the HM, which are elements found in low contents in the agroecosystems (He et al., 2005). Some elements, including copper (Cu) and molybdenum (Mo), are essential to plant growth and are referred to as micronutrients (He et al., 2005). Some HM, such as cobalt (Co) and selenium (Se), are not essential to plant growth, but are necessary for animals and humans. Others, like cadmium (Cd), lead (Pb), chromium (Cr), nickel (Ni) and arsenic (As), have toxic effects on living organisms and are frequently considered as contaminants (Wan ngah and Hanafiah, 2008). HM can enter agroecosystems through the inheritance from the parent material of the soil or through human activities (Kimberly and William, 1999; Li et al., 2001; Wan ngah and Hanafiah, 2008). Soil contamination with HM and toxic elements, due to parent material or point sources, often occurs in a limited area and is easily identified. The repetitive use of chemical products rich in metals, fertilizers and organic correctives, such as sewage sludge, as well as wastewaters, can cause large-scale contamination (He et al., 2005).

https://doi.org/10.1016/j.geodrs.2018.e00173 2352-0094/© 2018 Elsevier B.V. All rights reserved.

Please cite this article as: Anjos, D.C., et al., Microbial community and heavy metals content in soils along the Curu River in Ceará, Brazil, (2018), https://doi.org/10.1016/j.geodrs.2018.e00173

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D.C. Anjos et al. / Geoderma Regional 14 (2018) e00173

Irrigation constitutes a significant source of soil contamination by HM, especially when the water used comes from rivers that have received high pollutant loads (Arora et al., 2008; Liu et al., 2005). Environmental and microbiological factors can influence the toxicity of HM, through degradation, biosorption and bioaccumulation processes in organelles or linked to proteins of the cellular interior of the microorganisms (Mapanda et al., 2015; Pereira and de Freitas, 2012). However, not all microorganisms have the same resistance to HM. Therefore, when a soil is contaminated with HM, a large part of soil microorganisms are the first living organisms to suffer the impacts. The HM can decrease the microbial biomass by directly killing or biochemically deactivating soil organisms (He et al., 2005; Wang et al., 2007). Soil microorganisms help in the regulation of environmental processes, such as soil aggregation, nutrient cycling, degradation of xenobiotic compounds and gas balance (Fernandes and Chaer, 2010; Viana et al., 2011). According to the observations, the management of these processes occurs through complex interactions between different microbial groups, and currently there is the understanding on the necessity of adopting the microbial communities (MC) as basic units in the ecology studies (Drenovsky et al., 2010; Fernandes and Chaer, 2010). One of the potential techniques to detect the toxic effects caused by HM on soil microorganisms is the analysis of profiles of phospholipid fatty acids, or soil PLFA (Frostegård et al., 2011). PLFA analysis is a technique very sensitive to variations in soil MC and has been used in studies on microbial ecology. In addition, it is widely used to observe variations that indicate physiological changes in the MC in response to an environmental stress factor (Fernandes and Chaer, 2010). Despite not allowing differentiation of microorganisms at the level of species, PLFA analysis provides a quantitative evaluation of the many groups inside the MC of the soil (Viana et al., 2011). PLFA analysis provides most of the information required for the characterization of MC in the environment, including biomass, composition, nutritional status and metabolic activity (White, 1983). Despite the economic, social and environmental importance of the Curu River watershed for a significant part of the population inhabiting its surroundings and as a source of agricultural products for the supply of the metropolitan area of Fortaleza-CE, little information is found on

