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Effects of platinum from vehicle exhaust catalyst on carbon andnitrogen mineralization in soils

Karsten Kalbitza,⁎, David Schwesigb, Wenxia Wanga,c

aDepartment of Soil Ecology, University of Bayreuth, 95440 Bayreuth, GermanybIWW Water Centre, Moritzstr. 26, 45476 Mülheim an der Ruhr, GermanycCollege of Resource & Environmental Sciences, Northwest A&F University, Yangling, Shaanxi 712100, China

A R T I C L E I N F O

⁎ Corresponding author. Tel.: +49 921 55 5624E-mail address: karsten.kalbitz@uni-bayreu

0048-9697/$ – see front matter © 2008 Elsevidoi:10.1016/j.scitotenv.2008.06.044

A B S T R A C T

Article history:Received 10 March 2008Received in revised form 23 June 2008Accepted 24 June 2008Available online 3 August 2008

There is strong evidence of continuously increasing contamination of soils with platinumgroup elements (PGE), in particular with platinum (Pt) from vehicle exhaust catalysts inroadside soils. However, knowledge about the effects of Pt contamination on soil processesis very limited. The objective of this study was to investigate whether the contamination ofsoils with Pt at realistic environmental levels leads to observable adverse effects on selectedindicator parameters of the carbon and nitrogen turnover in soils. Incubation experimentswith artificially contaminated soils and solutions containing dissolved organic matter(DOM) were carried out by the use of milled material from a Pt-containing vehicle exhaustcatalyst. Interaction of the catalyst material with the soil resulted in amobilization of Pt intothe dissolved phase reaching up to 0.1% of the added Pt. The amount of Pt mobilizationseemed to be mainly driven by the pH of the soil. Mineralization of carbon and nitrogen didnot reveal any significant adverse effect of the Pt addition as compared to the controlsamples. Future studies dealing with Pt effects on soil processes should focus onenvironmental conditions favoring Pt mobilization, e.g. such as very low pH values orlarge concentrations of DOM.

© 2008 Elsevier B.V. All rights reserved.

Keywords:BioavailabilityDissolved organic matterPGESorption

1. Introduction

During the last decades there has been an increasing use ofplatinum group elements (PGE) for a number of applicationssuch as vehicle exhaust catalysts, chemical catalysis, jew-ellery and anticancer therapy (Hoppstock and Sures, 2004;Ravindra et al., 2004). As a consequence, increasing concen-trations of platinum (Pt), palladium (Pd) and rhodium (Rh)have been reported for most environmental compartments, inparticular for soils, road dust, sediments and sewage sludge(Laschka and Nachtwey, 1997; Helmers et al., 1998; Schäferet al., 1999; Pyrzyńska, 2000; Haus et al., 2007).

In the first years after introducing vehicle exhaust catalystsit was believed that PGE are relatively inert. However, recentstudies have provided evidence that PGE bound to ceramic

; fax: +49 921 55 5799.th.de (K. Kalbitz).

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particles from car catalysts can be solubilized under environ-mental conditions, and that even in their particulate form theyare at least partly bioavailable to both plants and animals(Moldovan et al., 2001; Ravindra et al., 2004; Zimmermannet al., 2005). Most available studies on bioaccumulation of PGEare focused on fish, mussels, freshwater isopods or algae(reviewed inRavindra et al., 2004). Little is knownon theuptakeof PGE emitted fromvehicle exhaust catalysts by the terrestrialbiosphere except for a few studies showing some uptake byplants (Hooda et al., 2007; Ravindra et al., 2004). In particular,up to nownothing is knownabout the effects of increasing PGEcontents in terrestrial ecosystems on soil microbial processessuch as carbon (C) and nitrogen (N) mineralization. Regardingthe still increasing Pt levels of currently up to 200 µg/kg inroadside soils (Zereini et al., 2007), one may hypothesize that

.

Table 1 –Main properties of the used soil samples

Horizon Clay Silt Sand pH TOCa TNb

(%) (CaCl2) g kg−1 g kg−1

Arable soil 1 (conventionally managed) Ap 23 70 7 7.4 26.5 1.5Arable soil 2 (organically managed) Ap 32 66 2 7.3 28.9 1.8Deciduous forest soil Oe/Oa n.d. n.d. n.d. 3.4 258.6 12.0Coniferous forest soil Oa n.d. n.d. n.d. 2.6 294.1 13.4

n.d.=Not determined.a Total organic carbon.b Total nitrogen.

