Geoderma 136 (2006) 20 – 27 www.elsevier.com/locate/geoderma PAHs and trace elements in volcanic urban and natural soils Giulia Maisto a,⁎, Flavia De Nicola a , Paola Iovieno c , Maria Vittoria Prati b , Anna Alfani c a Dipartimento di Biologia Strutturale e Funzionale, Università degli Studi di Napoli Federico II, Complesso Universitario di Monte Sant'Angelo, via Cinthia, 80126 Napoli, Italy b Istituto Motori del CNR, via Marconi 8, 80125 Napoli, Italy c Dipartimento di Chimica, Università di Salerno, via S. Allende, 84081 Baronissi (SA), Italy Received 23 December 2004; received in revised form 16 December 2005; accepted 19 January 2006 Available online 3 March 2006 Abstract Surface volcanic soils (0–5 cm) were analysed for 8 trace elements (Cd, Cr, Cu, Fe, Mn, Pb, V, Zn) and 27 PAHs. Soils were collected every four months at five urban sites in Naples and at a control site in the Vesuvius National Park from May 2001 to May 2002. Cr, V and Fe were more abundant in the control soils while Pb, Zn and ΣPAH were higher in the urban soils. Pb was 2–4 and ΣPAH 2–20 fold higher in urban than in control soil, whereas V was 2 fold higher in the control than in urban soils. Among the urban soils, both trace element and PAH accumulations also differed. Atmospheric deposition is responsible for trace element and PAH accumulation in urban soils. Phenanthrene, fluoranthene, pyrene, benzo[a]pyrene, benzo[b+k+j]fluoranthene and benzo[g,h,i] perylene were the most abundant PAHs both in control and urban soils. Naphthalene was also high in the control soil. Coronene was the most abundant at the urban site near the highway. The accumulation of PAHs in soils appeared to be affected by soil organic matter and microbial metabolism. © 2006 Elsevier B.V. All rights reserved. Keywords: Trace elements; PAHs; Soil accumulation; PAH profiles; Naples 1. Introduction Soils may be affected by deposition of various kinds of air pollutants, such as polycyclic aromatic hydrocarbons (PAHs) and trace elements (Adriano, 1986; Alfani et al., 1996; Sheu et al., 1997; Maisto et al., 2004a). PAHs derive from combustion processes both of natural and anthropogenic origin; trace elements are emitted, conspicuously, by human activities. The magnitude of ⁎ Corresponding author. Tel.: +39 081 679095; fax: +39 081 679233. E-mail address: g.maisto@unina.it (G. Maisto). 0016-7061/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2006.01.009 the contamination depends on different parameters such as air pollutant concentrations, prevailing atmospheric conditions and size distributions of the particulate fraction (Baek et al., 1991). Soil concentrations of trace elements and PAHs result from the integration of air contamination over long-term exposure (Maisto et al., 2003, 2004b). However, litterfall, soil microbial activities and soil texture may also affect the accumulation of both trace elements and PAHs in surface soils (Kabata-Pendias and Pendias, 1992; Tuháćková et al., 2001). Our research aimed to detect the concentrations of some trace elements and PAHs in urban and control 21 G. Maisto et al. / Geoderma 136 (2006) 20–27 2001) and the microbial biomass C, by substrate-induced respiration method (Sparling, 1995), were also determined in triplicate, measuring the CO2 evolution rate in non-amended and glucose-amended samples by gas chromatography, after 4 h of incubation at 25 °C in darkness. Microbial biomass C (μg C g− 1 d.w. soil) was calculated as 50.4 × glucose respiration rate (μl CO2 g− 1 d.w. soil h− 1). soils in order to evaluate how air pollutant deposition influences the level of the investigated compounds. The relationship between soil organic matter, microbial activity and PAH accumulation was also investigated. 