the current conditions of the environmental quality of the Curu River watershed. Given the above, this study aimed to survey the soils surrounding the Curu River with respect to the contents of HM and MC biomass and structure, besides evaluating the effects of the heavy metals contents on the MC, through soil PLFA analysis. 2. Material and methods The survey was performed in the Curu River watershed, which extends for 198 km and has its source located in the Serra do Machado (42°47′73 W; 94°97′519 S) and the mouth of the river in the Atlantic Ocean, between the cities of Paraipaba and Paracuru-CE, Brazil (49°38′ 0.87 W; 96°22′884 S). The area has average annual temperature of 26.3 °C, with maximum of 35 °C and minimum of 18 °C, and average annual rainfall of 800 mm, exposed to the effects of the irregular rainfalls of the semiarid region. The Curu River watershed has a predominant moderate to strong relief. This watershed was chosen because it was the first one to be created in the state of Ceará and because of the little information on the current conditions of the environmental quality of the Curu River. For the survey of the contents of HM and the MC, 22 sites were georeferenced close to the main roads with intense traffic of vehicles and to urban, industrial and agricultural areas (Table 1). According to Kimberly and William (1999), Li et al. (2001) and Wan ngah and Hanafiah (2008), areas with these characteristics are considered as potential sources of heavy metals. The geographic coordinates of the 22 sampling sites along the Curu River are shown in Fig. 1. Samples were collected in July 2013, using a hand riverside auger in the layers of 0–5 cm and 5–30 cm, close to the river shore. The collected samples were preserved in ice until arrival at the laboratory, where they were separated into two fractions. One fraction was air-dried, its clods were broken and it was sieved (2-mm grid) for the analyses of soil physical and chemical characteristics. The other fraction was used for soil microbiological analysis and was carefully packed in plastic recipients with capacity for 40 g, cooled at 4 °C and sent to the Water Management Research Laboratory of the USDA, Parlier-CA, USA.

Table 1 Soil classification, characteristics and land use types of sample collection sites along the Curu River – Ceará, Brazil. SiBCSa

Soil taxonomyb

Site Characteristics of site

Neossolos Litólicos

Entisols Lithic

Luvissolos Crômicos

Aridisols

1 2 3

Argissolos Vermelho-Amarelos Eutróficos

Oxisols

4 5 6 7 8 9 10 11 12

Argissolos Vermelho-Amarelos Distróficos

Neossolos Quartzarênicos Órticos

a b

Ultisols

13 14 15 16 17 18 Entisols 19 Quartzipsamments 20 21 22

It is located at the source of the river, has natural vegetation and does not human action Has a source of water for animals that are created close and does not human action There is a weir, region of leisure for people, with intense antropic action, no vegetation and visible appearance of being a compacted area Area there is an animal leather processing factory and has no vegetation Area under a site of intense traffic of vehicles and realization of agriculture Recreation area, the population washes clothes and receives some of the city's sewage Area used in agriculture There is a dam for water storage for human use and it is an area of agriculture It is an area of intense urbanization and the neighboring areas are used with intensive agriculture that receive heavy loads of fertilizers and chemical defenses Area used with agriculture receiving large loads of fertilizers and chemical pesticides Area used with agriculture that receive heavy loads of fertilizers and chemical pesticides. Area used with intensive agriculture, receives great amount of fertilizers and chemical defenses and easy visualization of the contamination of the river water by eutrophication Area under a site of intense traffic of vehicles and realization of agriculture Area close to the city, but with soil covered by secondary vegetation Area close to the city and receives part of the sewage of the city Area used for storing water to be used in agriculture and receiving large amounts of fertilizers and chemical pesticides Area used with intensive agriculture, receives great amount of fertilizers and chemical defenses Area of intense anthropic use, used for recreation and extraction of river area Deforested areas, without human use and used for grazing animals Areas with characteristics of dunes near a shrimp farm Area of wetland and has little human intervention The mouth of the Curu River

Brazilian System of Soil Classification. Soil Taxonomy Classification System from USDA-NRCS.



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Fig. 1. Map of the Curu River watershed with the location of the soil sampling sites along the Curu River shore – Ceará, Brazil.

The following parameters were analyzed: granulometry, pH, electrical conductivity (EC) of the soil saturation extract and the organic matter (OM) contents, according to the analytical methodologies described by EMBRAPA (2011) (Table 3). After the soils were classified and grouped by soil types, according to the Brazilian System of Soil Classification from EMBRAPA (2014) and Soil Taxonomy Classification System from USDA-NRCS (Soil Survey Staff, 2014) (Table 1). For the determination of As, Cd, Co, Cr, Cu, Mo, Ni, Pb and Se, the samples were prepared according to the analytical methodology for HM extraction USEPA 3050, in its updates. The total contents of HM in the samples were quantified at the Water Management Research Laboratory of the USDA, Parlier-CA, USA, using an inductively coupled plasma optical emission spectrometer (IPC-OES), with radial configuration (VISTA PRO-CCD, Varian, Mulgrave, Australia) and an automatic sample introduction system (SPS 5 Varian, Mulgrave, Australia). Only high-purity reagents were used in this study and ultra-pure water (Millipore Direct-Q 3 system) was used in the preparation of the solutions. The calibration curves for the determination of the metals were prepared using a 1000 mg L−1 standard solution (TITRISOL®, Merck) and ultra-pure water for the dilution. In order to evaluate the procedures in the determination of the soil chemical elements, the standard NIST 2709 (San Joaquin soil SRM 2709, Gaithersburg, MD, EUA) was used every 15 samples. The National Institute of Standards and Technology (NIST, 2002) serves to guarantee and control the quality of the analyses. After the determination, the contents of HM in soil samples were evaluated through the comparison with the guiding values of the Environmental Company of São Paulo State (CETESB, 2014) (Table 2). These guiding values of CETESB were used due to the lack of information on the natural contents or the background values for HM for the Ceará State, which were related to the concentration of chemical substances derived using numerical criteria and data from the international scientific literature, in order to subsidize actions of pollution prevention and control, aiming the protection of soil quality and the management of contaminated areas.