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adverse effects of Pt on the soil microbial community mayoccur, and may lead to changes in the turnover of soil organicmatter. Profoundknowledge about these factors andprocessesis a prerequisite for any risk assessment of the currentsituation in PGE polluted sites, and also for any remediationmeasures to improve soil and water quality. Micro-toxicitytests have shown that the EC50 of platinum chloride for Pho-tobacterium phosphoreum was 25 µg L−1 which was much lowerthan that of Cu (200 µg L−1; Wei and Morrison, 1994).

Carbon and N mineralization are crucial processes of soilorganic C and N dynamics and affect the amounts of C and Naccumulating in the soil and leached into deeper soil horizons.In turn, rates of mineralization have been extensively used asassays to study metal toxicity to soil microorganisms (Gilleret al., 1998). Mineralization of soil organicmatter appears to besensitive to metal contamination in forest soils whereas foragricultural soils the results are conflicting (Giller et al., 1998).However, threshold values where decreasing mineralizationrates could be detected vary in a large range because of factorsmodifying the toxicity of metals and differences in sensitivityof microorganisms (Giller et al., 1998). Carbon and N miner-alization depend strongly on the size of the labile C pool inaddition to the effects of the microbial community and abioticenvironmental conditions like temperature and moisture(Davidson et al., 1998; Reichstein et al., 2000; Fierer andSchimel, 2002; Park and Matzner, 2006). Although dissolvedorganic matter (DOM) makes up only a small portion of totalsoil organic matter (mostly less than 1%; Zsolnay, 1996;Chantigny, 2003) it may provide a measure of microbiallyavailable C (Burford and Bremner, 1975; Marschner andBredow, 2002; Rees and Parker, 2005). Concurrently, thewater-soluble fraction of Pt deposited onto soils might be asuitable indicator for adverse effects of Pt contamination onsoil microbial processes.

In our study we expected adverse effects of soil contamina-tion with Pt from vehicle exhaust catalyst on soil functions. Wehypothesized increasing concentrations of water-soluble Ptafter contamination with Pt accompanied by decreasing C andN mineralization. Furthermore, we expected a decreased bio-degradation of DOM as a direct consequence of Pt contamina-tion. We added shredded and milled vehicle exhaust catalystmaterial to 4 different soils adjusting a contamination level thatis typical for roadside soils. As controls we used the ceramicsupport material of the vehicle exhaust catalyst without thePGE-containingwashcoat-layer and untreated soils. In a secondexperimental approach we added the catalyst material todifferent DOM samples and quantified its biodegradation.

2. Materials and methods

2.1. Soil and solution samples

We used two agricultural and two forest soils to test effects ofPt contamination on C and N mineralization from soils. Theused soils differed largely in their properties (Table 1) allowingsome general conclusions. The arable soils were collectedfrom a conventionally and an organically managed field inColle Valle Agrinatura, Italy (Marinari et al., 2007). The fieldmoist samples representing composite samples of six sub-plots per field were sieved (2 mm) and air-dried. The samplefrom the conventionally managed field is hereafter referred toas ‘arable soil 1’ and the sample from the organicallymanagedfield as ‘arable soil 2’. We also used the Oa horizon of a Norwayspruce forest in Bavaria, Germany (Waldstein site: ‘coniferousforest soil’; Kalbitz et al., 2004) and the Oe/Oa horizon of amixed beech/oak forest in Bavaria, Germany (Steigerwald:‘deciduous forest soil’; Kalbitz et al., 2007). The organicmaterial was sampled just before the experiment started infall 2006 and sieved (5 mm) at field moisture.

Water extracts of the Oi andOa horizon of the forest floor ofthe Norway spruce forest from the Waldstein site (‘coniferousforest soil’) were prepared at 5 °C with a soil to (ultrapure)water ratio of 1:10 (Seralpur Pro 90CN; Kalbitz et al., 2003).These extracts were considered as an acceptable surrogate tosoil solution DOM collected in situ (Chantigny et al., 2007), andwere used for testing the effect of Pt contamination onbiodegradation of DOM.