2. Materials and methods Samplings of surface soils (0–5 cm) were carried out, after litter removal, at five urban sites in Naples (U1–U5), and at a control site in woodland in the Vesuvius National Park (C), 12 km from Naples at about 750 m a.s.l. The sampled soils originated from pyroclastic materials coming from various Vesuvius eruptions (Di Gennaro, 2002; De Nicola et al., 2003). The sites are characterised by a typical Mediterranean climate. The urban sites, located in downtown Naples, are affected by high traffic flow and differ greatly in some characteristics: two of them have soft soils and are rich in organic matter with dense, extensive grass cover and abundant litter (U2 and U5), one with a non-mature soil profile is covered by lawns and has no litter, since it was periodically removed (U1), and two have compact soils, little grass cover and scant litter (U3 and U4). However, all the soils have volcanic origins insofar as they were imported from the Vesuvius area. Soil samples were collected at the base of the same plant species (Quercus ilex L.) in order to minimize the effect of vegetation upon soil characteristics. Soils were sampled at the stem flow microsites to enhance the effect of air depositions. The samplings were carried out from May 2001 to May 2002 every four months. At each site the soils were sampled, from the base of at least 8 specimens, through a PVC cylinder and a Plexiglas trowel in order to avoid the introduction of metal contamination. In the laboratory, the soil sub-samples collected at each site were mixed to obtain a homogeneous sample and sieved (2 mm). Organic matter contents by loss on ignition, pH by electrometric method in a water suspension (1 : 2.5 = soil : water), water contents by weighing before and after oven drying at 105 °C were all measured in the collected soils in triplicate. The basal respiration (Schipper et al., 2.1. Detection of PAH concentrations Air-dried soil samples (ca. 30 g wet weight) were mixed with equal quantities of sodium sulphate anhydrous. A mix of deuterated PAHs (naphthalene D8, acenaphthene D10, phenanthrene D10, chrysene D10 and perylene D12) was added before extraction for quantification. The samples were extracted in a mixture of dichloromethane : acetone (1 : 1 = v : v) by 3 consecutive sonications (Misonix, XL2020). After the first sonication (3 min) in 100 ml of solvent mixture, the samples were recovered by vacuum filtration and the extracts were stored in a flask. The recovered samples were sonicated (3 min) again, adding 100 ml of solvent mixture, and consecutively a third sonication (3 min) was carried out adding another 100 ml of the solvent mixture. After the third sonication, the samples were vacuum filtered and the extracts were added to the extracts from the first sonication. The extracts were rotary evaporated to ca. 5 ml, filtered (0.2 μm), and completely dried under a gentle nitrogen stream. Subsequently, they were diluted with 4 ml of dichloromethane to be analysed. We developed our method using EPA 3550c as a starting point (Index to EPA test methods, 2003). The analyses were carried out by GC-MS (Hewlett Packard 5890 GC with on-column manual injection, coupled to a Hewlett Packard 5971 mass-selective detector, and equipped with an HP-5MS capillary column 30 m × 0.25 mm i.d. with a 0.25 μm film thickness and a phase ratio of 250). Oven temperature program was held at 33 °C for 1 min, then ramped at 20 °C min− 1 to 280 °C and held for 15 min. The on-column inlet Table 1 Organic matter content (mg g− 1 d.w.), basal respiration (μg CO2 g− 1 d.w. h− 1), microbial biomass C (μg C g− 1 d.w.), pH, water content (mg g− 1 d.w.) mean values (s.e.), in the control and urban soils over the investigated period C Organic matter Basal respiration Microbial biomass C pH Water content a 287.7 (10.5) 35.77 (2.94) 2159 (185) 5.51 (0.26) 586.5 (111.4) Data referred to September 2001. U1 91.8 (7.9) 3.13 a 549 a 7.66 (0.17) 350.8 (39.1) U2 261.9 (12.2) 32.52 a 1617 a 6.90 (0.13) 384.4 (59.4) U3 80.6 (2.8) 12.19 a 1158 a 6.37 (0.16) 178.8 (24.4) U4 76.3 (7.2) 8.02 a 875 a 6.16 (0.25) 120.5 (25.9) U5 161.5 (6.4) 24.22 a 2198 a 5.90 (0.22) 304.0 (36.1) 22 G. Maisto et al. / Geoderma 136 (2006) 20–27 Table 2 Mean concentrations (s.e.) of Cd, Cr, Cu, Mn, Pb, V (μg g− 1 d.w.), Fe, Zn (mg g− 1 d.w.), and ΣPAHs (ng g− 1 d.w.) in the control and urban soils over the investigated period Cd Cr Cu Fe Mn Pb V Zn ΣPAHs C U1 U2 U3 U4 U5 0.42 (0.06) 47.84 (3.46) 64.45 (3.03) 41.65 (1.87) 950.00 (37.88) 52.16 (6.97) 223.41 (17.73) 103.50 (10.65) 264.84 (28.03) 0.31 (0.05) 29.95 (2.08) 44.04 (4.50) 34.19 (1.44) 1050.00 (31.18) 79.12 (8.06) 118.64 (7.24) 115.33 (5.33) 677.14 (45.99) 0.72 (0.08) 22.82 (2.00) 178.83 (12.11) 30.77 (1.37) 941.67 (39.97) 198.16 (18.50) 100.68 (4.09) 189.17 (7.39) 1462.94 (133.40) 0.52 (0.07) 45.55 (4.13) 104.94 (9.11) 38.65 (0.69) 983.33 (29.66) 136.69 (12.44) 142.64 (18.35) 197.67 (15.54) 5293.90 (988.06) 0.60 (0.03) 26.92 (3.14) 109.80 (7.93) 38.81 (1.43) 995.83 (35.60) 156.01 (13.66) 111.43 (15.94) 181.17 (7.76) 3576.08 (267.51) 0.64 (0.03) 15.82 (1.05) 46.77 (1.45) 28.52 (1.08) 887.50 (46.34) 222.10 (3.56) 81.92 (8.03) 220.83 (15.95) 874.42 (22.81) temperature was set in oven track, with an injector temperature 3 °C higher than the oven temperature at all times. The carrier gas was helium (constant flow rate: 1 ml min− 1). The SIM (selected ion monitoring) modality of acquisition was used. The 27 investigated PAHs were naphthalene (Nap), acenaphthylene (Acy), acenaphthene (Ace), fluorene (Fle), phenanthrene (Phe), anthracene (Ant), fluoran- thene (Fla), pyrene (Pyr), 2,3-benzofluorene (BFle), benzo[c]phenanthrene (BPhe), benzo[g,h,i]fluoranthene (BghiF), benzo[b]nafto(1,2-d)tiophene (BnT), cyclopenta[c,d]pyrene (CpPyr), benzo[a]anthracene (BaA), chrysene + triphenylene (CT), benzo[b+k+j]fluoranthene (BbkjF), benzo[e]pyrene (BeP), benzo[a]pyrene (BaP), 1,3,5-triphenylbenzene (TpB), perylene (Per), indeno[1,2,3-c,d]pyrene (Ind), benzo[b] Fig. 1. Boxplot of trace element and total PAH concentrations in the urban soils collected in May and September 2001, and in January and May 2002. The box indicates the 25th and 75th percentiles, the continuous lines indicate the median values. The line represents concentrations in the control soil. G. Maisto et al. / Geoderma 136 (2006) 20–27 chrysene ce:hsp sp="0.12"/>+ picene (BbCP), dibenzo [a,c]anthracene (DacA), dibenzo[a,h]anthracene (DahA), benzo[g,h,i]perylene (BghiP), anthanthrene (Anth) and coronene (Cor). They were quantified after a calibration curve had been performed for each PAH; the analyses were carried out in triplicate. Concentrations of all the PAHs were calculated considering the recovery of deuterated PAHs that ranged from 40% to 80%, with the highest losses for naphthalene and acenaphthene. The ΣPAHs reported in this study represent the sum of the concentrations of all compounds. The analytical detection limit was approximately 2 pg μl− 1, for each analysed PAH. 2.2. Detection of trace element concentrations The oven-dried (75 °C) soil samples were ground into a fine powder by an agate pocket (Fritsch pulverisette) and 250 mg of each sample were mineralised with the addition of 2 ml of HF (50%) and 4 ml of HNO3 (65%) in a micro-wave oven (Milestone mls 1200, Microwave Laboratory Systems). Total concentrations of the elements were measured by atomic absorption spectrometry (SpectrAA 20, Varian) via graphite furnace (Cd, Cr, Cu, Pb and V) or via flame (Fe, Mn and Zn). The analyses were carried out in triplicate. Accuracy of the trace element measurements was checked by concurrent analysis of standard reference materials from the Community Bureau of Reference from the Commission of the European Communities (BCR No. 142R): recovery rates ranged from 86% to 98%. 23 Over the period studied, in the urban soils, for Cd, Cu, Pb, Zn and ΣPAHs, higher values were generally detected than in the control soil; by contrast, Cr, Fe and V had lower values in the urban soils than in the control soil. Mn exhibited similar values in urban and control soils (Table 2). At each sampling, there were differences in trace element concentrations between control and urban soils depending on each investigated element (Fig. 1). Cu and Mn soil concentrations at the control site were in the range of the values detected in the urban soils (Fig. 1), although Cu concentrations in the control soil showed values close to the lowest in the urban soils. Cr and V concentrations were higher in the control than urban soils, as was Fe