The responses of soil microbial structures to the different soil uses were evaluated through the PLFA analysis, in 5 g of soil samples, using the modified method of Bligh-Dyer (Buyer et al., 2010). The fatty acids were directly extracted from the soil samples using the mixture of chloroform: methanol: phosphate buffer (1:2:0.8). Then, the fatty acids were separated into neutral and glycolipids in the solid phase of columns and then alkalinized with methanol and potassium hydroxide 0.2 M. The PLFA of the samples was analyzed qualitatively and quantitatively using the gas chromatograph Agilent 6890 (Agilent Technologies, Palo Alto, CA) and the fatty acids were identified by the retention time according to the MIDI eukaryotic method (MIDI Inc., Newark, NJ). The individual markers of the PLFA were used to identify the concentration of the specific microbiological groups (Gram + bacteria, Gram - bacteria, arbuscular mycorrhizal fungi, saprophytic fungi, eukaryotes and actinomycetes) of the soil samples (Frostegård et al., 2011). HM contents were analyzed using a descriptive analysis, considering the parameters of mean, minimum and maximum values between the different soil sampling sites. Tukey test was used for the comparison of means of PLFA total values in the samples. Multiple comparisons were performed through canonical correlation analysis, by adopting three groups: variables related to soil physical and chemical characteristics (granulometry, pH, EC and OM), variables consisting of the soil

Table 2 Definition of the classification of guiding values of heavy metals contents in soil, according to the Environmental Company of São Paulo State (CETESB). Source: CETESB, 2014. Classification

Definition

Quality reference value (QRV)

The content of certain substance that defines a soil as clean The content of certain substance above which harmful changes in soil quality can occur The content of certain substance in the soil above which potential risks, direct or indirect, to human health can occur, considering a general exposure scenario

Prevention value (PV) Intervention value (agricultural, residential and industrial) (IV)


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HM (As, Cd, Co, Cr, Cu, Mo, Ni, Pb and Se) and variables related to soil microbiological attributes (total PLFA, Gram + bacteria, Gram - bacterias, arbuscular mycorrhizal fungi, actinomycetes, eukaryotes and saprophytic fungi). For the correlation between HM and soil chemical characteristics and the correlation between the contents of HM and soil MC. The effects of heavy metals on soil MC were evaluated using the software IBM SPSS Statistics 21.0 Desktop for Windows (IBM, 2015).

3. Results and discussion 3.1. Soil characteristics in the study area The granulometric analysis of the samples, using the pipette method (EMBRAPA, 2011), showed that sand particles prevailed in most of the sites, ranging from 69.3% to 98%. Silt represented 0.3–15.8% of the texture of the soil samples, while clay was observed in low contents (0.2–19.2%) (Table 3). Most of the samples were classified as sand, loamy sand and sandy loam; thus, these soils have low clay contents in their constitution. The high sand contents occurred because the sampling sites are close to the river shore, an area under great influence of

Table 3 Soil physical-chemical characteristics: pH, electrical conductivity (EC), organic matter (OM), sand, silt and clay in the 22 sampling sites along the Curu River shore, Ceará, Brazil. Site

Layer

pH

cm 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

0–5 5–30 0–5 5–30 0–5 5–30 0–5 5–30 0–5 5–30 0–5 5–30 0–5 5–30 0–5 5–30 0–5 5–30 0–5 5–30 0–5 5–30 0–5 5–30 0–5 5–30 0–5 5–30 0–5 5–30 0–5 5–30 0–5 5–30 0–5 5–30 0–5 5–30 0–5 5–30 0–5 5–30 0–5 5–30