2.2. Incubation of soils to determine C andNmineralization

For soil incubation we used 25 g and 10 g of soil (dry massbasis) of the arable and forest soils, respectively. The sampleswith an adjusted water content of 50% of maximum waterholding capacity were placed into 250 ml vessels of theRESPICOND apparatus to monitor CO2 evolution continuously(Nordgren, 1988). After 6 days of pre-incubation and astabilization of the respiration rate we added vehicle exhaustcatalyst material to the soil samples. The catalyst ready formounting in the automobile industry had a Pt content of500 mg kg−1. Contents of other PGE were 130 mg kg−1 Pd and170mg kg−1 Rh. As a control we used the ceramicmaterial thatis used as the support medium of vehicle exhaust catalyst(synthetic cordierite, supplied by Imerys Tableware; Selb,Germany). This material without Pt-coating had a Pt content

241S C I E N C E O F T H E T O T A L E N V I R O N M E N T 4 0 5 ( 2 0 0 8 ) 2 3 9 – 2 4 5

of less than 5 µg kg−1. We used such a control because wewanted to be sure that possible measured effects are notcaused by the surface area of the ceramic particles. Bothmaterials were treated in the same way. The catalysts wereshredded and milled in order to get a size distributionrepresentative for emitted particles. Particle size distributionwas measured by laser obscuration (extinction) of the singlesuspended particles. Particles with a diameter N10 µm con-stitute approximately 80% of the particle mass in the milledmaterial. This is in good agreement with the findings on theparticle size distribution of platinum emissions from enginetest bench experiments (Artelt et al., 1999). Particles between0.7 and 20 µm constitute approximately 80% of the specificsurface area of thematerial which is about 9.8m2 g−1. After themilling, the material was muffled at 700 °C for 4 h in order toremove any organic contamination. We added 0.4 mg of thecatalyst per 1 g of soil to adjust a soil contamination of 200 µgPt kg−1 soil as found in contaminated roadside soils. The soilsand the catalyst material were well mixed in the incubationvessels. Soil without addition of any material was our secondcontrol. The Pt content of unpolluted soils is usually below10 µg kg−1. In the organic soil samples from the two Germansites, Pt contentswere even b3 µg kg−1. One set of samples wastransferred back to the RESPICOND apparatus to measure Cmineralization. The incubation was done at 20 °C for further31 days, with three replicates per treatment. A second set ofsamples was immediately extracted with 5 mM CaCl2 solution(pH=5.26) as an estimate of water-soluble Pt, C and N at thebeginning of soil incubation (Chantigny et al., 2007). We used asoil-solution ratio of 1:5 for the arable soils and 1:10 for theforest soils. Extraction was done at 5 °C for 24 h with 3 times ofmanual stirring of the suspensions. The suspensions werefiltered through pre-rinsed 0.45 μm cellulose-acetate mem-branes (Schleicher & Schuell; OE 67). The filtered solutionswere stored frozen until analysis for C and N. For thedetermination of Pt the solutions were stored in vesselsmade of perfluoroalkoxy polymer (PFA) rinsed with dilutedaqua regia and stabilized with nitric acid. At the end of the soilincubation,we carriedout a secondextractionwith 5mMCaCl2solution as described above. Differences in mineral N specieswere used as an estimate of N mineralization.

In our study we focused on the effects of Pt because it wasthe dominant platinum group element in the investigatedcatalyst and particular the contents of Pt increased in recentyears in airborne dust (Wichmann et al., 2007). Amounts ofwater-soluble Pt were determined before and after incubation(day 0 and day 31) by inductively coupled plasma-massspectrometry (ICP-MS) with an Elan DRC II (PerkinElmer,Sciex, Ontario, Canada) using the standard mode. The threemain Pt isotopes 194Pt, 195Pt and 196Pt were measured and theresults were checked for consistency. The main isotopes ofhafnium (180Hf) and tungsten (184W) were also measured asoxides of these elements are known to interfere with themeasurement of Pt isotopes, if present at high concentration.A concentric quartz nebulizer (Meinhard-type) with a cyclonicspray chamber was used for sample introduction to theplasma (nebulizer gas flow was 0.9 L/min, sample uptake1 ml/min). Iridium (1 µg/L) was used as internal standard.Other instrumental parameters were set according to stan-dard recommendations of the manufacturer. Furthermore we

measured (i) pH, (ii) dissolved organic C (DOC) (TOC 5050,Shimadzu; Tokyo, Japan), (iii) NH4

+, and (iv) NO3− by a

segmented flow analyzer (Skalar Sanplus Analyzer, SkalarAnalytical BV., Breda, Netherlands). In all extractswe analyzedspecific UV absorbance at 280 nm (UVIKON 930, BIO-TEKInstruments, Bad Friedrichshall, Germany) and recordedemission fluorescence spectra (SFM 25, BIO-TEK Instruments,Bad Friedrichshall, Germany) in order to get information aboutthe composition of DOM (Kalbitz et al., 2004).