with the exception of January (Fig. 1); Pb and Zn were higher in the urban soils, as was Cd with the exception of September (Fig. 1). The PAHs reflected the same trend as Pb and Zn (Fig. 1), showing at each sampling higher values in the urban than in the control soils. Among the urban soils large concentration differences were evident for Cd, Cr, Cu, Pb and Zn (Table 2). Differences in ΣPAH concentrations were more substantial, with the highest values at sites U3 and U4 and the lowest at sites U1 and U5 (Table 2). At the control site, Cd, Cr, Mn, Pb and Zn concentrations showed similar trends over time with the highest values in September (Fig. 1). At the urban sites no similar trend was observed among sites or contaminants. The profiles of the detected PAHs were almost the same in the control and urban soils as shown in 2.3. Statistical analyses The data were processed by statistical tests. The correlations between each element and ΣPAH concentrations in the soils as well as the correlations between the investigated pollutants and the physico-chemical soil characteristics were evaluated by Spearman's test, using the SigmaStat package. 3. Results Organic matter content and water content were higher in the control than urban soils; by contrast, pH was lower (Table 1). Among the urban soils, three sites showed the lowest basal respiration and microbial biomass, accordingly with the lowest values of soil organic matter, while at the other two sites basal respiration and microbial biomass showed values close to the control site. Fig. 2. Percentages of each PAH over the total in urban soils (U1–U5) and in the control soil (C) at the January 2002 sampling. The trend of the profiles was repetitive among the samplings. 24 G. Maisto et al. / Geoderma 136 (2006) 20–27 Table 3 Spearman correlation coefficients between element concentrations and between each element and total PAH concentrations in the soils: *P b 0.05, **P b 0.01, ***P b 0.001 Cd Cr Cu Fe Mn Pb V Zn ΣPAHs 1 − 0.570** 0.426* NS NS 0.770*** − 0.519** 0.740*** NS Cd 1 NS 0.730*** NS − 0.730*** 0.780*** − 0.503* NS Cr 1 NS NS NS NS NS 0.606** Cu 1 NS − 0.541** 0.700*** NS NS Fe Fig. 2 and were repetitive among the samplings. Phe, Fla, Pyr, BbkjF, BaP and BghiP were among the most abundant PAHs both in the control and urban soils (Fig. 2). Nap showed high concentrations in the control soils, whereas benzo[g,h,i]perylene showed high concentrations in the urban soils (Fig. 2). In the soil at the site near the highway (U5), Cor showed the highest percentage contribution to the ΣPAH concentration (Fig. 2). In Table 3 the correlations are reported among the element concentrations and between each element and ΣPAH concentrations in the soils. Various elements were correlated to one another and the ΣPAHs were correlated to Cu, Pb and Zn (Table 3). The soil trace element concentrations were not correlated to organic matter content and pH, with the exception of Mn that was negatively correlated to organic matter (P b 0.05) and positively correlated to pH (P b 0.01). ΣPAH concentrations were negatively correlated to soil organic matter (P b 0.001). The same results were obtained for each PAH. 4. Discussion The findings of the investigated compounds show a clear separation between the control and urban soils. The highest concentrations of both some trace elements and PAHs indicated an accumulation in urban soils, attributable to air deposition, although a contribution to the total concentrations of some investigated elements could also come from the bedrock. The higher concentrations of Cr, V and Fe in the control soils are attributable to the strictly volcanic origin of the substrate. Particularly conspicuous was Cr content, about two fold higher than the content in each urban soil. De Nicola et al. (2003) found higher Cr and V concentrations in the deeper layer (15–20 cm) than in the surface layers at various sites of the Vesuvius National Park, suggesting clear derivation of these 1 NS NS NS NS Mn 1 − 0.730*** 0.880*** 0.485* Pb 1 − 0.511* NS V 1 0.600** Zn 1 ΣPAHs elements from the volcanic