6.91 6.80 6.35 6.98 6.48 5.98 5.24 6.25 6.11 6.38 7.30 5.00 7.62 5.79 7.46 7.10 6.33 4.96 8.06 6.89 7.61 6.70 8.41 5.42 6.91 6.05 7.05 7.86 7.10 7.23 4.96 6.94 7.46 6.92 6.64 6.69 7.33 5.58 6.86 6.76 7.04 5.88 7.20 5.30

EC

OM

Sand

dS m−1

g kg−1

%

0.19 0.31 0.17 0.04 0.10 0.13 0.10 0.20 0.11 0.14 0.41 0.18 0.12 0.37 0.19 0.30 0.28 0.12 0.31 0.28 0.12 0.30 0.41 0.14 0.10 0.61 0.35 0.35 0.06 0.14 0.65 0.08 0.32 0.10 0.12 0.14 0.30 0.07 0.80 0.07 0.08 0.07 0.04 0.07

20.0 37.0 11.1 20.8 9.6 28.7 14.3 10.3 15.9 14.2 14.2 17.0 15.9 33.9 16.8 31.5 5.4 20.1 27.3 34.5 8.7 10.3 32.2 24.2 4.7 14.2 12.9 13.9 3.2 8.4 12.9 7.7 4.6 12.6 7.0 18.9 5.9 2.1 5.7 5.4 3.8 4.6 21.0 16.1

92.8 87.8 83.4 75.9 88.2 85.3 92.9 86.0 95.7 97.0 71.2 79.3 77.6 74.3 96.4 90.3 90.3 85.7 94.7 98.0 83.0 77.1 94.5 97.3 89.7 80.0 86.9 91.9 89.1 82.2 88.4 83.3 94.4 95.8 95.0 97.7 79.9 73.1 81.8 94.7 92.4 94.2 92.4 90.3

Silt

Clay

4.2 6.4 8.2 13.3 7.8 8.9 5.7 8.6 3.5 1.5 5.8 2.3 8.2 7.1 1.2 6.7 8.9 9.5 3.5 0.6 3.4 9.9 3.9 1.1 1.1 8.0 2.5 7.5 7.9 5.6 8.2 9.9 1.8 0.4 1.0 0.3 1.9 7.7 5.8 2.5 6.6 2.4 4.0 5.9

3.0 5.8 8.4 10.8 4.0 5.8 1.4 5.4 0.8 1.5 23.0 18.4 14.2 18.6 2.4 3.0 0.8 4.8 1.8 1.4 13.6 13.0 1.6 1.6 9.2 12.0 10.6 0.6 3.0 12.2 3.4 6.8 3.8 3.8 4.0 2.0 18.2 19.2 12.4 2.8 1.0 3.4 3.6 3.8

the increase in the water level, which suffers with the siltation caused by deforestation. As to the soil chemical characteristics, pH varied from 4.96 to 8.41, showing a great variation between layers and sampling sites in the study area (Table 3). Soil pH is an important characteristic, because it is related to the main reactions in the soil, among which the availability of HM and the decomposition of soil OM (Pavinato and Rosolem, 2008; Violante et al., 2010). The soil samples showed great variation in EC (0.04 to 0.80 dS m−1) (Table 3), which is related to the contents of ions in the soil. Since these soils occur in areas close to the river, they receive a significant load of colloidal material from the higher areas, causing the accumulation of both ions and OM (Hua et al., 2011). The OM contents in the analyzed samples varied between layers and the different sampling sites (2.1 to 40.8 g kg−1) (Table 3). In a great number of sampling sites, there was a higher OM accumulation in the layer of 5–30 cm, compared with 0–5 cm. This accumulation can be related to the fact that the areas close to rivers are rich in sediments, which accumulate close to or at the bottom of the rivers over time. 3.2. Canonical correlation analysis between soil physical-chemical characteristics and heavy metals The correlations and canonical pairs between physical-chemical characteristics and the contents of heavy metals in the soil samples are shown in Table 4. The canonical correlations in the first canonical pair were highly significant (Pr = 0.01) by the chi-squared test (0.97) and the considered groups are not independent (Table 4). Group I explains group II in the first canonical pair. The elements Pb (R = 0.90), Cr (R = 0.89,), Cu (R = 0.88), Co (R = 0.78) and Ni (R = 0.78) are associated with clay (0.91), OM, (0.83), silt (0.81), pH (0.78) and sand (0.74), i.e., they increase with the following increase order: clay, OM, silt, pH and sand. The other pairs (2, 3 and 4) were not important (R b 0.5). These results corroborate those obtained by Payê et al. (2012) and Cunha et al. (2014), who also observed that the contents of HM can vary in the soil due to the its texture and OM content, since these factors have a great influence on soil CEC, which promotes HM retention in the soil. On the other hand, pH has a strong influence on the dynamics of metallic ions, which are more mobile in conditions of low pH (Violante et al., 2010).