In the soils we measured the C and N content by an auto-mated analyzer (CNS analyzer, Vario EL, Elementar, Hanau,Germany). The Pt content of the catalysts was measured by thesame method as used for Pt measurements in the soil extracts,butafteraquaregiamicrowavedigestion.To thispurpose, 250mgsample (dry weight) were suspended in 7 mL aqua regia andheated up to 190 °C for 90min in sealed PTFE containers. Digestswere diluted by water (1:20 at aminimum) prior to themeasure-ment. Validation and quality control of the method was per-formed by measurement of Pt in the certified reference materialfor Pt in road dust (BCR-723). On each measurement day, analiquot of BCR-723was subject to thewhole analytical procedure(including the microwave digestion step), and results werealways within the certified range (81.3+/−3.3 µg kg−1).

2.3. Incubation of soil extracts to determine biodegradationof dissolved organic matter

In a second experimental approach we quantified biodegrada-tion of dissolved organic matter (DOM) as affected by thevehicle exhaust catalyst. We incubated 60 mL of the waterextracts obtained from the Oi and Oa horizon (see above) in120mL incubation flasks and adjusted the C/N and C/P ratio to20 by adding NH4NO3 and K2HPO4 (analytical grade, Merck,Darmstadt, Germany). The initial DOC concentrations were60.5 mg C L−1 and 34.4mg C L−1 in the extracts of the Oi and Oahorizons, respectively. We had two treatments: vehicleexhaust catalyst (with Pt) and the ceramic core of the catalyst(Pt-free cordierite). In each treatment we added 10 mg of therespective material to the solutions, which was equal to about80 µg Pt L−1. DOM solutions without any addition of catalystmaterials were used as controls. Everything was done intriplicates. The solutions were inoculated by 0.6 mL unfilteredextracts from the Oa horizon. Samples were shaken manuallyeach day and incubated at 20 °C for 31 days. Biodegradation ofwater-extractable C was quantified by DOC measurementsafter the 31-day incubation.

We carried out an additional sorption experiment with thesame solutions and the same amounts of added catalystmaterial in order to quantify to what extent the declining DOCconcentrations during incubationwere due to sorption of DOMby the catalyst and the ceramic material. We shook thesamples horizontally at 5 °C for 24 h and quantified sorptionby DOC measurements of filtered samples in comparison tosamples without addition of catalyst materials.

2.4. Statistics

We examined Student's test for independent samples toexamine whether the addition of the catalyst materials

242 S C I E N C E O F T H E T O T A L E N V I R O N M E N T 4 0 5 ( 2 0 0 8 ) 2 3 9 – 2 4 5

significantly affected the measured properties of soils andsolutions. Furthermore, we conducted linear and non-linearcorrelation and regression analysis to detect controllingfactors on water-soluble Pt and C and N mineralization.

3. Results and discussion

3.1. Water-soluble Pt

At the beginning of soil incubation, concentrations of water-soluble Pt were significantly larger in all soils treated with thevehicle exhaust catalyst in comparison to the controls,whereas Pt concentrations of the two controls (3 ng L−1) didnot differ significantly. This means that the addition of theceramic core of the catalyst did not result in increasingconcentrations of water-soluble Pt. Platinum concentrationsdiffered significantly between the four treated soils with largerconcentrations in the forest soils in comparison to the arableones (Fig. 1).

The pH seems to be the key soil property responsible for thedifferent mobility of Pt in the studied soils. In the arable soils,the pH of the aqueous extracts was 7.3–7.4 resulting in averagePt concentrations of 8 ng L−1. Even a pH of 3.8 as measured inthe deciduous forest soil resulted in only slightly increasingconcentrations of water-soluble Pt (12 ng L−1). Only very lowpH values of about 3.0 asmeasured in the coniferous forest soilresulted in relatively large Pt concentrations of 46 ng L−1. Inthis soil, 0.09% of the added Pt could be found in the aqueousextract (Fig. 2). This proportion was only 0.01% in the arablesoils and 0.02% in the deciduous forest soil, and we found an

Fig. 1 –Water-soluble Pt and DOC concentrations in four soils amceramic core of the catalyst containing no Pt (ceramic) in comparwith 5 mM CaCl2 solution at the beginning of soil incubation (me

exponential relationship between pH and the water-solublepercentage of the Pt added with the catalyst (Fig. 2).