substrate. Giammanco et al. (1996) found soil Fe and V resulting from the volcanic substrates of Mount Etna (Sicily). The higher concentrations of Pb, Zn, Cd and Cu in the urban than control soils are due to air deposition. Cr and Fe are also abundant in the air deposition in the urban area of Naples (Alfani et al., 2000) but the high background values hide the atmospheric contribution. Pb and Zn showed great accumulation in urban soils with concentrations almost two fold higher than in control soil. Pb and Zn were more abundant at site U5 near the highway. It is well known that trace element contamination of urban soils is caused especially by vehicular traffic (Berthelsen and Steinnes, 1995; Bloemen et al., 1995; Alfani et al., 1996; Pichtel et al., 2000; Maisto et al., 2004a). Typical soil concentrations of ΣPAHs from rural areas are estimated at about 100 ng g− 1 d.w. (Edwards, 1983; Trapido, 1999); the higher concentrations detected in the urban soils suggest that the latter sites are affected by considerable air deposition. Our findings show PAH contents in urban soils 2–20 times higher than in control soils. The comparable trend of each PAH in the urban soils suggests that air deposition influences the PAH soil profile in the same way. The high percentage of benzo[g,h,i]perylene and coronene in the urban soils is attributable to their continuous emission by vehicular traffic and consequent deposition (Cretney et al., 1985; Yang et al., 1991; Amagai et al., 1999). Moreover, the relatively high resistance of these compounds to degradation contributes to their accumulation in soils (Johnsen et al., 2005). In the soil collected at site U5 (near the highway, affected by heavy continuous traffic flow), the high contribution of coronene could be attributed to the load of vehicles that use diesel fuel, whose combustion emits high levels of this PAH (Mastral et al., 2003). The higher percentages of some PAHs, such as naphthalene, in the control soil, suggest a source other than urban. Naphthalene would seem to G. Maisto et al. / Geoderma 136 (2006) 20–27 derive from natural sources such as plant and microbial metabolism (Borneff et al., 1968; Wilcke et al., 1999). The higher percentages of naphthalene found in urban soils rich in organic matter (U1 and U5) with a dense plant cover also support a biological source in urban soils (Wilcke et al., 1999; Atanassova and Brümmer, 2004). The correlations found between PAHs and Cu, Pb and Zn concentrations suggest a common source for some PAHs and trace metals. Indeed, several authors show the relationship of some PAHs and trace elements with high traffic load (Bloemen et al., 1995; Chen et al., 1997; Pichtel et al., 2000). The wide range of the organic matter in the urban soils appears to depend on litter removal. At the urban sites, U2 and U5, with rich grass cover and abundant litter, the organic matter contents show values similar to those found in the control soils. Variations in soil water contents are related to organic matter due to the capability of the latter to hold water. The urban soils rich in organic matter and with good water availability also exhibited a higher microbial biomass and oxidative metabolism, as shown by basal respiration. The negative correlation between PAH soil concentrations and soil organic matter contents could be explained by the accumulation of PAHs due to scarce microbial activity depending on scarce availability of resources from litter. The site along the highway (U5) with the highest PAH depositions, evaluated by PAH Q. ilex leaf concentrations (Maisto et al., 2004b), showed values in the soils which were among the lowest at the urban sites. At this site, the rich amount of soil organic matter and rapid microbial activity appear to enhance high PAH biodegradation that determines a low soil accumulation of PAHs, while trace elements, particularly Pb and Zn, exhibited the highest accumulation. Sites U3 and U4 with a PAH deposition comparable to site U5 (unpublished data), but a scant microbial metabolism, exhibited the highest PAH accumulation in the soil. Compact