Table 4 Canonical correlations and canonical pairs between the characteristics of group I (pH, EC, OM, sand, silt and clay) and group II (As, Cd, Co, Cr, Cu, Mo, Ni, Pb and Se) for the 22 sampling sites along the Curu River shore, Ceará, Brazil. Groups

Attributes

Canonical pairs 1

2

3

4

−0.14 0.30 −0.28 −0.25 −0.48 0.28 −0.29 −0.16 −0.21 −0.41 −0.26 −0.24 −0.27 −0.09 −0.20 0.69n.s. 39.24 32

0.07 −0.25 0.38 0.16 −0.25 −0.12 −0.25 −0.37 0.12 0.08 0.02 0.14 0.12 −0.07 −0.28 0.48n.s. 17.42 12

Canonical correlations GI

GII

pH EC OM Sand Silt Clay As Cd Co Cr Cu Mo Ni Pb Se Canonical R Chi-squared DF

0.79 0.27 0.84 0.75 0.82 0.91 0.26 0.13 0.78 0.90 0.82 0.06 0.79 0.91 0.16 0.97⁎⁎ 163.66 60

−0.43 0.52 −0.07 −0.24 −0.10 −0.11 0.02 0.33 0.03 −0.05 −0.07 0.07 0.00 0.15 0.38 0.65n.s. 68.00 45

DF – Degree of freedom; ns – not significant, respectively. ⁎⁎ Canonical correlations ≥ 0.5 were considered significant for interpretation purposes.



3.3. Survey of the heavy metals contents The quality control of the analyzes was performed using the soil sample with certified values for NIST SRM 2709 San Joaquin soil metals. Certified levels of the metals of the reference soil sample were determined by methods using the combined action of acids, including hydrofluoric, capable of decomposing soil silicates, or non-destructive methods such as X-ray fluorescence; thus, they determine the total metals contents. The values defined by the EPA 3050 method provide the pseudo-metal of the metals in the soil. Therefore, the NIST recommends the comparison with recoveries based on leached values (Biondi et al., 2011). The rates certified soil recovery compared to leachate values were satisfactory for all metals (Table 5). The recovery values of metals and elements for the pseudototal solubilization of the samples ranged from 48 to 110%, which are within the recommended range for most metals and elements when compared to recovered values and, mainly, when compared to leached values. These results attest to the quality of the methods used to open the samples and the dosage of heavy metals in soils. The behavior of the studied metals was different along the sampling sites, considering the relations of each heavy metal with the chemical and physical characteristics of the soil, as well as the competition between these metals for the soil adsorption sites. 3.3.1. Arsenic Arsenic was one of the elements observed in low contents along the Curu River and its values varied widely between the different sampling sites and soil layers (0.01 to 0.71 mg kg−1), with values below the ICPOES detection level in some sites (Fig. 2). According to CETESB (2014), all the sampling sites along the river showed very low As contents, compared with the quality reference value (3.5 mg kg−1). The low As contents in the sampling sites are due to the low exposure of these areas to anthropic activities using this element and the arsenic-poor mineralogy of the analyzed soils, since As contents range from 5 to 10 mg kg−1 for most soils in the world (Smedley and Kinniburgh, 2002). 3.3.2. Cadmium Cadmium was the element with the lowest contents among the studied HM (0.01 to 0.21 mg kg−1), with values below the ICP-OES detection level in some sites (Fig. 2). The quality reference value for Cd indicated by CETESB (2014) (b0.5 mg kg−1) was higher than the value observed in the sampling sites, showing that the soil is Cd-free. The low Cd contents in the samples can be related to management adopted in the surrounding areas, which show a low Cd contamination level, and the constitution of these soils. These soils have a great amount of sand,