Zereini et al. (1997) found a similar percentage of solubi-lized Pt after contacting a crushed vehicle exhaust catalystwith rainwater in the pH range from 3–9 (0.01–0.025% of addedPt). In this study, solubilization of Pt increased exponentiallyat pH b3 and reached 0.35–0.5% of added Pt at pH 1. However,only 0.008–0.012% of the added Pt could be solubilized in amixture of rainwater and soil because of sorption processes(Zereini et al., 1997). Readsorption to soil particles of Ptmobilized from the catalyst should be responsible for decreas-ing water-soluble Pt concentrations during 31-days of incuba-tion in all soils reaching similar levels as the two controls (datanot shown). We found no evidence for post-depositionaltransformations resulting in increased mobility of Pt asproposed by Hooda et al. (2007).

Although Bowles and Gize (2005) reported similar propor-tions of water-soluble Pt as in our study, much larger propor-tions were reported in the review paper by Ek et al. (2004)reaching 3.9%water-soluble Pt. Thismight be caused by effectsof organic matter, because Pt can be solubilized by naturalcomplexing agents, resulting in bioavailable platinum species(Ek et al., 2004). Dahlheimer et al. (2007) demonstrated thesolubilizing effect of naturally occurring siderophores. Menzelet al. (2001) concluded that dissolved organic matter cancontribute to the redistribution of PGE in the environment.However, the effects of organic matter on Pt are discussedcontroversially, and in our study we did not find evidence forenhancement of Pt solubility by organicmatter at DOCconcen-trations typical for arable and forest soils. Based on the carboncontents in the aqueous extracts, the largest Pt concentrationswere expected to occur at the deciduous forest soil because of

ended with vehicle exhaust catalyst (Cat+Pt) and with theison to non-treated soil (control) as determined by extractionan and standard error of three replicates).

Fig. 2 –Proportions of water-soluble Pt as % of the addedamounts of Pt with vehicle exhaust catalyst to 4 soils independence on the pH in the aqueous extract(y=a*exp(−b*x)).

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the by far largest DOC concentrations (35 mg C L−1; Fig. 1).However, the largest Pt concentrations occurred in the extractsof the coniferous forest soil although DOC concentrations(20 mg C L−1) were smaller than at the deciduous forest soil. Itseems that the pH is more important for the mobility of Pt insoils than its solubilization by organic matter. However, itremains unclear whether much larger concentrations of DOCas occurring e.g. during snow melting, after litterfall or afterlong dry periods can mobilize additional Pt.

Fig. 3 –Cumulative C and N mineralization after 31 days of incubcatalyst (Cat+Pt) and with the ceramic core of the catalyst containmean and standard error of three replicates).

3.2. Effects of Pt on carbon and nitrogen mineralization

Carbon mineralization differed significantly between the fourstudied soils. We found the largest CO2 production (mg C per gof soil organic C) at the deciduous forest soil and smallestvalues at the coniferous forest soil (Fig. 3). In absolutenumbers, the four soils covered a large range in C mineraliza-tion (0.2–12 mg C g−1 soil) illustrating their suitability for thisstudy. However, we did not find a consistent effect of catalystaddition. Carbon mineralization decreased only in the arablesoil sampled from an organically managed site (arable soil 2).However, the decrease was not significant, neither in compar-ison to the soils where we added the ceramic core of thecatalyst nor to the untreated soil. Mineralization ratesdecreased also in the arable soil 2 during the first days afterthe addition of the catalyst material (data not shown),however, the decrease was not significant. The only signifi-cant effect we found was an unexpected increase in C miner-alization from the deciduous forest soil after addition of thePt-free ceramic material.

We did not find a consistent picture of the effects of Ptaddition comparing the DOC contents in the extracts rangingbetween 0.6 and 1.4 mg C per g soil C (Fig. 1). In the arable soilsDOC concentrations variedmore between the treatments thanin the forest soils. However, only the decrease in DOCconcentrations after addition of the ceramic core of thecatalyst to arable soil 1 was statistically significant. Propertiesof dissolved organic matter determined by UV and fluores-cence spectroscopy were also not significantly affected by anyof the treatments (data not shown). Therefore, amounts andproperties of DOM support the results of C mineralization.

ation at 20 °C in four soils amended with vehicle exhausting no Pt (Ceramic) in comparison to non-treated soil (control;

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Nitrogen mineralization was much larger in forest soilsthan in arable ones, with the largest N mineralizationoccurring in the deciduous forest soil (Fig. 3). This patterncoincided with Cmineralization per soil mass. Again, no effectof Pt addition could be detected.