soils, with scant oxygen, low water and low organic matter content are unfavourable to soil microbial activity and PAH biodegradation (Blakely et al., 2002; Johnsen et al., 2005). In soils with low organic matter content microbial growth is limited, and microbial biomass and activity are lower than in soils rich in organic matter. Moreover in OM-poor soils, fungi could produce lower amounts of oxidative enzymes (Gramss et al., 1999) such as lignin peroxidases and manganese peroxidases which are presumed to be involved in PAH degradation (Augustin and Muncnerova, 1994). PAH degrading fungi isolated from contaminated soils showed different abilities to degrade PAHs of different 25 molecular weight. Some of these fungi show a surprising ability to degrade highly condensed PAHs more efficiently than less condensed ones (Potin et al., 2004). However, the pollutant accumulation level found in Naples does not appear to have inhibited soil microbial activity that in some urban soils appears as considerable as in control soil. Also Johnsen et al. (2005) reported that diffuse pollution along roads is not toxic for PAH-mobilizing organisms. Not only biodegradation but also irreversible sorption, volatilisation and leaching are possible pathways that control soil PAH concentrations (Reilley et al., 1996). The highest element concentrations in the control soils recorded in September could be due to a further input of these elements in the surface soils from May to September due to the accumulation of dead leaves of Q. ilex L. that fall in late spring. In the urban soils, the highest element concentrations are not always detected in the same sampling, probably due to the heterogeneity of the urban soils and to various anthropogenic factors, such as quality and quantity of emissions, and possible litter removal, depending on the investigated site. The higher PAH concentrations in the control soils in January could be attributed to the low air temperatures that increase the affinity of the PAHs to the matrix with respect to the air. Variation in PAH air/leaf and air/soil partitioning with seasonality is widely reported (Kömp and McLachlan, 1997; Howsam et al., 2001; De Nicola et al., 2005). The PAH concentrations detected in the urban soils did not show the same temporal trend of the control soil, attributable to the heterogeneity of the urban soils, depending on the organic matter content (Kohl and Rice, 1999), the grass and litter cover (Cousins et al., 1999; Maisto et al., 2004b) and, probably, on continuous PAH emissions over the year at the urban sites. However, our findings demonstrate a clear spatial separation not only between control and urban soils but also among urban soils. In the volcanic studied soil, PAH net accumulation is affected not only by deposition but also by microbial decomposition. In fact, the urban soils with highest organic matter and microbial activity showed lowest PAH accumulation. Trace elements from the volcanic pedogenic substrate appear to hide air deposition of trace elements in the urban soils. 5. Conclusions Pb, Zn and ΣPAHs showed a substantial accumulation in the urban soils whereas Cr, V and Fe showed higher values in control soils. The distribution in soils of PAHs, unlike that of trace elements, would seem 26 G. Maisto et al. / Geoderma 136 (2006) 20–27 affected by soil organic matter and microbial metabolism. In addition to air deposition, the pedogenic substrate affects trace element concentrations in the soil. Further investigation could be required to highlight the main pathways of inputs and outputs of both trace elements and PAHs, regulating their accumulation in the soils. Acknowledgements This work was financially supported by MIUR (PRIN 2000 — MM05188955_001). References Adriano, D.C., 1986. Trace Elements in the Terrestrial Environment. Springer Press, Berlin, Heidelberg, New York. Alfani, A., Bartoli, G., Rutigliano, F.A., Maisto, G., Virzo De Santo, A., 1996. Trace metal biomonitoring in the soil and the leaves of Quercus ilex in the urban area of Naples. 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