Table 5 Mean recovery of heavy metals at soil in standard reference soil NIST SRM 2709 San Joaquin soil, used in the analyzes, values certified and recovered by leaching. Metal

Value Determined

Recovery Certifieda

mg kg−1 As Cd Co Cr Cu Mo Ni Pb Se

8.87 0.42 10.2 61.9 26.0 4.7 65.2 9.82 1.43

Determinedb

By leachatec

% 17.7 ± 0.8 0.38 ± 0.1 13.4 ± 0.7 130 ± 4 34.6 ± 0.7 ND 88 ± 5.00 19.8 ± 0.5 1.57 ± 0.08

51 110 77 48 76 ND 75 52 91

ND ND 90 61 92 ND 89 69 ND

ND = Values not determined by NIST. a NIST: National Institute of Standards and Technology. b % Recovery (determined) = (determined value/certified value) × 100. c % Recovery by leachate = (median leachate (NIST)/certified value) × 100.

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which favors a low CEC, a determinant factor for Cd retention in the soil (Silveira et al., 2008), as observed in the layer of 0–5 cm of the site 6, in which the highest contents of clay (23%) and Cd (0.21 mg kg−1) were observed. 3.3.3. Cobalt Cobalt contents showed varied widely in the analyzed soil layers and in the different sampling sites (0.23 to 10.0 mg kg−1) (Fig. 2). All the sampling sites along the river showed Co contents below the quality reference value (13.0 mg kg−1), according to CETESB (2014). A higher Co content was observed in the layer of 5–30 cm of all the sampling sites, which points to the parent material as the source of Co. If the source had anthropic origin, the Co content would be higher in the layer of 0–5 cm, since the element has a low mobility in the soil (Seliman et al., 2010). 3.3.4. Chromium Cr showed different contents in the different sampling sites (1.1 to 36.1 mg kg−1). All the sampling sites along the river showed Cr contents below the quality reference value (40.0 mg kg−1) determined by CETESB (2014). Cr showed a behavior similar to that of Co (Fig. 2), which can be explained by the fact that Cr is strongly bonded to the soil through internal adsorption mechanisms, i.e., strong bonds that limit its movement in the soil (Araújo et al., 2002). 3.3.5. Copper Cu contents ranged from 0.3 to 17.1 mg kg−1 in the different soil layers and sampling sites (Fig. 2), which were below the quality reference value (35.0 mg kg−1) for this element (CETESB, 2014). Similarly to Co and Cr, with low mobility in the soil, Cu also showed great variation between soil layers, which can be related to the concentration with the content of OM, since Cu has a high affinity to OM and is retained in forms of low mobility in the soil or forms soluble complexes when bonded to high-solubility OM (Silveira et al., 2008). 3.3.6. Molybdenum Molybdenum contents ranged from 0.004 to 0.629 mg kg−1 (Fig. 2), which were very low compared with the quality reference value (b4.0 mg kg−1) (CETESB, 2014). This behavior can be related to the low Mo content in the parent material (Wichard et al., 2009) and the low use of Mo in anthropic activities, as observed in the sites 1 to 15. As to the sites 16 to 22, an increase in Mo contents was observed, due to anthropic activities that, together with soil pH, lead to higher mobility of this element, since its availability increases as soil pH increases (Wichard et al., 2009). 3.3.7. Nickel Nickel contents ranged from 0.5 to 23.9 mg kg−1 and varied between soil layers and sampling sites (Fig. 2). Unlike the other sites, the site 3 in the layers of 0–5 cm and 5–30 cm and the site 6 in the layer of 5–30 cm showed higher Ni contents (16.3, 23.9 and 13.3 mg kg−1, respectively), compared with the reference value for soil quality (13 mg kg−1) (CETESB, 2014). The reference values of CETESB (2014) show that these areas are contaminated and receive sewage loads from the surrounding populations. According to Revoredo and Melo (2006), sewage sludge from metropolitan regions has high Ni contents. The increase in Ni content is related to the anthropic action, since these sites are intensively used by the population. The site 3, known as ‘Balneário do Açude General Sampaio’, is a recreation area and the site 6 (upstream of the wet passageway of the Curu River in Apunhares), which, besides being a recreation area, is used by the people for laundry and received part of the municipal sewage (Table 1). In most of the studied sites, Ni showed a lower accumulation in the layer of 0–5 cm in relation to 5–30 cm. This can be explained by the low competition of Ni for the adsorption sites with the other metals, which causes its leaching in the soil (Oorts et al., 2007).