The absence of any observable effects of Pt addition on Cand Nmineralizationmight be the result of sorption processesof the addedand solubilized Pt to the soilmatrix as proposedbyZereini et al. (1997). Therefore, we added the vehicle exhaustcatalyst directly to water extracts with large differences in thecomposition and degradability of organic matter. However,also this approach did not reveal any significant effect of Pton C degradation (Fig. 4). Biodegradation of water-soluble Cdepended on its source with much larger degradability of or-ganic matter extracted from the Oi horizon in comparison tothe Oa horizon as already reported by Kalbitz et al. (2003). Eventhe percentages of biodegraded organic matter were similar inthis study and in the study of Kalbitz et al. (2003), althoughdifferent methods were used for quantification (CO2 versusDOC measurements). We observed a slightly larger apparentdegradation of DOM after addition of the catalyst and of itsceramic corematerial. The observed difference of about 5mgCL−1 comparing solutions with and without addition of catalystmaterials (Fig. 4) was slightly larger than the proportion ofsorbed organic matter to these materials. Removal of DOM bythe catalyst materials amounted to 1–4% for both solutionsresulting in C contents of the catalyst materials between 2 and14 mg C g−1. Sorption of organic matter to mineral surfacesshould result in its stabilization against microbial decay(Kalbitz et al., 2005; Mikutta et al., 2007). However, we do notknow the long-term fate of organic matter sorbed to emittedand deposited particles from car exhaust catalysts. An enrich-ment of roadside soils to Pt levels of 200 µg kg−1 requires adeposition of 400 to 3000 times larger amounts of ceramicmaterials assuming an average Pt content of the catalystmaterial in the range of 0.036 to 0.236% (Böck, 2006). This factor

Fig. 4 –Degradation of dissolved organic C obtained from theOi and Oa horizon of a coniferous forest after 31 days ofincubation of the solutions at 20 °C; solutions were amendedwith vehicle exhaust catalyst (Cat+Pt) and with the ceramiccore of the catalyst containing no Pt (ceramic). Untreatedsolutions were used as a control (mean and standard error ofthree replicates).

might evenbe considerably larger assuming that thePt contentof road dust is usually less than 1 mg kg−1 (Ravindra et al.,2004). In sandy soils with often small contents of pedogenicoxides (0.5 to 20 g kg−1 soil) this material might contributesignificantly to the sorption capacity. Therefore, the long-termfate of organic matter sorbed to deposited particles fromcar exhaust catalysts should be of environmental relevanceparticularly in sandy soils.

4. Conclusions

Although Pt was mobilized from the added catalyst to a differ-ent extent in all soils, it had no significant effect on any of thestudied soil properties. We chose C and N mineralization andcontents of DOM as three indicators of labile organic matter insoils, because they reflect the diverse but central effects that theorganic matter has on soil properties and processes (Haynes,2005). Obviously, the concentrations of water-soluble Pt in thesoils which did not exceed 60 ng L−1 were too low to have anyobservable effect on microbiological soil processes. Even theaddition of the same amounts of the vehicle exhaust catalyst tosoil solution did not adversely affect biodegradation of organicC. In our studywemimickeda realistic level of Pt contaminationof roadside soils and realistic environmental conditions. How-ever, our main findings have to be tested for larger soil con-tamination and for environmental conditions favoring Ptmobilization, e.g. such as very low pH values or large concen-trations of dissolved organic matter. Besides, effects of Pt onother soil processes and specific organisms or specific enzymesshould be studied. That means we have to modify the con-trolling factors of the toxicity of Pt and the diversity of micro-organisms in order to include the most important variablesresponsible for the observed large differences in the response ofmineralization to metal additions (Giller et al., 1998). The timescale of the experimentswas rather short. At present, we donothave profound knowledge about the effects of ageing processeson Pt contamination such as changes in speciation andmobilityof Pt and its long-term effects on soil processes. Furthermore,long-term effects of the deposited particles containing PGE onsoil processes like sorption of DOM and its subsequent miner-alization should be studied.

Acknowledgements

We gratefully acknowledge the financial support from DAAD,CSC, and National Science Foundation of China (No. 30370288and 40571087).We thank themembers of the Central AnalyticalDepartment of BayCEER of the University of Bayreuth for ana-lytical support.

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