6


Fig. 2. Contents of the chemical elements As, Cd, Co, Cr, Cu, Mo, Ni, Pb and Se in the soil samples collected in the layers of 0–5 cm and 5–30 cm, in different sites along the Curu River shore, Ceará, Brazil. *Quality reference values (CETESB, 2014).

3.3.8. Lead Lead showed the highest content (49.4 mg kg−1) among all the analyzed metals (Fig. 2), with values ranging from 0.50 to 49.4 mg kg−1 in the soil layers and sampling sites. According to the guiding values indicated by CETESB (2014), the only site with content higher than the reference for soil quality (17 mg kg−1) was the site 16 in the layer of 0–5 cm. According to He et al. (2005), the presence of HM and toxic elements in the soil, due to the parent material or point sources, occurs in

limited areas and is easily identified. As observed in the site 16, the higher content can be related to anthropic activity, since it occurs in the superficial layer, in an area under intensive use by the local community. The high Pb content in the superficial layer can be related to the low pH in the site 16 (4.96) and the great capacity of Pb to be retained in the soil through stable bonds, which reduces its mobility and leaching in the soil profile (Table 3) (He et al., 2005).



3.3.9. Selenium Selenium contents ranged from 0.01 to 0.35 mg kg−1 between the layers and the sampling sites (Fig. 2). The site 2 in the layer of 5–30 cm and the site 9 in the layer of 0–5 cm showed Se contents (0.35 and 0.26 mg kg−1, respectively) higher than the reference value (0.25 mg kg−1) for soil quality (CETESB, 2014). The Se content in the site 2 can be related to the parent material, since the area where the soil sample was collected is under little anthropic use and the Se content is within the limits determined in the literature for soils of different regions worldwide (b0.05 to 2.14 mg kg−1) (Kabata-Pendias and Pendias, 2001). In the site 9, the Se content can be related to the anthropic action, since the highest content was observed in the layer of 0–5 cm (Table 1). In addition, this is an area of intense urbanization and its surroundings are under intensive agriculture, receiving large loads of fertilizers and chemical pesticides that are sources of Se (He et al., 2005).

3.4. Canonical correlation between heavy metals and soil microorganisms There were significant canonical correlations (R = 0.8; P ≤ 5%) for the first canonical pair, by chi-squared test (Table 6). According to the coefficients of the first canonical pair, there was no significant difference between soil microbiological attributes and the contents of heavy metals (group II). These results suggest that these factors are independent in these soils. The other pairs (2, 3 and 4) were not important (R b 0.5). The relation between the attributes of group I (Total of microorganisms, Gram+ and Gram - bacteria) with group II (HM: As, Cd, Co, Cr, Cu, Mo, Ni, Pb and Se) was inversely proportional to the increase in the contents of heavy metals (He et al., 2005). These results agree with those observed by Wang et al. (2007), who suggest that HM can decrease microbial biomass by directly killing or biochemically deactivating soil organisms. However, microbes are also able to respond to metal contamination in the soil and maintain metabolic activity, apparently through changes in the structure of the microbial activity and selection of resistant species (Turpeinen et al., 2004).

Table 6 Canonical correlations and canonical pairs between the characteristics of group I (total of soil microorganisms, gram +, gram −, actinobacteria and saprophytic fungi) and group II (As, Cd, Co, Cr, Cu, Mo, Ni, Pb and Se) for the 22 sampling sites along the Curu River shore, Ceará, Brazil. Groups

Attributes

Canonical Pairs 1

2

3

4

0.49 0.35 0.46 0.73 0.54 0.40 −0.10 0.23 −0.12 0.17 0.43 0.16 0.35 0.03 0.53n.s. 15.56 24

0.16 0.05 0.16 0.20 0.69 −0.01 −0.19 −0.04 0.12 0.08 −0.02 −0.07 0.65 0.31 0.28n.s. 3.91 14

Canonical correlations GI

GII

Total Gram + Gram Actinobacteria Saprophytic Fungi As Cd Co Cr Cu Mo Ni Pb Se Canonical R Chi-squared DF

0.11 0.29 0.11 −0.53 −0.35 −0.01 −0.14 −0.24 −0.38 −0.39 −0.18 −0.38 −0.08 −0.39 0.74⁎ 64.54 50

0.00 0.20 −0.26 0.37 0.32 0.46 0.22 0.74 0.75 0.71 −0.35 0.79 −0.07 0.80 0.66n.s. 35.78 36

DF – Degree of freedom; ns – not significant, respectively. ⁎ Canonical correlations ≥ 0.1 were considered significant for interpretation purposes.

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3.5. Survey of microbial PLFA composition 3.5.1. Soil PLFA biomarker values Effects of HM on the total amount of PLFAs, as well as the compositions of the soil MC were clearly observed. The total microbial biomass ranged from 2.46 to 16.74 μg g−1 of soil, with large variation between soil layers and sampling sites (Fig. 3). The total microbial biomass was greater in the sites 9 and 14, located downstream of the city of São Luís do Curu, exhibiting the highest total concentrations of PLFA, equal to 16.74 and 16.6 μg g−1 of soil, respectively, in comparison to the sites 21 and 18, located in areas of intensive agriculture and shrimp farming, respectively (Table 1), with total concentrations of 2.40 and 2.64 μg g−1 of soil, respectively, in the layer of 0–5 cm. Similar to total PLFA, in sites 9 and 14, the amount of saprophytic fungal PLFA (0.911 and 0.448 μg g−1 soil, respectively), AMF (0.543 and 0.594 μg g−1 soil, respectively), Gram + bacterial PLFA (5.005 and 5.307 μg g−1 soil, respectively), Gram – bacterial PLFA (7.840 and 7.521 μg g−1 soil, respectively) and actinomycetes PLFA (2.179 and 2.301 μg g−1 soil, respectively) were higher compared to other sites in the layer of 0–5 cm. In the deepest soil layer (5–30 cm), the total PLFAs and the amounts of PLFAs of fungi, AMF, gram-positive bacteria, gramnegative bacteria and actinomycetes were higher in the site 9, in comparison to the other sites, showing the highest total concentration of PLFA, 20 μg g−1 of soil (Fig. 3). These results demonstrate the effect of the anthropic action on the total amount of microorganisms in the soil, since the sites 9 and 14 are areas close to the city, but with the soil covered by a secondary vegetation (Table 1), which creates favorable conditions for the increase in soil moisture and concentration of OM, which are essential for the development of microorganisms. On the other hand, in the sites 18 and 21, there is an intensive use of the areas by the activities of agriculture and shrimp farming (Table 1), which constantly require soil disturbance through plowing and harrowing, performed by heavy machines that compromise the development of soil MC. Such variation between the use and occupation of these areas demonstrates the sensitivity of the MC structure to the anthropic action and that the soil PLFA technique identifies the variations and physiological alterations of the MC in response to an environmental stress factor (Fernandes and Chaer, 2010). According to the marker PLFAs used to indicate specific microbial groups, Gram + and Gram - bacteria were the ones that most varied with the use and management adopted in the sites 3 and 14 (Fig. 3). The PLFA profile provides a quantitative evaluation of the various groups in the MC of the soil (Viana et al., 2011; White, 1983). 4. Conclusion The soil attributes clay, OM, silt, pH and sand contributed to explain the contents of heavy metals in the studied soil samples. However, the attributes related to soil CEC showed the following order of importance in the retention of metals: clay N OM N silt N pH N sand. According to the national standards of CETESB (2014), established for the maximum contents of heavy metals in the soil, some sites in the studied area are contaminated by heavy metals. The areas surrounding the Curu River need constant monitoring in order to avoid the increase in the contents of heavy metals to levels that could cause harmful changes in the quality of the soil in the region. The microbiological attributes and the contents of heavy metals suggest that these factors are independent in these soils. Acknowledgements We acknowledge the participation of California State University, Fresno (CSU Fresno) undergraduate and graduate students who jointly worked on this study as members of the Bañuelos research team. This work was funded by National Institute of Science and Technology in Salinity (INCTSal) (grant number 573884/2008-0) and National Council


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Fig. 3. Saprophytic fungi, AMF, Gram + bacteria, Gram – bacteria, actinomycetes, eukaryotic and total amount of PLFAs for the 22 sampling sites along the Curu River shore, Ceará, Brazil at 0–5 cm and 5–30 cm depth.

for Scientific and Technological Development (CNPq) (grant number 246916/2012-5). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.geodrs.2018.e00173.

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